“Almost Pong” for the Commodore 64

Original version: https://www.lessmilk.com/almost-pong/

I recently came across a funky version of Pong called “Almost Pong”. I really liked it and thought it would be fun to re-create it for the Commodore 64. I went about this entirely in BASIC, without writing any assembler routines — by using a fancy compiler that I recently came across: https://egonolsen71.github.io/basicv2/. (Before you get too excited, the physics in my game are quite different — I don’t think doing the same physics calculations would work in realtime on the C64, but using lookup tables it might be possible to produce very similar physics.)

So is this a usable game? I don’t know, when I play the above-mentioned Almost Pong it kind of feels more fun, maybe it’s the physics, maybe it’s because my game is even more bare-bones, or maybe I’m just biased against my own game? Anyway, without further ado:

  • Works in VICE and on real hardware
  • Press fire button on joystick #2 to jump
  • The source code has lines that are longer than 80 lines and will therefore produce syntax errors if you type it into a C64 verbatim
  • It’ll also be way too slow to play if you don’t compile it using Basicv2

Here’s the source code:

0 in=0
1 poke 53280,1:poke 53281,0:poke 646,1
2 v=53248:lv=1:co=1:c=0:fr=0:x=160:y=141:rem center
3 if in=0 then print chr$(147):print " press fire to play":wait 56320,16,16:wait 56320,16:in=1
4 for t=12288 to 12415 step 1:poke t,0:next
5 for t=12289 to 12350 step 3:poke t,255:next:for t=12373 to 12397 step 3:poke t,255:next
6 poke v+21,7:poke v+39,1:poke v+40,1:pokev+41,co
7 poke v,72:poke v+16,1:poke v+2,16:poke v+4,x:poke 53271,3
8 poke v+1,y:poke v+3,y:poke v+5,y
9 poke 2040,192:poke 2041,192:poke 2042,193
10 s=54272:w=17:poke s+5,97: poke s+6,200: poke s+4,w:poke s+24,15:rem sound
15 ox=4:oy=1:od=1:jm=4
20 sx=ox:sy=oy:dy=od
21 gosub 1500
30 for i=1 to 2 step 0:rem infinite loop
35 sc=peek(53278):rem sprite collision lag workaround
40 x=x+sx
50 y=y+0
60 if x>255 then poke v+16,5:poke v+4,x-256:goto 70
65 if x<=255 then poke v+16,1:poke v+4,x
70 y=y+sy:sy=sy+dy
73 if y>234 then y=234:poke v+5,y:gosub 1000:rem todo could add poke v+5,y to the subroutine
74 if y<43 then y=43:poke v+5,y:gosub 1010
75 poke v+5,y
80 rem joystick
90 f1=peek(56320) and 16
92 if f1<>0 then fr=0
93 poke 162,0:wait 162,2:rem wait 1/80s
99 rem only fire if fire wasn't pressed during last poll
100 if fr=0 and f1=0 then y=y-sy*jm:sy=oy:dy=od:fr=1:gosub 1700

110 rem bounce
120 if sx < 0 or x<=328 then goto 150
125 rem x=328:pokev+4,x-256
126 poke v+5,y
127 sc=peek(53278)
130 if sc <> 0 then goto 140:rem bounce if we collided
131 if x+sx < 344 then goto 180
135 gosub 1020:rem game over if no collision
140 gosub 1600:rem bounce
145 poke v+1,int(rnd(0)*158)+50

150 if sx > 0 or x>=32 then goto 180
155 rem x=32:pokev+4,x
156 poke v+5,y
159 sc=peek(53278)
160 if sc <> 0 then goto 170:rem bounce if we collided
161 if x+sx > 16 then goto 180
165 gosub 1030:rem game over if no collision
170 gosub 1600:rem bounce
175 poke v+3,int(rnd(0)*158)+50
180 gosub 1800:next:rem sound off
190 end

1000 print " you lose (don't fall into the abyss)":gosub 1100
1010 print " you lose (don't jump into the sky)":gosub 1100
1020 print " you lose (you need to bounce off the":print " right paddle)":gosub 1100
1030 print " you lose (you need to bounce off the":print " left paddle)":gosub 1100
1100 poke s,120:poke s+1,6:poke 162,0:wait 162,16
1110 wm=0
1120 gosub 1800
1130 print " press fire to play"
1140 wait 56320,16,16:wait 56320,16
1150 goto 1
1160 return:rem not reached

1500 rem print game status
1510 print chr$(147):print " level:", lv, "points:", c
1520 return

1600 rem bounce
1610 sx=-sx
1620 c=c+1
1630 if c/5 < lv then goto 1650
1640 lv=lv+1:co=co+1:poke v+41,co:ox=ox+1:if co=15 then co=1
1650 gosub 1500:rem print sc:poke 162,0:wait 162,8
1660 return

1700 rem fire button sound on
1710 poke s,133:poke s+1,11
1730 wm=3:rem num of sound off gosubs to wait before muting
1740 return

1800 rem sound off
1810 if wm>0 then wm=wm-1:goto 1830
1820 poke s,0:poke s+1,0
1830 return

Here’s the compiled .prg:

Here’s me playing the game.

Stupid game

How to load software from the internets on a real C64 using the Datasette drive

Now here’s a (zipped) .wav file that you can put on a tape (or much easier, stream through a cassette adapter into your datasette drive) and load on a real computer. I will describe how to generate .wav files from .prg/.tap/.d64 files in the next section.

Notes on using cassette adapters to load C64 software

When using a cassette adapter, you should make sure your playback device’s volume is neither too loud nor too quiet. On my computer, 50% volume appears to be the sweet spot. You may have to experiment a bit to find your own. When using a cassette adapter, you will have to pause streaming manually (after the C64 prints out “FOUND NAMEOFPROGRAM”, as whatever device you’re using to stream is completely unaware of whether the datasette drive’s motor is on or off and just continues streaming regardless. (I didn’t add a long enough pause between the header(?) and the actual data. You could maybe add a multiple-second pause yourself to automate this part.) It’s probably generally best to stream using Audacity (or similar software) and watch the datasette drive to see whether the motor is on or off. When it’s off, you press stop and when the motor starts running again, reposition the cursor into the nearest section of silence and press play again. I was able to load various kinds of software using this approach. Commercial software may have multiple bits of silence where the motor stops running for a bit.

In this Audacity screenshot, there’s a short bit of silence at the 11 second mark. This is where the C64 will stop the motor and print “FOUND NAMEOFPROGRAM”. (Note that in the above .wav file, the name of the program is blank, so the C64 will only print “FOUND”.) When the motor stops, press the stop button. When the motor starts running again, reposition the cursor somewhere in the middle of the silent section (11.139s in this example) and press play.

Here’s an Audacity screenshot for Great Giana Sisters (the tape version):

The cursor is positioned in the first short bit of silence, right after the “FOUND NAMEOFPROGRAM” but there are multiple points where the motor stops spinning and you will have to manually press stop/play each time.
Great Giana Sisters! I think you had to press space to continue loading beyond this screen, which means you’d again need to manually press stop/play in Audacity

Now that we have talked about how to load .wav files into the C64, here’s how to actually generate .wav files:

Generating .wav files from .tap/.prg/.d64 files

I spent a few hours evaluating multiple solutions, and have found that wav-prg (https://wav-prg.sourceforge.io/) did the best job.

Here’s how to build this program on Linux:

git clone https://git.code.sf.net/p/wav-prg/libtap
cd wav-prg-libtap
make libtapdecoder.so
make libtapencoder.so
cd ..
git clone https://git.code.sf.net/p/wav-prg/libaudiotap
cd wav-prg-libaudiotap
make clean # probably not needed but happened to be in my notes, possibly for a reason
make -j4 DEBUG=y LINUX64BIT=y libaudiotap.so
cd ..
git clone https://git.code.sf.net/p/wav-prg/code
cd wav-prg-code
make clean # probably not needed but happened to be in my notes, possibly for a reason
make -j4 cmdline/wav2prg DEBUG=y AUDIOTAP_HDR=../wav-prg-libaudiotap/ AUDIOTAP_LIB=../wav-prg-libaudiotap/
make -j4 cmdline/prg2wav DEBUG=y AUDIOTAP_HDR=../wav-prg-libaudiotap/ AUDIOTAP_LIB=../wav-prg-libaudiotap/

Here are some example invocations for an NTSC C64. You can also generate .wav files for use with the PET (full explanation here) or the VIC-20.

# (pwd is .../wav-prg-code)
LD_LIBRARY_PATH=../wav-prg-libaudiotap/:../wav-prg-libtap/ cmdline/prg2wav -m c64ntsc -t filename.tap /path/to/prg # convert prg to tap image for use in vice etc.
LD_LIBRARY_PATH=../wav-prg-libaudiotap/:../wav-prg-libtap/ cmdline/prg2wav -m c64ntsc -w filename.wav /path/to/prg # convert prg directly to wav file

Here’s how to extract a .prg file from a .d64 floppy image, which you can then convert to a .wav file. All you need is VICE, which comes with a tool to extract data from .d64 files:

./c1541 -attach ~/retro/c64/software/BLOCKNB1.D64 -list
0 "ass presents:   "      
66    "block'n'bubble"    prg 
6     "block'n'bub. dox"  prg 
1     "doc-maker.code"    prg 
1     "scores"            prg 
590 blocks free.

“ass”? :p Anyway, judging by the number of blocks, “block’n’bubble” seems like the program we’re most interested in. (You can try extracting the others and running them in an emulator.) To extract and to convert, run the following commands:

# (pwd is .../vice-3.5/src)
./c1541 -attach ~/retro/c64/software/BLOCKNB1.D64 -read "block'n'bubble" bnb.prg
reading file `block'n'bubble' from unit 8

# (change directory to .../wav-prg-code)
LD_LIBRARY_PATH=../wav-prg-libaudiotap/:../wav-prg-libtap/ cmdline/prg2wav -m c64ntsc -w bnb.wav bnb.prg # adjust input and output file paths

I don’t remember how to play, but I remember the intro screen music. Felt good to hear it played from a real Commodore 64 for the first time in 15-20 years! Here’s a clip. Sorry for the copyright violation:

Block’n’Bubble title music (looped)

The game’s playable as-is, but at least on my hardware the first time I started a game I got some weird colors. (The second time round the colors were normal.) Could be a PAL-vs.-NTSC issue, or could have something to do with the above conversion (for example, a program could assume that the C64’s tape buffer is available for temporary usage, but when loading from tape… it isn’t).

Note that large commercial software is often divided into multiple files, e.g., consisting of a loader and a main program, and perhaps a high score file. In that case, you will not be able to easily convert the software. (You would have to re-write the parts where the loader starts loading from the floppy, etc.)

For example, here’s what’s in the Sokoban .d64:

./c1541 -attach ~/retro/c64/SOKOBAN0.D64 -list
0 "ass presents:   "      
1     "soko-ban"          prg 
40    "soko-ban docs"     prg 
56    "sokoban"           prg 
41    "title"             prg 
2     "maze01"            prg 
2     "maze55"            prg 
9     "start"             prg 
3     "flip"              prg 
1     "test"              prg 
509 blocks free.

It would probably require some hacking to convert this to tape.

However, not all commercial software is this complex. For example, using this method, I was able to convert the main .prg file in “JupiterLander_1982_Commodore.d64” to .tap and load this in VICE (didn’t try on real hardware but should work) and play as normal.

~/src/vice-3.5/src/c1541 -attach JupiterLander_1982_Commodore.d64 -list
0 "1982 commodore  " -136-
30    "j.lander +2  /n0"  prg 
5     "j.lander docs/n0"  prg 
629 blocks free.

~/src/vice-3.5/src/c1541 -attach JupiterLander_1982_Commodore.d64 -read "j.lander +2  /n0.prg" jupiterlander.prg

LD_LIBRARY_PATH=~/src/wav-prg-libaudiotap/:~/src/wav-prg-libtap/ ~/src/wav-prg-code/cmdline/prg2wav -t jupiterlander.tap jupiterlander.prg
Stupid game

Commodore PET 2001 repair

I recently had a look (multiple long looks) at a Commodore PET 2001 (with the ~first version of BASIC etc.) and a special character ROM with Japanese katakana instead of lower-case characters. I was told it had worked in around 2000 when it was last turned on. I went right in without knowing much about the Commodore PET.

Wobbly screen, junk characters on boot

The wobbly/unstable/warped screen was caused by an oxidized and slightly broken molex connector supplying power to the board. This caused the voltages to jump between 4-5 volts multiple times per second. Well, nothing’s going to work that way. (Ideally I would have fixed this first, but I’d already noticed a bunch of other bad connections and didn’t think the molex connector would go bad.)

Stable screen, junk characters on boot

All chips that cost more than a couple cents (don’t know the prices in 1977 of course) were socketed. But boy, these are bad sockets. The one redeeming feature of the sockets that were used by Commodore at this time is that it’s easy to check if the inserted chip is making a connection with the socket. Just put your multimeter in continuity mode, one multimeter probe on the chip’s pin, and the other on the slightly exposed metal part belonging to the socket. No beep — bingo, that needs to be fixed.

This isn’t my machine and I’m not allowed to use contact cleaner (except IPA of course). I’m sure that contact cleaner would have helped here though. Anyway, not all of the bad connections were due to oxidization as far as I can tell — it seems like the socket and the pin just aren’t making physical contact, even after judicial use of IPA.

In these cases you can either replace the bad socket, or you can try to bend the chip’s legs to get better contact. I’d advise you not to do that with the chips in the PET; most of them were rather brittle in my case. Another thing you can do, and which I did here, is to put a new socket right on top of the existing socket. In most cases that’ll improve things immediately.

So I did that on all RAM chips after testing continuity almost everywhere (there was at least one pin that didn’t make contact on most chips) and all ROM chips except one (one didn’t have any bad contacts for some reason. The middle one, so maybe that one was more protected from the elements compared to ones closer to the edge?), and one of the 6520s. For the CPU and other chips, just re-seating did the trick.

Double sockets almost everywhere. I used extra high-quality sockets for the left-most RAM chips and the video RAM at the back, and (accidentally) in one other location.

One thing you have to know about the PET 2001 is that it’s possible to boot with just 2K (or even 1K?) of RAM. So make sure to put working RAM (making good contact) into the leftmost RAM sockets. If you have slightly faulty RAM, it doesn’t matter so much if you have that in the sockets beyond 2K. Your PET (if it boots) will tell you how many bytes of RAM you have free on boot. If it says 7167 bytes (on an 8K model), that means your RAM is probably fine. If it’s less than that, you probably have faulty RAM somewhere. (Broken RAM that misbehaves grossly may cause problems though.)

Keyboard fixes

If your PET has successfully booted, you should test all keys. If there are keys that don’t appear to work, try holding the key down for a little, or repeatedly pressing the key. If none of that helps, you will need to take apart the keyboard and clean it up using IPA. That fixed all problems for me.

Clean these contacts with IPA
I also cleaned some of these conductive rubber? thingies with IPA, but only the ones on keys that were kind of problematic.

Testing RAM and ROM with a short BASIC program

If your PET has successfully booted, you may want to test your RAM and ROM chips. I found a BASIC script on the web to test RAM, but wasn’t able to run it back when my PET reported it only had 363 (or so) bytes free — the program was just too long. Original program from https://www.commodore.ca/commodore-manuals/commodore-pet-memory-test/. Here’s my modified version, which will work with much less RAM, has some visual feedback and isn’t that tough to type in.

21 FORY=1TO4
25 GOSUB200
30 DATA0,85,170,255
120 PRINT "."
121 NEXT
140 END

Note that the PET firmware will place the BASIC code at address 1025+, variables go between 1946-2071, and arrays go from 2072-2231. As long as our BASIC code doesn’t use arrays, not too many variables, and isn’t too long, the system won’t need to access memory beyond 2K. So it’s safe to run this program on all RAM, from 2048 to 8191. (It’s pretty slow BTW, you may want to run it multiple times, in 2K steps.)

Found some RAM errors! Stuck bit.
More RAM errors. Bit 3 (counting from 0) appears to be stuck at 0 here.

If you get errors, check the schematics for your board revision, re-test continuity between chip and socket on the supposedly faulty chip, and if there’s no connection error, replace it.

Here’s the ROM test, in ASM and BASIC (with data lines). On the PET, the ROM isn’t accessible from BASIC using PEEK, so assembly is required. Make sure you don’t make any mistakes when typing in the data lines.

.ORG = $1000
LDA #0
LDY #$D0 ; stop when INC_ADDRESS+2 reaches this value
CLC ; clear carry flag, otherwise we'd add unneeded +1s
ADC $C800 ; add contents of memory address $C800 to A (this address will be modified below)
JSR PRINT_HEX ; print out checksum for every added byte
INC INC_ADDRESS+1 ; increments insignificant byte
BNE LOOP ; if that address hasn't overflowed back to 0, go to LOOP
INC INC_ADDRESS+2 ; increments significant byte
CPY INC_ADDRESS+2 ; compare significant byte with Y register (into which we loaded the significant byte of the end address earlier)
BNE LOOP ; if unequal, go to LOOP
; JSR PRINT_HEX ; if we blazed past both BNEs then we're done so output checksum ; commented out because we currently print out the checksum for every added byte
RTS ; return from subroutine

PHA ; copy A to stack
LSR ; shift right 4 times so we get significant nibble
PLA ; get fresh copy of A from stack
PHA ; and also write it back on stack
AND #$0f ; get lower nibble
LDA #32 ; code for space
JSR $FFD2 ; print a space

CMP #10 ; if we're >= 10
CLC ; clear carry flag, otherwise we'd add unneeded +1s
ADC #48 ; otherwise, add 48 to turn into a number
JSR $FFD2 ; print number
RTS ; return
HEX_ABCDEF: ; this is the a-f branch
CLC ; clear carry flag, otherwise we'd add unneeded +1s
ADC #55 ; we're at range 10-15 but want range 65-70, so add some
JSR $FFD2 ; print a-f


10 t=4096
15 input "decimal msb of start address"; sa
16 input "decimal msb of end address "; ea
17 input "carry over "; co
20 read x
30 if x=-1 then sys 4096:end
40 if x=-2 then poke t,sa:goto 80
50 if x=-3 then poke t,ea:goto 80
60 if x=-4 then poke t,co:goto 80
70 poke t,x
80 t=t+1
90 goto 20

Here’s a very lazy script (bin2data.sh) to assemble (using acme) and generate a simple BASIC loader:


echo rem assuming compilation with acme --setpc 4096 -o foo check_rom.asm

echo '10 t=4096'
echo '20 read x:if x<>-1 then poke t,x:t=t+1:goto 20'
(od -Anone -tx1 $1 | perl -pe 's/([0-9a-fA-F]{2})/hex($1).","/eg' | sed -r -e 's/^/data/' -e 's/,$//'; echo 'data -1') | nl -i 10 -v 1000 -nln | sed -e 's/ //g' -e 's/ //g'

Here’s how to assemble using acme and use the bin2data.sh script:

acme --setpc 4096 -o check_rom.bin check_rom.asm; bin2data.sh check_rom.bin

Note that the BASIC listing above is almost identical to the output of the assembler script, except that I manually modified the output to place -4, -3, and -2 in the DATA lines. This is where the start/end addresses and the carry over go.

What this program does is sum up all values in the ROM. It also prints out the intermediate sum for each byte. Note that in the above loader, we poke the machine code into addresses 4096+. If you are on a 4K PET, that will not work. Just modify the .ORG, –setpc, and t=4096 lines in the assembly source, acme command line, and bin2data.sh, respectively.

The ROMs on my machine are 2K each. Their addresses are 0xC000-0xC7FF (most significant byte in decimal: 192), 0xC800-0xCFFF (200), 0xD000-0xD7FF (208), 0xD800-0xDFFF (216), 0xE000-0xE7FF (224), 0xF000-0xF7FF (240), 0xF800-0xFFFF (248).

So you enter 192 for the start address, 200 for the (non-inclusive) end address, 0 for carry over. On my machine (and in VICE), the last few numbers are 8F EF 91 CB. Note: the ROMs at https://www.zimmers.net/anonftp/pub/cbm/firmware/computers/pet/index.html yield different checksums for this region.

I had the same checksums as the emulator for C000-C7FF, C800-CFFF, D000-D7FF, D800-DFFF, E000-E7FF, but different values for F000-F7FF and F800-FFFF. However, the checksums for F000 to FFFF were identical with rom-1-f000.901439-04.bin and rom/rom-1-f800.901439-07.bin downloaded from the above site.

Same values, yay

Here’s a very lazy script to compute the checksums using perl:

od -Anone -tx1 roms/rom-1-f800.901439-07.bin | perl -ne 'while (s/([0-9a-fA-F]{2} ?)//) { $sum = ($sum + hex($1)); printf("%x\n", $sum%256) }'

Repairing the tape drive

First of all, one thing that is useful to know is that you can connect the C64’s Datasette to the edge connector for the first tape drive on the PET’s mainboard. It’ll work just the same as the built-in drive. (However the Datasette’s plastic case may get in the way if you attempt to connect it to the edge connector for the second tape drive.)

Datasette drive connected to PET edge connector

You can even close the PET in this state without pinching the cable; I think there is around 1 cm of empty space between the base and the “lid” of the PET. (Which might be the reason everything is so dusty and oxidized in there.) So if you have a working Datasette drive, you may want to see if you can load/save programs using that.

On the PET’s internal drive I had to replace the drive belt, which had snapped. Here’s someone who created replacement belts using their 3d printer: https://www.insentricity.com/a.cl/275/making-belts-with-a-3d-printer / https://www.thingiverse.com/thing:2600198/files Armed with this person’s measurements, I chose a pack of replacement drive belts from Amazon that seemed like they should have fitting ones (they did, though the belts were a bit thinner than advertised): https://www.amazon.co.jp/gp/product/B08JP7J5VX/. Cleaning the heads and the capstan and pinch roller (https://en.wikipedia.org/wiki/Tape_transport) with IPA made things work.

That was one brittle belt

Running some software

With a Datasette drive (for use with C64s and similar machines), it’s very easy to take off the cover (just lift it a bit further from it’s open position). With the cover removed, it’s very easy to put software on the PET using a 3.5mm/cassette adapter.

Datasette drive with cover removed

Most software written for the PET doesn’t seem to work on the PET 2001 (even the 8K model), but there are some games on http://www.zimmers.net/anonftp/pub/cbm/pet/games/english/index.html that work. (Search the page for ‘2001’)

There’s one more step however, as these are .prg files. I used wav-prg to convert the .prg files into .wav files. Quick setup:

git clone https://git.code.sf.net/p/wav-prg/libaudiotap
git clone https://git.code.sf.net/p/wav-prg/libtap
git clone https://git.code.sf.net/p/wav-prg/code
cd wav-prg-libtap
make libtapdecoder.so
make libtapencoder.so
cd ../wav-prg-libaudiotap
make clean # may not be necessary but it's in my notes, perhaps for a reason
make -j4 DEBUG=y LINUX64BIT=y libaudiotap.so # DEBUG=y is in my notes, perhaps for a reason
cd ../wav-prg-code
make clean
make -j4 cmdline/wav2prg DEBUG=y AUDIOTAP_HDR=../wav-prg-libaudiotap/ AUDIOTAP_LIB=../wav-prg-libaudiotap/
make -j4 cmdline/prg2wav DEBUG=y AUDIOTAP_HDR=../wav-prg-libaudiotap/ AUDIOTAP_LIB=../wav-prg-libaudiotap/

# for PET: (-s seems to be required)
LD_LIBRARY_PATH=../wav-prg-libaudiotap/:../wav-prg-libtap/ cmdline/prg2wav -s -w space\ invader.wav space\ invader.prg

I don’t remember why, but I’d enabled debugging in my Makefiles as follows. It’s unlikely that these changes are needed.


diff --git a/Makefile b/Makefile
index 61b0bac..7f3e573 100644
--- a/Makefile
+++ b/Makefile
@@ -14,7 +14,7 @@ libaudiotap.so: libaudiotap.o libaudiotap_external_symbols.o pthread_wait_event.
        $(CC) -shared -o $@ $^ -ldl $(LDFLAGS)
 ifdef DEBUG
- CFLAGS+=-g
+ CFLAGS+=-g -O0


diff --git a/Makefile b/Makefile
index f96b135..2442e6c 100644
--- a/Makefile
+++ b/Makefile
@@ -66,7 +66,7 @@ ifdef AUDIOTAP_HDR
 ifdef DEBUG
-  CFLAGS+=-g
+  CFLAGS+=-g3 -O0

Once you have .wav files, you can just use a cassette tape adapter for car stereos as described above. Unfortunately the edge connector for the PET’s second datasette drive doesn’t have enough clearance, so I disconnected the internal drive and once again connected the Datasette drive instead. Then you just type “load” and enter and press play, then you play back the .wav file from your computer.

Here’s the title screen of a random game that I found that just so happened to almost work on the PET 2001 with some minor bugs (perhaps it was developed with a different machine in mind), Diamond Hunt II:

Yay it’s working!

More two-channel oscilloscope-based RAM testing

In a previous article we were able to see that one of our 4164 RAM chips didn’t produce a clear 0 or a clear 1 when asked to read from its memory. In this article we’ll hunt for more broken RAM chips by comparing the output from a known good 4164 RAM chip with the output of each installed 4164 RAM chip.

Basically, we place the known good 4164 RAM chip on a breadboard next to the computer to test, and then connect all address, RAS, CAS, WRITE and D pins to the equivalent pins on one of the 4164 RAM chips on the computer (by using tiny test clips, of which you will need at least 12, but more would be better). Then, we attach one oscilloscope probe to the Q pin on the known good 4164 RAM chip and one probe on the Q pin on the RAM chip to test (we’ll test each one, one by one). (Note that D and Q pins are often shorted, so if that is the case we can also attach the probe to the D pin on the RAM chip under test.)

We should then hold down the computer’s reset button and (while doing so) set the oscilloscope to trigger on a positive edge from the known good RAM chip’s Q pin, zoom out so we can capture a good amount of activity, and then release the reset button. We expect both chip’s outputs to be exactly the same. If there is a discrepancy, either the connections are bad, or we have found a broken RAM chip. (Be sure to check your connections using your multimeter’s continuity mode, and try hard not to touch the cables while probing around.)

Here’s a picture of my setup:

The keyboard connector on the Hitachi MB-H2 provides power. I’m measuring the right-most chip in this example, and to avoid having to touch the cables I’m taking D7 from a completely different chip. I’m also taking some of the address pins from a different RAM chip, just because that was easier.

If your testing shows that the RAM chips under test never produce the same signal as the reference known-good chip, then your wiring may be messed up. If on the other hand the signals always match up perfectly, then it seems likely that none of your RAM chips are bad. Finally, if there is a single RAM chip that often (but perhaps not always) produces a late signal or a different signal, you may have found a bad RAM chip.

In my case, most chips seemed to produce consistent signals. However, touching the cables often messed up signal integrity and I had to re-measure multiple times. I believe that the RAM chip for D7 was bad, as it often produced late or wrong signals even though the connections should have been stable.

In the below video I’m scrolling along an oscilloscope capture. You can see that when the yellow line (which measures the output of the reference chip) goes high or low, the blue line also goes to (or stays at) the same level. This means that we’re getting the same output on both, which is good.

(There is a lot of other activity on the blue line, which we don’t know much about — some of it’ll be I/O to other chips, some will be writes to the RAM. The only thing we have that we can compare to is the Q output on the reference chip, so all we can do is check whether the transitions from 0 to 1 and from 1 to 0 on the output of the chip under test are correct. We are not able to check 1 to 1 or 0 to 0 outputs.)

For the less video-inclined readers, here is a screengrab showing a correct signal:

Confusing but okay signal

We have two transitions in the above screenshot. The first transition looks confusing at first because the blue signal takes a nosedive right after the yellow reference signal goes up — but that’s not too important, what matters is that they both have the same level for a short while. The blue line was already at “high” due to other I/O. (Also ignore the fact that the yellow line goes slightly higher than the blue one.) In the second transition we have a very neat and synchronized nosedive on both lines.

Here is a screengrab showing an erroneous signal:

Bad signal

Here we have two transitions on the yellow channel, and both times the blue channel goes to the exact opposite logic level. This isn’t perfect proof of a bad RAM chip — first of all we should make sure that this is repeatable and not due to wobbly probes. Second there is a slight possibility that some other device thinks it’s its turn to play with the bus (but normally that would show up as a voltage level somewhere between 0V and 5V).

4164 tester on the Raspberry Pi Pico and mini-review

I recently lost access to my Arduino for a couple days and decided to finally start playing around with my Raspberry Pi Pico. In case you don’t know, the Raspberry Pi Pico is even cheaper than most Arduinos. I think I paid 550 Japanese yen at a brick-and-mortar Marutsu for mine), and packs a lot of pins and quite some performance. (Amazon is way more expensive.)

So before we get to the 4164 tester, here’s my mini-review of the Raspberry Pi Pico from the perspective of someone who has never used a Raspberry Pi before and is used to his Arduino Nano:

The header pins weren’t attached so I had to solder them myself. I guess it’s possible to buy Picos with header pins soldered on, and I guess it’s also possible to buy Arduino Nanos without header pins. (Confirmed on amazon.co.jp). Soldering wasn’t too hard and I didn’t break anything.

Getting the (C/C++) development environment set up (on Debian Linux) wasn’t too hard for me personally, but I do this kind of thing a lot and would expect this to be much more difficult for someone who doesn’t have as much experience, especially if they are on a different distro. What I did was look at this script: https://raw.githubusercontent.com/raspberrypi/pico-setup/master/pico_setup.sh and instead of just running it, did the same things the script would have done — i.e., installed the prerequisite packages, cloned the relevant repositories, compiled and installed. If you can read bash scripts reasonably well you should be able to do this — otherwise you can just try and run the script verbatim and hope for the best. Note that the script is made to be run on a (non-Pico) Raspberry Pi.

The Arduino has an LED that is always on as long as there is power. The Pico has an onboard LED but it’s completely software-controlled. That’s great for non-beginners and perhaps not so great for beginners. (Is this thing even working?) To flash a program, you hold down a button and then connect USB. The Pico will identify as a sort of flash drive and you can copy over a .uf2 file. Once your OS is done copying (i.e. almost instantly) the Pico will immediately reboot and immediately start running your code. To the OS it will look like the flash drive suddenly disappeared. (This took me a little while to figure out.)

On the Arduino, you can be connected via serial at all times, though you’ll get a grey screen while flashing. On the Pico, USB serial feels much more “software-defined”, and you can’t be connected while re-flashing AFAICT. If you write a program that outputs something to serial right after starting, you probably won’t ever be able to see that output because your computer will take a while to notice there is something on USB. (For this reason, I added a 25 second pause at the beginning in my RAM tester program.)

So some parts of the development process are slightly more annoying than on the Arduino Nano, but the features may make up for it I think — more pins (but fewer analog I/O pins), much more performance, and step-by-step debugging via GDB (haven’t tried this yet).

One more very important thing to be aware of is that the Pico’s GPIO pins’ high logic level is 3.3V, not 5V. This doesn’t matter (IME) when driving the 4164’s input pins (address/RAS/CAS/WRITE/data in), but it’s likely to matter for the single GPIO pin connected to the 4164’s Q output pin. So you will need a resistor divider to bring the 4164’s 5V output down to 3.3V.

The code consists of two files, a very standard CMakeLists.txt (pretty much a straight amalgam of https://github.com/raspberrypi/pico-examples/blob/master/CMakeLists.txt and https://github.com/raspberrypi/pico-examples/blob/master/hello_world/usb/CMakeLists.txt) (with -O0 added to make sure we don’t optimize out the NOP delay) and 4164_test.c. Some of the code in 4164_test.c has been adapted from https://ezcontents.org/4164-dynamic-ram-arduino.


cmake_minimum_required(VERSION 3.12)

# Pull in SDK (must be before project)

project(pico_examples C CXX ASM)

    message(FATAL_ERROR "Raspberry Pi Pico SDK version 1.3.0 (or later) required. Your version is ${PICO_SDK_VERSION_STRING}")


# Initialize the SDK


add_compile_options(-Wall -Wextra
        -Wno-format          # int != int32_t as far as the compiler is concerned because gcc has int32_t as long int
        -Wno-unused-function # we have some for the docs that aren't called


# pull in common dependencies
target_link_libraries(4164_test pico_stdlib)

# enable usb output, disable uart output
pico_enable_stdio_usb(4164_test 1)
pico_enable_stdio_uart(4164_test 0)

# create map/bin/hex file etc.

# add url via pico_set_program_url


#include <stdio.h>
#include "pico/stdlib.h"

#define D           0
#define WRITE       1
#define RAS         2
#define A0          3
#define A2          4
#define A1          5

#define A7          16
#define A5          17
#define A4          18
#define A3          19
#define A6          20
#define Q           21 // don't forget the voltage divider for this pin!
#define CAS         22

#define HIGH        1
#define LOW         0

#error blink requires a board with a regular LED

// #define STATUS_LED 15

#define BUS_SIZE 8
#define MAX_ERRORS 20

const unsigned int a_bus[BUS_SIZE] = {
  A0, A1, A2, A3, A4, A5, A6, A7

#define println printf

void nop(void) {

void set_bus(unsigned int a) {
    int i;
    /* Write lowest bit into lowest address line first, then next-lowest bit, etc. */
    for (i = 0; i < BUS_SIZE; i++) {
        gpio_put(a_bus[i], a & 1);
        a >>= 1;

void write_address(int row, int col, bool val) {
    // Pull RAS and CAS HIGH
    gpio_put(RAS, HIGH);
    gpio_put(CAS, HIGH);

    // Set row address

    // Pull RAS LOW
    gpio_put(RAS, LOW);
    nop(); // need to wait 15 ns before setting column address
    nop(); // need to wait 15 ns before setting column address
    nop(); // need to wait 15 ns before setting column address
    nop(); // need to wait 15 ns before setting column address

    // Set column address

    // Pull CAS LOW
    gpio_put(CAS, LOW);

    // Set Data in pin to HIGH (write a one)
    gpio_put(D, val);

    // Pull Write LOW (Enables write)
    gpio_put(WRITE, LOW);

    gpio_put(WRITE, HIGH);
    gpio_put(CAS, HIGH);
    gpio_put(RAS, HIGH);

void read_address(int row, int col, bool *val) {
    // Pull RAS and CAS and WRITE HIGH
    gpio_put(RAS, HIGH);
    gpio_put(CAS, HIGH);
    gpio_put(WRITE, HIGH);

    // Set row address

    // Pull RAS LOW
    gpio_put(RAS, LOW);
    nop(); // need to wait 15 ns before setting column address
    nop(); // need to wait 15 ns before setting column address
    nop(); // need to wait 15 ns before setting column address
    nop(); // need to wait 15 ns before setting column address

    // Set column address

    // Pull CAS LOW
    gpio_put(CAS, LOW);


    *val = gpio_get(Q);

    gpio_put(WRITE, HIGH);
    gpio_put(CAS, HIGH);
    gpio_put(RAS, HIGH);

void setup() {
    bool dummy = false;
    int i;



    gpio_set_dir(D, GPIO_OUT);
    gpio_set_dir(WRITE, GPIO_OUT);
    gpio_set_dir(RAS, GPIO_OUT);
    gpio_set_dir(A0, GPIO_OUT);
    gpio_set_dir(A2, GPIO_OUT);
    gpio_set_dir(A1, GPIO_OUT);

    gpio_set_dir(A7, GPIO_OUT);
    gpio_set_dir(A5, GPIO_OUT);
    gpio_set_dir(A4, GPIO_OUT);
    gpio_set_dir(A3, GPIO_OUT);
    gpio_set_dir(A6, GPIO_OUT);
    gpio_set_dir(Q, GPIO_IN);
    gpio_set_dir(CAS, GPIO_OUT);

    gpio_set_dir(STATUS_LED, GPIO_OUT);

    gpio_put(RAS, HIGH);
    gpio_put(CAS, HIGH);
    gpio_put(WRITE, HIGH);
    gpio_put(D, LOW);
    gpio_put(A0, LOW);
    gpio_put(A1, LOW);
    gpio_put(A2, LOW);
    gpio_put(A3, LOW);
    gpio_put(A4, LOW);
    gpio_put(A5, LOW);
    gpio_put(A6, LOW);
    gpio_put(A7, LOW);
    for (i = 0; i < 8; i++) {
        read_address(0, 0, &dummy);

int main() {
    int row, col;
    int errors = 0;
    bool val = true;
    bool read_val = false;


    sleep_ms(25000); // wait until /dev/ttyACM0 device is ready on host
    println("Starting test...\n");

    for (row = 0; row <= 255; row++) {
        for (col = 0; col <= 255; col++) {
            println("Testing row: %d, col: %d\n", row, col);
            write_address(row, col, val);
            read_address(row, col, &read_val);
            if (val != read_val) {
                println("ERROR: row %d col %d read %d but expected %d\n", row, col, read_val, val);
                if (++errors > MAX_ERRORS) {
                    while (true) {
                        gpio_put(STATUS_LED, HIGH);
                        gpio_put(STATUS_LED, LOW);
            val = !val;
        gpio_put(STATUS_LED, HIGH);
        gpio_put(STATUS_LED, LOW);

    println("Test DONE. All OK!\n");
    while (true) {
        gpio_put(STATUS_LED, HIGH);
        gpio_put(STATUS_LED, LOW);
No cross connections

The Raspberry Pi Pico has a lot more pins than the Arduino Nano, which means that we can connect everything in a very straight and orderly manner. The pins on the left of the IC are connected to pins on the left of the Pico, and the pins on the right of the IC are connected to pins on the right of the Pico, and there are no cross connections. That’s a huge plus, and (as in our case) if you’ve got just one pin that requires level shifting, I’m not sure I’d choose the Arduino Nano if I already knew and owned both. Adjust the #defines at the top if you want to connect things differently.

Also, the Pico could theoretically allow much faster testing than the Nano, but my test is pretty slow, especially when using USB serial output. I also added a couple of delays to be super sure we don’t go out of spec. As the library only has millisecond and microsecond delays, I added a NOP-based delay (which probably delays things much more than required). It should be safe to reduce the number of nop() calls to just a single call, as the -O0 prevents this function from being inlined and the single NOP turns into a function call and a number of stack operations.

Note that this code won’t do anything the first 25 seconds, this delay allows the host computer to identify the USB serial device (should appear as /dev/ttyACM0). If you then execute, e.g., ‘minicom -D /dev/ttyACM0’ you should be able to see output as the test progresses. It’s also possible to see whether the test passed by looking at the onboard LED — fast blinking means error, slow blinking means success. If you prefer running tests using the LED, I suggest you do the following:

  • Comment out the sleep_ms(25000); call
  • Remove ‘#define println printf’, add ‘#define println(…) ;’ instead
  • Set MAX_ERRORS to 0 (or remove the if (++error > MAX_ERRORS) logic entirely)

(The MAX_ERRORS logic causes the program to ignore the first 20 errors, if you don’t want that, feel free to remove.)

To compile, copy the above files into a new directory, create a ‘build’ subdirectory and cd to it, run cmake and make:

mkdir build
cd build
cmake ../ -DCMAKE_BUILD_TYPE=Debug
make -j4
# hold BOOTSEL button while plugging in Pico USB
cp 4164_test.uf2 /media/.../RPI-RP2/

Hitachi MB H2 (MSX) partial schematics and partial repair

I recently got hold of a Hitachi MB H2 that wouldn’t work. Attaching a composite cable, I’d just see a black screen with some faint colored vertical bars. I probed around the computer with an oscilloscope and found that the computer actually appears to execute code in the first few microseconds or milliseconds after powering it on (or resetting). There didn’t appear to be any bad connections.

I saw some very pronounced unusual voltages on the data bus; judging by the color intensity on the oscilloscope screen I got 33% 0V, 33% about 1-2 V, 33% 5V. The 1-2 V bar could just be everything attached being in a “don’t care”/”high impedance” state, so I proceeded to look for some schematics on the internets. But there were none as far as I can tell! (Anyway, it seems normal to have some not-one-but-not-quite-zero-either activity on this computer as I found out later.)

Having no prior MSX experience, I proceeded to trace out a lot of the connections from the CPU to the other chips. It turned out that most data lines are directly connected to the CPU, implying that most of the chips should have some kind of “don’t care”/”high impedance” state. From the tracing I did I managed to create some partial schematics, until I found I was wise enough to figure some stuff out.

Partial Hitachi MB-H2 schematics
Partial Hitachi MB-H2 schematics

So what’s wrong with my MSX?

There is one thing that seemed very wrong to me on my MSX, I had no activity on the Z80’s IOREQ pin. I also never had activity on the VDP’s pins that interface with the CPU. So I set a rising edge trigger on the Z80’s IOREQ pin and pressed the reset button and saw that there were a couple very early IOREQ signals, suggesting that the computer works normally for at least a while.

A Z80 starts executing from address #0, and the code at this address will have to be on ROM. This machine has 32 KB + 16 KB of ROM, and 64 KB of RAM. A Z80 can only address 64 KB, so we will have to have some kind of mechanism to switch between ROM and RAM. I am not entirely sure how this works on the MSX, but to me it just seemed likely that the early boot code would perhaps copy the ROM contents to RAM and then execute code from there. And if the RAM is defective somehow we’ll be in a bit of a situation… (Even if that isn’t correct, broken RAM should cause things to go haywire early in the boot process.)

So I suspected the (non-socketed) 4164 RAM is probably broken (I think this is a very common defect for early computers), and set up a small circuit to help me test that theory.

RAM can break in a number of ways, some RAM chips get hot, some pull the data pin to 0 or perhaps 1, and some are perpetually high-impedance, i.e. they do not affect the voltage of the data pin at all. (None of the RAM chips were hot in my case. The video chip is very hot, but the datasheet mentioned something about 70 degrees Celsius so that’s probably fine.)

Unfortunately my oscilloscope only has two inputs. However, to verify if a RAM chip actually does something when somebody reads from it, we may need more, depending on the RAM chip in question of course. Here are some screenshots from the datasheet (TMS4164-datasheet-texas-instruments.pdf):

4164 pinout
4164 pinout
4164 read sequence
4164 read sequence

In the read sequence, we see that \CAS (usually high) is low and \W (usually “don’t care”) is high, and after a short moment we see either a low or a high on Q (normally “high impedance”).

So with just two oscilloscope inputs we don’t get very far, we’d need three (for \W, \CAS and Q). However, with a very simple circuit we can get by with two inputs.

All we need is a NOT gate and an AND gate. Then we can combine \W and \CAS and produce H if and only if (\W is high and \CAS is (not high)). So we wire things like this:

       |    |--\CAS------NOT----| 
--\W-- |    |--Q--probe         AND--probe
|      |4164|                   |
|      |----|                   |

All you need is a breadboard, 74LS04 and a 74LS08 chip. Here’s a picture of the setup:

MB-H2 4164 RAM under test
MB-H2 4164 RAM under test

So let’s have a look at the resulting oscilloscope captures to see if we can find anything interesting. In the captures, the output of the AND gate is in yellow, and the output Q (which is shorted to D on this computer) is blue. Maybe take a look and try to find the problem. The answer is right below the last image so, spoiler warning. ;)


Highlight (and possibly copy and paste somewhere) the next paragraph to read the answer:

D0. Yellow goes high, but blue doesn’t budge at all. We’re trying to read from this chip but the chip isn’t outputting anything!

I piggy-backed (and made sure this particular problem went away) and replaced the suspect chip, but unfortunately it appears that that isn’t all that is wrong with this computer. :/ (If you see this text instead of a link to additional blog posts, that means that I haven’t figured out the problem yet, or haven’t gotten around to creating a write-up for it yet.)






Testing a ZX81 RAM pack with an Arduino (and repair)

For a quick overview of what I did to the ZX81 before arriving at this point, see this post: ZX81 repair (no video, some keys not working, and bad RAM pack)

I recently got hold of a Spectrum ZX81 RAM pack that when plugged in, produced a garbled screen on boot. I decided to check what’s wrong before ordering any chips. To do that, I first looked at the schematics and made sure there were no bad connections. This was a laborious process, but fortunately all RAM chips share all pins except the data pins, so you should have continuity between all pins except two on all RAM chips.

I finally thought I found something broken — but it turned out that there’s just a slight difference between my board and the schematics: only three NAND gates are used on the quad NAND IC, and logically and electrically it doesn’t matter which gates you use and which one you leave unsoldered. Well, for some reason my board used different pins (i.e., left a different gate unsoldered) than the ones in the schematics.

Below you will find my annotated schematics of the RAM pack.

Annotated schematics of the ZX81 RAM expansion pack

Here is an actual picture (with the bad RAM chip replaced) that shows which chips are where:

The ZX81 RAM pack is made of two circuit boards. These circuit boards are sandwiched together. The pins connecting the two boards are very flexible, so you can just apply a small amount of force and bend the two boards apart. One board has logic chips (the aforementioned NAND chip, an OR chip, four data selector chips (74LS157) and a dual 4-bit counter chip (74LS393)). The other has the DRAM chips and some circuitry to generate -5V and 12V from 5V and 9V input. My voltages were all good and I didn’t see anything unusual there, so I didn’t really look into it too much. If you need to debug the power circuitry, you may need to know how to generate negative voltage (https://www.allaboutcircuits.com/projects/build-your-own-negative-voltage-generator/). I also created a rough simulation of the power circuitry on https://www.falstad.com/circuit/. If you are interested, go to File -> Import from Text and paste the following code, but I don’t think I’m using the correct transformer and there may be other issues:

$ 1 0.000005 24.46919322642204 50 5 43 5e-11
169 112 112 192 112 0 4 9 -1.3552527156068805e-20 0.05437017461335131 0.022386130031495474 0.99
R 192 112 128 64 0 0 40 9 0 0 0.5
w 192 144 272 144 0
w 272 144 272 256 0
t 240 272 272 272 0 -1 17.389821061615624 -0.6849346284276479 100 default
w 272 288 272 336 0
w 192 224 288 224 0
d 336 224 288 224 2 default
34 zener-12 1 1.7143528192810002e-7 0 2.0000000000000084 12 1
z 336 224 400 224 2 zener-12
d 336 224 336 256 2 default
w 336 256 336 336 0
r 192 224 192 272 0 100
w 192 272 240 272 0
r 192 272 192 336 0 2200
g 192 336 144 336 0 0
w 192 336 272 336 0
w 304 336 336 336 0
w 192 176 224 176 0
w 224 176 224 384 0
w 272 336 304 336 0
d 304 384 304 336 2 default
d 352 384 304 384 2 default
r 352 384 416 384 0 2200
34 zener-5.1 1 1.7143528192810002e-7 0 2.0000000000000084 5.1 1
z 416 384 416 336 2 zener-5.1
209 352 336 352 384 0 0.000001 5.679241726295006 1 1
w 224 176 352 176 0
d 352 176 400 176 2 default
w 400 176 400 224 0
d 400 112 400 176 2 default
w 192 112 400 112 0
w 400 112 464 112 0
w 416 384 448 384 0
w 416 336 464 336 0
w 464 336 464 256 0
209 464 208 464 256 0 0.000022000000000000003 8.999999999994335 1 1
w 464 112 464 160 0
c 400 224 400 336 0 0.00009999999999999999 9.397384509781268 0.001
w 352 336 400 336 0
w 336 336 352 336 0
w 400 336 416 336 0
O 384 416 432 416 1 0
O 400 224 448 224 1 0
c 192 176 192 224 0 0.000022 -18.406729087662764 0.001
c 224 384 304 384 0 0.000001 -0.8460828370149334 0.001
r 464 160 464 208 0 1000
r 448 384 512 384 0 1000000
g 512 384 560 384 0 0
x 9 10 431 32 4 16 ZX81\sRAM\spack\spower\ssupply\scircuit\s(9V\s->\s-5,\s12V).\\nChanged\ssome\scapacitors\sto\snon-polarized
x 489 171 629 212 4 16 Added\s1k\sresistor\\nto\sprevent\sshort\\ncircuit

The four 74LS157 selector chips on the non-DRAM board work as two separate entities, that is, the “selector” inputs are tied together for the lower two chips and tied together for the higher two chips. When you look for 74LS157 pinouts on the internet, you’ll often find an OCR’d and slightly wrong pinout. The pin labelled I1d on the bottom side should be labelled I1a instead:

Wrong 74ls157 pinout; lower I1d should be I1a
Two I1d pins? Yeah right! The lower one is actually I1a.

The 74LS393 is used by the ZX81 to refresh the DRAM. According to the datasheet, the DRAM has to be refreshed at least every 2 ms. I am guessing that the CPU or ULA periodically generates the RFSH signal, but we don’t have to worry about that in the context of this repair. Each time RFSH goes low (low because there is a NOT gate built from one of the NAND gates between RFSH and pin 1 (“clock”) of the 74LS393 counter chip), the counter chip adds +1 to its internal state. Additionally, the RFSH signal also goes into the first pair of selector chips, which causes the output of the counter to be selected as the output. Otherwise, address lines A0-A6 are used as the output.

The second pair of selector chips has address lines A7-A13 as one set of inputs, and the output of the previous selector chip as the other set of inputs. The circuit that goes into the selector pin is somewhat complicated, as it uses four different inputs to decide which set of inputs to select. I decided to make a truth table to better understand it. If you need to understand this circuit, the truth table or OpenDocument / Excel files below may help a little bit:

Write operation:        
A14 temp2
00010 111
00011 002
00110 113
00111 004
01010 115
01011 016
01110 117
01111 018
Read operation:        
A14 temp2
10010 119
10011 0010
10100 1111
10101 1112
11010 1113
11011 0114
11100 1115
11101 1116

The circuit contains a number of RC delay circuits to make the timing work, but as the delay is on the order of 10-20 ns, I don’t have to worry about those when driving this circuit using an Arduino — I’m using digitalRead() and digitalWrite(), and these functions take a couple of microseconds to complete. Looking at the timing diagram in the DRAM IC’s datasheet however, it is relatively obvious that these delays are needed.

As stated above, the DRAMs are all connected in parallel on all pins except the data pins. And while the DRAM chips have separate pins for input and output, the RAM pack ties these together as they are of course not used at the same time — you either read or write.

Some more notes on the timing — programming the Arduino like this will drive the chips very slowly, but according to the datasheet, we don’t really have to worry about being too slow in most cases. Some parameters have “max” values on the order of 10s or 100s of ns, but the notes alleviate most concerns in that area. The maximum RAS/CAS pulse width of 32000/10000 ns should be okay with just digitalRead()/digitalWrite() (I didn’t measure too much though, to be honest). Here is the code doing the write and CAS pulses, and what we know about digitalWrite(), this should be just under 10000 ns:

void writeAddress(...) {
  /* write */
  digitalWrite(WR, LOW);

  /* tRCD max is 50 ns, but footnote 10 states:
   * "If tRCD is greater than the maximum recommended value shown in this table, tRAC will increase by the amount that tRCD exceeds the value shown."
   * Therefore this is not a hard maximum and we don't have to worry too much about being too slow */
  digitalWrite(XA14, HIGH); /* pulls CAS low after 10-20ns */

  digitalWrite(WR, HIGH);
  digitalWrite(XA14, LOW);

Here’s an oscilloscope screenshot for just the WR pulse (which should have the same timing), which is approximately… 10 microseconds!

-Width=10.00us :o

There is code out there to test 4116 RAM ICs. However, the chips in my RAM pack weren’t socketed so I couldn’t take them out very easily. And it’s not certain if we can just attach the Arduino directly to the DRAM chips’ pins — if we apply power to the board we will power up the rest of the circuitry and that could interfere with our testing — the selector chips might produce 1s when we want 0s, or vice versa. I took this code and modified it to work with the rest of the circuitry. I originally planned on testing two bits at once (i.e., two DRAM chips at once), but I ran out of cables. I’ve left in the code however, commented.

Since we don’t have a lot of pins on the Arduino (or connectors that we can use to connect the Arduino with the RAM pack), I decided to enlist the binary counter chip’s help to generate the addresses. Check out the advanceRow() function to see how easy this is — we just need to manipulate RFSH. (Note that “row” means the same thing as it does in the datasheet — the DRAM chip is organized into 128 “rows” and 128 “columns”, 128×128 = 16384 bits.)

I also decided to write two different values in two successive addresses before reading back from these addresses. This is important because otherwise the Arduino may just read whatever it just put on the wire itself. I.e., if you take an Arduino that isn’t connected to anything at all and do something like the following, your digitalRead may return whatever you wrote using digitalWrite!

digitalWrite(13, HIGH);
val = digitalRead(13); // val may be 1 now!

Which is why we instead do something like this (c is column, v is value, row is set elsewhere):

        writeAddress(c, v, v);
        writeAddress(c+1, !v, !v);
        readAddress(c, &read_v0_0, &read_v1_0);
        readAddress(c+1, &read_v0_1, &read_v1_1);

I also changed the error() and ok() functions. ok() will make a (preferably green) LED blink slowly, error() will made a (preferably red) LED and the other LED blink alternatingly.

ZX81 RAM pack memory test using an Arduino
Diagnostic surgery in progress.

Here is the code:

/* Modified by sneep to test the Sinclair ZX81 RAM pack.
 * Original code is at http://labs.frostbox.net/2020/03/24/4116-d-ram-tester-with-schematics-and-code/
 * The Arduino doesn't have enough pins to check all outputs at
 * the same time so we'll test one (out of eight) at a time;
 * rewiring is required between tests.
 * Unlike the previous version of this source code, we go through
 * the onboard logic (a couple of ORs, ANDs, multiplexers, and a
 * counter for refresh) rather than talking to the 4116 RAM ICs
 * directly.
 * It's probably not possible to check the 4116 chips in-circuit
 * using the original source code, as we would apply power to
 * everything and would then cause our address signals to fight
 * against the multiplexer's outputs.
 * NOTE: As we are using digitalWrite, this is a very slow test.
 * We go beyond the 'max' value recommended in the datasheet for
 * one thing, and go way beyond the 'min' values -- borderline
 * chips could pass our tests but fail when driven by the ZX81.
 * NOTE: At least the init refresh cycles may stop working if we
 * replace digitalWrite by something faster (init refresh).

//This is for an arduino nano to test 4116 ram ic. Please see video https://youtu.be/MVZYB54VD2g and blogpost
//Cerated in november 2017. Code commented and posted march 2020. 
//Most of the code and design is from http://forum.defence-force.org/viewtopic.php?p=15035&sid=17bf402b9c2fd97c8779668b8dde2044
//by forum member "iss"" and modified to work with 4116 D ram by me Uffe Lund-Hansen, Frostbox Labs. 
//This is version 2 of the code. Version 1 had a very seroisl bug at approx. line 43 which meant it only checked ram address 0 

//#include <SoftwareSerial.h>

#define XD0         A1
#define MREQ        5
#define WR          6
#define RFSH        10

#define XA7         4
#define XA8         2
#define XA9         3
#define XA10        A3 // orange
#define XA11        A4 // yellow
#define XA12        A5 // green
#define XA13        A2
#define XA14        A0

#define R_LED       13    // Arduino Nano on-board LED
#define G_LED       8

//Use the reset button to start the test on solder an external momentary button between RST pin and GND pin on arduino.

#define BUS_SIZE     7

#define NO_DEBUG 0
#define VERBOSE_1 1
#define VERBOSE_2 2
#define VERBOSE_3 3
#define VERBOSE_4 4
#define VERBOSE_MAX 5

#define DEBUG_LED_DELAY 0 /* Set to 0 for normal operation. Adds a delay inbetween when double-toggling fast signals, e.g. RFSH */

int g_row = 0;

const unsigned int a_bus[BUS_SIZE] = {
  XA7, XA8, XA9, XA10, XA11, XA12, XA13

void setBus(unsigned int a) {
  int i;
  /* Write lowest bit into lowest address line first, then next-lowest bit, etc. */
  for (i = 0; i < BUS_SIZE; i++) {
    digitalWrite(a_bus[i], a & 1);
    a /= 2;

void advanceRow() {
    /* Keep track of which row we're on so we can put that in our debug output */
    g_row = (g_row + 1) % (1<<BUS_SIZE);
    /* Counter chip should be fast enough.
     * NOTE there is a NOT gate between arduino pin and counter chip */
    digitalWrite(RFSH, LOW);
    if (DEBUG_LED_DELAY) {
    digitalWrite(RFSH, HIGH);

void writeAddress(unsigned int c, int v0, int v1) {
  /* Set column address in advance (arduino may be too slow to set this later) (won't appear on the RAM chip pins yet) */

    Serial.print("Writing v0 ");
//    Serial.print("Writing v1 ");
//    Serial.println(v1);
  /* Set val in advance (arduino may be too slow to set this later) (chip doesn't care what's on this pin except when it's looking) */
  pinMode(XD0, OUTPUT);
//  pinMode(XD1, OUTPUT);
  digitalWrite(XD0, (v0 & 1)? HIGH : LOW);
//  digitalWrite(XD1, (v1 & 1)? HIGH : LOW);

  digitalWrite(MREQ, LOW); /* pulls RAS low */

  /* write */
  digitalWrite(WR, LOW);

  /* tRCD max is 50 ns, but footnote 10 states:
   * "If tRCD is greater than the maximum recommended value shown in this table, tRAC will increase by the amount that tRCD exceeds the value shown."
   * Therefore this is not a hard maximum and we don't have to worry too much about being too slow */
  digitalWrite(XA14, HIGH); /* pulls CAS low after 10-20ns */

  digitalWrite(WR, HIGH);
  digitalWrite(XA14, LOW);
  digitalWrite(MREQ, HIGH);

  pinMode(XD0, INPUT);
//  pinMode(XD1, INPUT);

void readAddress(unsigned int c, int *ret0, int *ret1) {
  /* set column address (won't appear on the RAM chip pins yet) */
  digitalWrite(MREQ, LOW); /* pulls RAS low, row address will be read in after tRAH (20-25 ns) */

  /* Need to wait tRCD (RAS to CAS delay time), min. 20ns max. 50 ns, but a footnote implies that we can go over the max */
  digitalWrite(XA14, HIGH); /* sets S to high and pulls CAS low after 10-20ns (it's correct to have the column address on the bus before pulling CAS low) */

  /* Need to wait tCAC (time CAS-low to data-valid), but Arduino is slow enough for our purposes */

  /* get current value
   * datasheet "DATA OUTPUT CONTROL", p. 8:
   * "Once having gone active, the output will remain valid until CAS is taken to the precharge (logic 1) state, whether or not RAS goes into precharge."
  *ret0 = digitalRead(XD0);
//  *ret1 = digitalRead(XD1);

  digitalWrite(XA14, LOW);
  digitalWrite(MREQ, HIGH);

void error(int c, int v, int read_v0_0, int read_v1_0, int read_v0_1, int read_v1_1)
  unsigned long a = ((unsigned long)c << BUS_SIZE) + g_row;
  Serial.print(" FAILED $");
  Serial.println(a, HEX);
  Serial.print("Wrote v/!v: ");
  Serial.print("Read v0_0: ");
//  Serial.print("Read v1_0: ");
//  Serial.println(read_v1_0);
  Serial.print("Read v0_1: ");
//  Serial.print("Read v1_1: ");
//  Serial.println(read_v1_1);
  while (1) {

void ok(void)
  digitalWrite(R_LED, LOW);
  digitalWrite(G_LED, LOW);
  Serial.println(" OK!");
  while (1) {

void blink_abekobe(int interval)
  digitalWrite(R_LED, LOW);
  digitalWrite(G_LED, HIGH);
  digitalWrite(R_LED, HIGH);
  digitalWrite(G_LED, LOW);

void blink_green(int interval)
  digitalWrite(G_LED, HIGH);
  digitalWrite(G_LED, LOW);

void blink_redgreen(int interval)
  digitalWrite(R_LED, HIGH);
  digitalWrite(G_LED, HIGH);
  digitalWrite(R_LED, LOW);
  digitalWrite(G_LED, LOW);

void green(int v) {
  digitalWrite(G_LED, v);

void fill(int v) {
  int i, r, c, g = 0;
  int read_v0_0, read_v1_0;
  int read_v0_1, read_v1_1;

  if (DEBUG >= VERBOSE_1) {
    Serial.print("Writing v: ");
  for (r = 0; r < (1<<BUS_SIZE); r++) {
    if (DEBUG >= VERBOSE_1) {
      Serial.print("Writing to row ");

    for (c = 0; c < (1<<BUS_SIZE); c++) {
        if (DEBUG >= VERBOSE_4) {
          Serial.print("Writing to column ");
        green(g ? HIGH : LOW);
        /* The same two data pins are used for both read and write,
         * so when nothing is connected we would just read the value we just wrote.
         * So let's write 0 and 1 (or 1 and 0) to two addresses and read them back.
         * We should get 0 and 1, but if there's nothing connected we'd get 1 and 0,
         * which 
        writeAddress(c, v, v);
        writeAddress(c+1, !v, !v);
        readAddress(c, &read_v0_0, &read_v1_0);
        readAddress(c+1, &read_v0_1, &read_v1_1);
        if (DEBUG >= VERBOSE_3) {
          Serial.print("Read v0_0: ");
//          Serial.print("Read v1_0: ");
//          Serial.println(read_v1_0);
          Serial.print("Read v0_1: ");
//          Serial.print("Read v1_1: ");
//          Serial.println(read_v1_1);
        if ((read_v0_0 != v) || // (read_v1_0 != v) ||
            (read_v0_1 != !v)) { //|| (read_v1_1 != v)) {
          error(c, v,
        g ^= 1;


  for (i = 0; i < 50; i++) {

void setup() {
  int i;

  while (!Serial)
    ; /* wait */

  Serial.print("ZX81 RAM PACK TESTER");

  for (i = 0; i < BUS_SIZE; i++)
    pinMode(a_bus[i], OUTPUT);

  pinMode(XA14, OUTPUT);
  pinMode(MREQ, OUTPUT);
  pinMode(WR, OUTPUT);

  pinMode(R_LED, OUTPUT);
  pinMode(G_LED, OUTPUT);

  /* Input and output is tied together on RAM pack.
   * We'll leave the pinMode on INPUT for most of the time and only set to OUTPUT when writing.
  pinMode(XD0, INPUT);
//  pinMode(XD1, INPUT);

  digitalWrite(WR, HIGH);
  digitalWrite(MREQ, HIGH);
  digitalWrite(XA14, HIGH);


  digitalWrite(R_LED, LOW);
  digitalWrite(G_LED, LOW);


  /* Datasheet says: "Several cycles are required after power-up before proper device operation is achieved. Any 8 cycles which perform refresh are adequate for this purpose."
   * We'll just perform a refresh on all rows. */
  for (i = 0; i < (1<<BUS_SIZE); i++) {
    /* Should work fine timing-wise with standard Arduino digitalWrite() (tRC min: 375 ns, no max apparently) */
    Serial.print("init: refreshing row ");
    digitalWrite(MREQ, LOW);
    digitalWrite(MREQ, HIGH);

void loop() {
  interrupts(); Serial.print("."); Serial.flush(); noInterrupts(); fill(0);
  interrupts(); Serial.print("."); Serial.flush(); noInterrupts(); fill(1);

In my case, all DRAM chips passed the test except the one controlling D5. Even the very first read wouldn’t work out. I therefore replaced that one and hooray, things worked again! Here’s a pic of a 3d maze game running with the repaired RAM.

Some random notes on how to do the actual replacement

Before replacing the defective RAM chip I also tried piggybacking, but that didn’t make the test pass. I was planning on using my oscilloscope to get an idea of what’s going wrong when piggybacking, but things were just too finicky and I abandoned that plan. If you try yourself, make sure to put your multimeter in continuity mode and check that your piggybacked RAM chip is actually making contact.

I cut off the legs of the chip I 99% knew was bad and then desoldered the legs. Applying heat using a soldering iron from above and using a desoldering pump from below (or the other way round) worked reasonably well.

It should be okay to use a socket on most chips. Here’s a photo of the boards sandwiched up again after the replacement. You can see that there’s quite some clearance left:

Let me know if you have any questions about this repair.

ZX81 repair (no video, some keys not working, and bad RAM pack)

Over the last few weeks~months, I have been repairing a Sinclair ZX81.

This was the first time I had a look at a ZX81. I am actually not 100% sure if there really was no video, but based on the things I did after I finally figured out what I was doing, it’s reasonably likely that I didn’t accidentally “repair” (and break and then repair again) something that wasn’t broken in the first place.


There was no video — well, I don’t know if that was due to the TV somehow not being able to tune into the channel it said it was tuned into. Anyway, I took out all the chips from their sockets and cleaned them thoroughly, as that is what fixed a VIC-20 I worked on earlier. Still no luck, so I checked continuity between the sockets’ pins’ solder blobs on the underside of the mainboard and the chip pins, and identified a lot of bad connections. I used a screwdriver to forcefully bend the chips’ pins to have them make contact with the sockets, and in the end I got continuity everywhere and plausible activity on the oscilloscope.

But still no video output, so we took the signal on the ULA’s pin 16 and fed that into the TV’s composite input. Nothing… Or wait, wrong, that’s just a very dark picture, i.e., black on dark-grey.


Anyway, turning the TV’s brightness all the way up I was able to test a few things, and found that the keyboard had some keys that wouldn’t do anything.

The keyboard’s ribbon cable had a broken trace. This was easy to find using a combination of staring and a multimeter in continuity mode — first stare at a schematic to see which lines the keys that aren’t working are connected to, then put the two leads in the same spot and then go up the trace until it doesn’t beep anymore. Then stare some more and you should be able to see that the trace is indeed very slightly broken around that spot.

Note that opening and closing your ZX81 repeatedly will probably take its toll on this ribbon cable. At first I didn’t even know that it was possible to disconnect the ribbon cable without desoldering it. It’s very possible and you should probably do it — you tug at the ribbon cable to pull it out of the connector, not the connector out of the board. Anyway, I only had one broken trace at first, and after a couple times closing and re-opening the ZX81 (don’t do that in the first place if you can avoid it) I had two.

The first (original) one was very close to the connector, so you take a pair of scissors and cut off a a small bit, and it’s like you have a brand-new cable again. It takes some effort to put the ribbon cable back into the connector, but it’s not that hard.

The second broken trace was way higher up, but fortunately for me it was the right-most trace, which is probably easier to “fix” than other traces. Do not use a soldering iron. I did that, and it just melted the cable. It’s impossible to bodge wire this with solder as far as I can tell. What worked for me was conductive foil tape.


A lot of people seem to have had success just adding an emitter-follower between the TV and ULA pin 16, but that didn’t do anything at all. I also tried dropping the voltage using one or multiple diode drops right after the emitter-follower, but that didn’t seem to have much of an effect.

The video signal looked off on the oscilloscope, but to be honest I hadn’t seen a black and white composite signal on an oscilloscope before. I did some digging, and found the problem: there’s no black porch! Fortunately, that applies to many ZX81s, and there are people who have thought about the problem. For example, check out page: http://zx.zigg.net/misc-projects/ (or the accompanying video: https://www.youtube.com/watch?v=1irH3KuGyl0)

I used this person’s 555-based circuit, which wouldn’t immediately work. After some thinking and probing and staring at the oscilloscope, I found that the voltage during the horizontal blank wasn’t low enough to trigger the 555 (it has to be less than one third of the 555’s main input voltage). I added a resistor divider and suddenly had a beautiful signal!! This was the first time I ever saw a ZX81 boot up properly.

Resistor divider:
ULA pin 16 → 270 ohm resistor → 1k resistor → GND;
                              → rest of circuit

I downloaded a bunch of software from https://www.zx81.nl/ and converted some to an audio file using tapeutils.jar from http://www.zx81stuff.org.uk/zx81/tapeutils/overview.html, and was able to load 1KCHESS. It loaded all right.

16KB RAM pack

Unfortunately, the 16KB RAM pack didn’t work. Inserting it into the ZX81 would produce a garbled screen. So I reverse-engineered the RAM pack and tested its memory using an Arduino, and was able to identify the faulty IC and replace it. More on that in this post: Testing a ZX81 RAM pack with an Arduino

VIC-20 repair, oxidized pins

I recently had the chance to repair an NTSC VIC-20 that would not boot and just show a black screen.

The version I got to work on has a power brick containing a transformer. That power brick converts mains input to 9 volts AC, which then is fed directly into the computer. The rest of the (linear) power supply circuitry is inside the computer, including a very large capacitor. This capacitor wasn’t bulging but showed some signs of electrolyte leakage but that was not the cause of the problem. In fact I chose to skip replacing the capacitor for now.

In order to diagnose the problem I first checked the voltages, which all checked out perfectly.

Then I used an oscilloscope to check what the VIC and the CPU were doing.

The main clock signal  is generated by the VIC chip from the output of a 14.31818 MHz crystal. Everything looked perfect in this area.

I decided to reseat all the socketed IC chips (CPU, ROMs, IO chips — I was not able to extract the VIC chip even though it was socketed), but that did not help.

The CPU has a reset pin which is held low for a few seconds and then goes high, this worked perfectly too. There is a tiny 555 IC placed on the board responsible for doing this.

However there was no activity on the address lines at all; it seemed like all address lines were held high. (There is a possibility that some lines (A0~A3) were low, maybe I did not check carefully enough.) However, occasionally, right after clicking the power switch there seemed to be some normal-looking activity on the CPU’s address lines, which very quickly faded away. This happened maybe once in 10 or 20 power cycles.

So then my first suspicion was that the 6502 CPU might have given up. Thankfully, I had access to another board using a 6502 CPU. (Actually this one was a 65CS02 CPU.) As the voltages looked normal it seemed very low risk to swap the CPUs to see what would happen. Much to my dismay at first, the 6502 CPU extracted from the VIC-20 worked on the other board.

However, I was dismayed only for a few seconds, as the 65CS02 CPU, when put in the VIC-20, didn’t quite make the computer work but I was able to see a lot of activity on the address lines of the CPU now!

The new theory was that the IC pins were much more oxidized than expected. We extracted all the ICs (including VIC) again and gave them a clean up with concentrated alcohol. And it still did not work! However on the oscilloscope most pins now showed normal activity.

Thinking there might be another problem, perhaps with the ROMs, I decided to insert a game cartridge into the cartridge port.

And it booted up!

Okay, is the BASIC ROM busted?

Well. After turning the computer off and taking out the cartridge and turning it on again, it would successfully boot into BASIC! What the?!

I thought that simply reseating ICs would immediately take care of most “bad contact” problems, at least temporarily. Well, turns out that oxidation can be pretty serious sometimes!

We had even checked continuity between IC pins and the socket’s pins on the other side of the board, and got beeps as normal.

So it seems this is not a very good test. Well, today (yesterday actually) I learned.

Another note: the computer and power supply was made with 120V/60 Hz in mind according to the labels on the back, but it worked fine at 100V/50 Hz.