compucolor

Since we are on the subject, the purchase of the said disks motivated me to get my Compucolor working. I've had it for a couple of years but between moving, having other old computers to tend to, and the fact that it was reported to have produced smoke the last time it was powered up, I never got around to it.

Disks couldn't be formatted using other computers because of the hokey (although dirt cheap) disk interface. The Compucolor has a TMS 5501 multifunction interface chip which contains, in part, a serial port controller. This serial port controller is used for the RS-232 serial port of the machine, and it is good up to 9600 baud (but there is no hardware flow control). Anyway, ISC took advantage of an undocumented test mode of the chip to drive the serial port at 8x speed. The disk looks like a high speed serial channel and the data is simply conditioned and drives the r/w head of the disk drive. I haven't looked into it yet, but they must be encoding each real data byte into two transmitted bytes in order to ensure sufficient transition density and no accumulated DC bias. The net effect is that the disk holds only a bit over 50 KB.