My MCS-48 ECB Single Board Computer

In the beginning of ECB, the integration density enforced very modular systems with CPU, memory and even most essential I/O hardware each on their own card. The eighties already allowed to build single board computers with everything onboard and an ECB interface for extension cards. I decided to put what I consider the essential core of any system on one card: CPU, ROM, RAM, a battery buffered RTC, a serial console port and of course the ECB interface. The RAM is battery buffered, because that way I don't need NVRAM and the RTC needs battery backup anyway. If the CPU would not already contain a timer, I would have added one as well. Being used to rackmount systems, I added a message LED and a board temperature sensor. I cared about power consumption and although the hardware fills a whole eurocard (160x100mm), it only needs around 30mA at 5V. See the hardware description for details.

Back then, most systems offered either a simple boot monitor with one-letter commands, or BASIC, both written in assembler, with a few builtin device drivers allowing to boot from mass storage, and not made to be extended by more device drivers. Worse, changing the CPU architecture required rewriting everything. Sun showed how to solve this problem, so I went a similar route and implemented a simple Forth system as boot monitor.

The prototype

I admire people who build prototypes that don't look like a mess, but at least I am not the only one who does not master that art. At the left is a RJ45 connector for the serial console, using Cisco style pinout, at the right is the ECB connector. In between there is lots of wire and less than optimal placement as a result of various major changes, but: It works.

Build it

You can download schematics and layout for KiCad as ZIP archive. Further, there is the source for the Forth monitor and a couple diagnostic programs.