TRS-80 curiosity

Scott Kevill scott at
Fri Apr 14 20:16:13 CDT 2017

On 15/04/2017, at 1:45 AM, Fred Cisin via cctalk <cctalk at> wrote:
> I had thought that both ends of the Screen printer cable would be 40 pin, That's why I had speculated on Model 3.
> But I never spent $599 for one.
> (instead, I struggled with interfacing a DTC-300 HyType 1 serial printer)

I never had a Screen Printer either, but I don't think it would even be possible to use with a Model III.

I assumed the way it worked on the Model I was by either watching the CPU bus and sniffing writes to the video memory (mirroring to its own copy continuously), or reading the video memory when needed (trickier with synchronisation).

Unfortunately, despite the Model III having 50 pins instead of 40, 25 of them were ground and A0-A7 of the address lines were exposed. Even that was only enabled for IO ports via software first. The Model I was much for flexible for add-ons.


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