SC131 CP/M Computer

Back to the 1980s. Retro-Comp, in my case not so much collecting, restoring, or using older computer hardware and software, but rather revisiting my older designs and building, if possible, a modern version of an old computer.

My first encounter with computers was in late 1983 when my company’s computer club started a project to recreate the Ohio Scientific Inc. system using the MOS Technology 6502 microprocessor. I built several versions of this system and later expanded it with an EPROM programmer, an AX-25/Packet card, and a Z80 processor card. You can read a blog post about my experiments here.

So, in the mid-1980s, I learned to ‘handle’ CP/M and came to understand the Z80 a little better then I did while playing with the ZX81 and 16K RAM extension. 🙂

The system runs on RomWBW with a special configuration for the SCZ180_sc131. RomWBW is a complete implementation of CP/M operating systems and applications for modern Z80/180/280 retro-computing hardware systems. The SCS131 design includes a Z180 processor clocked at 18.432 MHz, 512 KB of RAM and 512 KB of Flash (ROM) with RomWBW, two serial ports, and an SPI port, primarily for SD card mass storage.

I purchased the kit through EU partner/VAT broker Lectronz. The package arrived within a week, without any delays or customs issues. The quality of the packaging and the contents of the kit are very good and professional.

I couldn’t wait to build the kit and get it working. So I got the soldering station, ESD protection, multimeter, and some tools on the workbench. The PCB solders very smoothly and the assembly guide is very clear. There are many explanatory documents on the SC131 page that are useful to read beforehand.

All static tests were good, and the smoke test indicated 4.87 Volts and 80 mA, so everything was fine. Unfortunately, the system wouldn’t boot. It hung on ROM VERIFY and wouldn’t proceed. This indicated a ROM error or problems with the address or data lines between the ROM, the RAM, and the Z180. I contacted Steve at SCC, and he responded within an hour with some ideas. He would check the batch of flashed ROM on his end, and I could checked the address and data lines, including A19 in the mean time.
Neither of us could find any abnormalities, and all the soldering was correct, and no short circuits were found. But then I realized I had measured op top of the Z180 socket and not on the chip itself, because those contacts are too deep hidden in the socket. Should I assume a bad contact or short between chip and socket pins? So…. would I dare to remove the Z180 as it sits very tight in the socket?
Short prayer…. removed the chip carefully (lever via the corners where there is special space for this), checked the socket for rubble, blew a bit in the socket and placed the Z180 firmly back. ‘Houston we have succes‘. The system booted! I have to admit I’ve never worked with a 68-pin PLCC before, so assembly-wise this was new. That same evening, on the day the kit arrived, I had this on my screen:

Excellent service from Steve and some electronics knowledge helped me successfully create this vintage system. The project was completed by mounting the SD card and placing the whole thing in the Hammond enclosure. I also printed an enclosure for the FTDI adapter between the Elite_sc131 and the USB cable to the Host PC which happens to be an HP EliteDesk. 🙂
I’m using ZOC Terminal in VT100 mode, but still need to experiment a bit to see if it meets all my needs.

So why retro computing? Not only because it’s a fun activity after retirement, but also because it brings back old memories. I’m working on getting a 6502 system up and running again, and I can think of a few more projects. The problem, however, is finding genuine components.

Isn’t it funny to boot up MBASIC on your newly built CP/M system and see that the last release date was 44 years ago?

Meanwhile, I’ve found an excellent CP/M landing page at The *HUMONGOUS* CP/M Software Archives. There you can start browsing through a large number of files to experiment with. Lawrence Woodman’s website also deserves special attention. There you’ll find explanatory videos.