Project: The "General Purpose MicroComputer"

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Since a long time I wanted to build my own computer, but I thought I would never be able to do that. After Ive seen the "RC2014" Computer, I realized that it's not that complicated and decided to recreate this Computer. The "Cactus" computer (which uses a 6502 CPU and features a nice looking front panel) heavily inspired this Computer, especially the front panel. So I started building it in the Summer of 2018, and slowly completed all cards. After almost three years later, I finally managed to get the computer working. If you wonder about the name, I have choosen this (unordinary) name because 1. I wanted a nice abbreviation 2. I wanted it to have "microcomputer" in it and 3. I wanted the name to indicate that it can perform all kinds of tasks (for example run BASIC, CP/M or just any assmbler code; be programmed through a serial terminal or the frontpanel; control all kinds of electronics through I/O ports) and 4. I don't want the abbreviation to be easily confound with something that already uses this term.

I decided to use a 19" Subrack and eurocards for it, with a 64-Pin Connector (DIN 41612) at the back.

This table shows all the cards (although some schematics and pictures are outdated, up-to-date schematics can be found further below):

Name Version Description Specs Picture Schematic
PSU 1.0 Power Supply Unit Input: ~+7-25V
Output: +5V
Current: ~1,5A
None(It's simple!)
CPU 1.0 Central Processing Unit Z80 CPU
Additional /MEMRD, /MEMWR, /IORD & /IOWR
CLK 1.0 Clock Generator
The clock speed is controlled by a 4-bit hexadecimal rotary switch whose output is demultiplexed(inverted output) and used to enable the corresponding clock speed by a NOR gate. (Wrong gates in schematic) To improve the signal quality it is sent through a NOT gate.
10 different clock speeds
Manual, ~10Hz, 0,1536-7,3728MHz
LED clock signal indicator
RAM 1.0 Random Access Memory 56k (64k paged)
Two 62256's
ROM 1.0 Read Only Memory
Note that the current circuit and the board have an error in it, pins 1 & 2 of the 74'393 need to be swapped and PAGE has to be conected to pin 3 instead of 6.
Accepts 2716 to 27512
Can be paged in and out
SIO 1.0 Serial Input Output Z80B SIO
CTS/RTS Handshake
2x RS-232 ports
FPL 1.0 Front Panel Logic
This board is used to control the front panel. The pictures is outdated; it already has two circuit boards piggybacked onto each other. It still has a few faults, but the indicators as well as memory deposit and examine already work.
Displays current bus status
Allows examine and deposit up and down

For almost all cards I used the schematics of the RC2014, although I modified some a bit and expanded the bus output. If you want to have a look at the KiCad schematics you can download them here. I try to keep them up to date, but some might differ from the current circuit. You can see the bus layout here:

After a lot of troubleshooting I finally got the computer working. Here are a few of the problems I found and fixed:

- The old "backplane", which was essentially just connectors interconnected by a ribbon cable, had many loose connections and made the whole computer malfunction. After replacing it with a real backplane I did not had another backplane-related problem.

- The Page circuitry was wired up completley wrong and caused the memory to malfunction.

- I forgot to connect the pull-up resistors on the CPU card with +5V, causing the CPU to malfunction.

- The RAM decoding circuit was wired wrong a couple of times, I had to redo it in order for the RAM to get decoded correctly.

In order to test the basic computer, I wrote a simple program that increases register A and outputs it to an 8-bit output port. After some small problems with the output-port circuitry and fiddling around with jumpers I got it working and was able to confirm the computer working. After that I added RAM and SIO and replaced the ROM with BASIC. To my surprise I was greeted with BASIC on the terminal! However, after a bit of testing I found out that the "OUT" statement sometimes crashes the computer. But not randomly, for example never when I output 00H or FFH to port 00H, but always when I output 01H or 04H to port 00H. I have no idea what causes the problem, there doesn't seem to be a similar problem existing on the RC2014.

But after all, I am quite happy that I built my own working RC2014. It was a fun project and I hope I can expand it even further. I still need to fix a few issues with the RC2014 front panel and the problem that some "OUT" statements crash the computer.

The first picture shows the completed case and the computer up and running; the second one the left sidepanel removed and the third one the wiring of the frontpanel.

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