The Simulated SelfScan

A project log for Rack8 MCM/70 rack machine replica

Goal: Build a replica, using an actual 8008-1 chip, of the "rack" machine used at Micro Computer Machines when developing the MCM/70.

camfarnellcamfarnell 03/23/2023 at 20:010 Comments

I don't have a SelfScan. eBay doesn't have any SelfScans. I would be delighted to source a working SelfScan and, in theory, it should plug into my Rack8 machine and work, provided I conjured up the required +250 Volt supply. If I had a real SelfScan I would be willing to do that. But in the mean time I've built a Simulated SelfScan which is plug-compatible with a real SelfScan other than not needing the high voltage.

So here is my simulated SelfScan showing a few of the special APL characters (and a Syntax error, but we will overlook that):

The photo really doesn't do it justice The crud at the bottom is reflections off the screen (which the camera picks up like crazy but in person is hardly noticeable) and the actual text is much closer to "SelfScan orange" than the photo would suggest. My simulated SelfScan consists of:

The displayed line is smaller than what a real SelfScan would show but it's way bigger than the usual text on my laptop so it's easily readable. A 7" display would probably result in something close to original SelfScan size.

In the photo above it is the 8008-1 CPU, and the SelfScan interface described in the previous log, doing all the work. All the Arduino is doing is pretending to be a SelfScan. If I ever do get a real SelfScan, that would be great, but for now this will do.

The code currently in the Arduino is proof-of-concept and quite slow: it takes about one second to do a full update of the SelfScan line. But we are turning on each LCD pixel individually and there are 6216 of them. There are various techniques that will speed this up a lot. Now that I know it works, writing that code is worthwhile.

The entire system, as it currently stands looks like this:

The simulated SelfScan is upper right with the SelfScan interface just below. There is actual text on the SelfScan but you will probably need the larger image to make it out.