Cheap Printout 2

by Mark Horsman

While an old Creed 75R proved adequate for listings etc, it mas hardly suitable for letters and reports. Therefore, seeing an ad. in Wireless World for IBM printers, i trekked to Essex and bought myself an old 3962 Terminal printer. I also received with it some suggestions on interfacing. These struck me as somewhat crude and rather incomplete, having no software with them. So, I set about designing my own.

Firstly, the hardware. Octal buffers to switch between characters and control and then 7-way Darlington drivers (ULN2003) to drive the operating coils. The timing contacts on my machine where not all c/o as per the handouts. So, they are just inverted as necessary and added to ARDY to generate a Busy signal. An enable signal for the octal buffers is derived and switched with bit 7 to determine control or character. The TTL is powered from the Nascom and the coils from a simple 48V ps. The worst part is stripping out and identifying all the wiring, then stuffing it back neatly.

Secondly, the software driver. (The nut behind the wheel). This is based on a look up table. Nulls are returned unused. A linefeed after a CR is also thrown away. Full stops are sent either case to improve speed. Character case is compared with the printer and any appropriate change sent. Finally, a routine for printer busy is ended with a short delay which I found necessary to ensure complete settling after Busy is finished.