For a change of pace, and a nice side project, I thought I’d design a SBC designed after, and compatible with the CoCo1/2. I decided to go with all through-hole components so that I’d be able to release it as a kit to interested builders.
Design goals included;
- small size
- no SMT components
- software and hardware compatible with the CoCo1/2
- built-in composite video
The first rev I wasn’t particularly happy with, because I had initially put the keyboard connector on the front of the board, and a toggle switch for power(being lazy because I had a supply of them I suppose). As I began to work on a case design (more on that later), I decided to quickly go to rev2 to make some changes. You can see some pictures of that build here.
Though the circuit is heavily modeled after a late-model CoCo2, there are a few minor differences. The design uses no ‘SALT’ chip, which on the CoCo handled power regulation, level shifting the serial IO to RS232 levels, and zero-crossing for the cassette interface.
As a result, the board is powered via an external +5v supply, serial IO is TTL level, and I designed an alternative zero-crossing circuit. The board also features circuitry for composite video output rather than an RF modulator. I initially included the 555 timer based artifact generating circuit, but later realized the improved composite circuit I’m using won’t require one, so that will be removed in the final revision.
The clone using the CoCo SDC and displaying artifact colors
A CoCo standard 40-pin expansion bus is included, as well as the usual joystick ports. The keyboard interface is essentially the same as the original, but with a different connector style, allowing the use of standard CoCo keyboards (or the CoCoMECH) with a simple adapter. This should also allow the use of a PS/2 or other keyboard adapter designed for the CoCo (with an appropriate connector).
Both the serial IO and cassette interface are available via pin headers on the board. There is no motor control relay for cassette IO, but the control pin for it will be available at the header on the next revision (TTL level). The new zero crossing circuit works great as you can see in the following video. I had been fiddling with the video adjustment on the board so the color is a bit off, but testing the cassette IO with a simple audio patch cable to a modern PC for saving and loading files this way was very successful. No problems there. 🙂
Cassette interface test
…more to come…