philpem.me.uk electronics :: 6502 computer  

The Great 6502 Computer Project

In yet another feeble attempt to stave off boredom, I've decided to build a computer. And not just any computer either - a 6502-based multi-board computer.

At the moment, all that's here are the designs for the CPU card and the UART (RS-232) interface. I'm planning to build a few more boards, too - one with an SPO256 speech synthesizer, one with a PC keyboard and I2C interface and maybe one with a GPIB interface, depending on how far I can make my budget stretch and just how bored I get :P

I've got a HP 1651B logic analyser and a Tek 466 oscilloscope - I also know how to use both of them properly. With a bit of luck, any minor faults should be easy to track down... I hope...

News

(10 AUG 2004) Found the glitches. The 6502 CPU card seems to be running code properly now - it's even running EhBASIC. Found a few more bugs in the address decoding GAL for the CPU card relating to RAM selection. This involved installing another wire between GAL pin 12 and RAM pin 22, cutting the link between RAM pins 20 and 22 in the process. I've also uploaded PNG versions of the schematics.

(28 JUL 2004) More glitches. Swap CS and OE on the EPROM socket (IC2) or the system won't run at full speed. CPU card schematic has been updated, now at rev 1.3. GAL equations for the CPU card (and the JEDEC fusefile) have been updated - a bug was found in the RAMCS equation. Also added schematics for my 8-way backplane PCB. Photos of CPU card and backplane uploaded.

(21 JUL 2004) I've found a minor glitch in the 6502 CPU card schematic. The pin numbers for XTAL (the 8MHz oscillator module) are incorrect. OUT is on pin 8, VSS is on pin 7. If you want to use 8-pin modules as well, you can also connect pins 4 and 7, then 8 and 11 on the XTAL socket together. Insert the 8-pin oscillator so pin 1 on the osc module connects to pin 1 on the XTAL socket (a normal 14-pin DIP socket). You could also connect pins 11 and 14 together instead, then insert the 8-pin osc at the bottom of the socket instead of at the top.

Schematics

CPU card schematic (PNG format)
UART card schematic (PNG format)

PLD data files

CPU card GAL16V8 (IC5) Atmel-WinCUPL source code
CPU card GAL16V8 (IC5) JEDEC fuse file
UART card GAL16V8 (IC2) Atmel-WinCUPL source code
UART card GAL16V8 (IC5) JEDEC fuse file

EAGLE CAD design files

EAGLE library - ATX power connector (PCB mount)
EAGLE PCB layout - backplane
EAGLE schematic - backplane

Photos

Backplane (component side)
Backplane (solder side)
CPU card (component side)
CPU card (solder side)

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