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computers suck

Single board PDP-8: take 2 …

A couple of years back, I said I was building a single board computer … and then things went quiet. Yes, I screwed up. A mix of dry joints and possibly burning through traces caused by following old instructions, impatience and a very unforgiving solder type made the original board almost unusable. I finally got a replacement board (thanks, Andrew!) and put in a humongous Digikey order for all the projects that I want to finish, and got going …

circuit board with many chips
I swapped out the 5 MHz crystal for a blazingly fast 8 MHz one

I took quite a bit more care building this, but it was still only a couple of evenings to put it together. While I still used lead-free solder, I hardly needed extra flux at all. The nice ($$$) turned-pin sockets hold the chips much more securely than the cheaper plain sockets I used before.

After a minor hiccup (homebrew null modem cable needs both RX and TX to be useful), it lives!

SBC6120 ROM Monitor V320 Checksum 3752 6072 3515 09-APR-10 21:15:39
Copyright (C) 1983-2010 by Spare Time Gizmos. All rights reserved.
NVR: Not detected
IDE: 489MB - LEXAR ATA FLASH
IOB: Not detected
B
-IDA0
.BASIC
NEW OR OLD--OLD
FILE NAME--ASCART
READY
LIST
ASCART BA 5B
100 FOR Y=-12 TO 12
110 FOR X=-39 TO 39
120 C1=X.0458 130 C2= Y.08333
140 A=C1
150 B=C2
160 FOR I=0 TO 15
170 T=AA-BB+C1
180 B=2AB+C2
190 A=T
200 IF (AA+BB)>4 GOTO 240
210 NEXT I
220 PRINT " ";
230 GOTO 270
240 IF I<=9 GOTO 260
250 I=I-57
260 PRINT CHR$(48+I);
270 NEXT X
280 PRINT
290 NEXT Y
300 END
READY
RUN
ASCART BA 5B
000000011111111111111111122222233347E7AB322222111100000000000000000000000000000
000001111111111111111122222222333557BF75433222211111000000000000000000000000000
000111111111111111112222222233445C 643332222111110000000000000000000000000
011111111111111111222222233444556C 654433332211111100000000000000000000000
11111111111111112222233346 D978 BCF DF9 6556F4221111110000000000000000000000
111111111111122223333334469 D 6322111111000000000000000000000
1111111111222333333334457DB 85332111111100000000000000000000
11111122234B744444455556A 96532211111110000000000000000000
122222233347BAA7AB776679 A32211111110000000000000000000
2222233334567 9A A532221111111000000000000000000
222333346679 9432221111111000000000000000000
234445568 F B5432221111111000000000000000000
864332221111111000000000000000000
234445568 F B5432221111111000000000000000000
222333346679 9432221111111000000000000000000
2222233334567 9A A532221111111000000000000000000
122222233347BAA7AB776679 A32211111110000000000000000000
11111122234B744444455556A 96532211111110000000000000000000
1111111111222333333334457DB 85332111111100000000000000000000
111111111111122223333334469 D 6322111111000000000000000000000
11111111111111112222233346 D978 BCF DF9 6556F4221111110000000000000000000000
011111111111111111222222233444556C 654433332211111100000000000000000000000
000111111111111111112222222233445C 643332222111110000000000000000000000000
000001111111111111111122222222333557BF75433222211111000000000000000000000000000
000000011111111111111111122222233347E7AB322222111100000000000000000000000000000
READY
ASCII art Mandelbrot set
If WordPress’s line wrapping has mangled the above, it should look like this

It compiles and runs a slightly modified ASCIIART.BAS Mandelbrot set benchmark in 144 seconds. This is comparable to many 8-bit computers. The modifications were:

  • PDP-8 BASIC doesn’t quite use ASCII. Its six-bit character set has digits 0–9 at decimal 48–57 like ASCII, but characters A–F are at decimal 1–6 (instead of 65–). The manual claims that CHR$() works modulo 64, so maybe I didn’t need to make this change.
  • Variable names can be called Letter+Number at most, so the original’s CA and CB had to become C1 and C2.
  • PDP-8 BASIC doesn’t support a familiar IF … THEN … structure, but only effectively an IF … GOTO …. I mean, sure, you can use THEN if you want, but only a line number or a GOTO … following it will avoid the dreaded terse NM error. ELSE? Who needs it?!
Categories
computers suck

SBC6120 build: day 1(-ish)

SBC6120 (RBC Edition) with all the sockets

Just started on a SBC6120 RBC Edition kit. It’s a DEC PDP-8-compatible single board computer that uses a CMOS chipset from the early 1980s. Yes, it will be very slow, even with the optional speedy 8 MHz oscillator installed. With a 12-bit processor and 32 kilo-words of RAM, this is definitely going to be a Slow Computing device.

Lots and lots of sockets. So many sockets. It’s quite soothing soldering them all in, one hole at a time. It looks like it’ll go more quickly than the Zeta did.

> Does anyone know what each of the pins on the 6502 CPU chip in the Apple II Plus does?
They all plug into the socket on the motherboard to keep the chip from drifting away. – c.s.a2 FAQ of yore

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photo

Volunteering at the Maker Festival this morning #soldering #magicsmoke #makerfestivalto #cantgetthestaffthesedays

Volunteering at the Maker Festival this morning #soldering #magicsmoke #makerfestivalto #cantgetthestaffthesedays

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