UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - S-100 Light Pen System

cmjones01 · 3rd Nov 2019, 2:41 pm

I remember playing with a home made light pen on my BBC Micro. It worked quite well, and was almost embarrassingly simple. The 6845 CRT controller chip had a 'light pen strobe' input which simply latched the values of the horizontal and vertical counters. That input was brought out to the computer's analogue input socket. Connecting a phototransistor stuck in the end of an old biro to this signal made a very low cost light pen!

Figuring out the light pen position was just a matter of reading a couple of registers in the 6845. I can't remember if there was any operating system support, but it was hardly necessary.

There was some software which took advantage of it, but not much. It was soon overtaken by conventional mice, which serendipitously also had native hardware support in the 6522 VIA chips, but that's another story!

Chris

3rd Nov 2019, 2:41 pm	#3
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Re: S-100 Light Pen System I remember playing with a home made light pen on my BBC Micro. It worked quite well, and was almost embarrassingly simple. The 6845 CRT controller chip had a 'light pen strobe' input which simply latched the values of the horizontal and vertical counters. That input was brought out to the computer's analogue input socket. Connecting a phototransistor stuck in the end of an old biro to this signal made a very low cost light pen! Figuring out the light pen position was just a matter of reading a couple of registers in the 6845. I can't remember if there was any operating system support, but it was hardly necessary. There was some software which took advantage of it, but not much. It was soon overtaken by conventional mice, which serendipitously also had native hardware support in the 6522 VIA chips, but that's another story! Chris __________________ What's going on in the workshop? http://martin-jones.com/