S100Computers.com System

Specifications

This system was assembled and built from bare PCBs over the course of 2 months or so in 2025 and consists of 8 cards:

Z80 CPU - V2.1
80286 CPU - V2
4 Megabyte Static RAM - V01
Propeller Console I/O - V2.1
Dual Compact Flash IDE - V3a
Serial I/O - V3.1
ZFDC Floppy Controller - V01
MS-DOS Suport - V3.0a

I first became aware of John Monahan's site s100computers.com around 2009 or so and purchased some of his early boards around then with the intention of building a complete system around them. The project was put on the backburner a number of times for one reason or another, mostly due to the enormity of the project and having to build several boards just to have a basic system. Instead, I concentrated on getting the Cromemco Z-1 running properly.

I eventually populated the slots in the backplane in 2016 but didn't do anything further until 2025 when I had enough Compupro cards to build a complete system. I suspected the Compupro 128k RAM card I had was faulty and this motivated me to build the 4 Megabyte Static RAM card followed by the termination circuitry on the backplane. From there, I successfully booted the Compupro system and this motivated me to continue soldering up the remaining cards I had - some of which I had bought upgraded versions since the initial PCB purchases.

The second card I built was the Propeller Console I/O which was a video/keyboard card which mostly emulated a serial port connected to a VT-100 terminal. I successfully tested this in my Cromemco and moved onto the Z80 CPU card which I got running at 10 MHz and gave me a basic system with a built in monitor program. The Dual Compact Flash IDE card was done next and I got it working after dropping the CPU speed down to 8 MHz. It was a small sacrifice to make in order to boot CP/M 3.

Next I built the Serial I/O card minus the speech synthesiser followed by the ZFDC Floppy controller which gave me trouble until I found that the 5V regulator was a counterfeit piece of garbage that was sitting just above 4V. The MS-DOS Support board was built after that, which gives me an additional serial port, an interrupt controller, timer chip, and real time clock.

The final card has an 80286 CPU running at 10MHz. I spent days getting this card working - firstly checking the jumpers, reflowing solder (which worked temporarily), and finally removing the CPU and cleaning the pins. I started with an 8 MHz oscillator and it was stable. I then increased it to 10 MHz and it was still good. Bringing it up to 12 MHz as described on the website didn't work regardless of how many wait states I put in. I'm happy with 10 MHz.

This configuration allows me to run several operating systems - CP/M 3 on the Z80, and either MS-DOS or CP/M-86+ on the 80286. Not having any proper video hardware limits what MS-DOS programs I am able to run - generally text-only programs that only use BIOS or system calls and no codepage 437 characters. It really doesn't leave much, hence why a PC-DOS 3.30 install on a 32M partition is more than adequate for my usage.

My only real criticism of the system is an incomplete implementation of the VT-100 escape codes on the Propeller Console I/O card, which lacks reverse and flashing or bold characters. This means some games I really love playing under CP/M, such as GORILLA.BAS, don't get drawn properly. My attempts to rectify this in the firmware weren't entirely successful, due to my lack of experience in Propeller Spin coding. Hell, I'm not really much of a programmer. It also would have been nice to have a centralised repository for the firmware and utilities to track updates and modifications. This, and the above, are just minor issues. John Monahan, as well as other contributors, has done an amazing job with this system and the website that documents it. It's by far the most complex and involved DIY computer project I've built and the community around it has been great.

One thing that I absolutely love about this system are the status LEDs on each card which give a lot of visual feedback that it's doing everything it's meant to be doing - reads, writes, card selection, etc. Some cards have LED bars for CPU signals, interrupts, and RS232 signals while the fancier ones have hexadecimal displays for disk sectors/tracks and ASCII codes. Running the computer puts on a bit of a light show.