The Program Coordinator for Rockville Science Center saw my exhibit at the Montgomery County Fair and invited me to do a talk and exhibit. The talk slides were a cut down version of a talk I did at VCF East 2017 with new material added. Had close to thirty people.
After the talk I discussed the equipment I had brought and demonstrated assembling a "hello world" program from paper tape then loading and running the binary tape output. Then did the signal processing demo. Had a request for games from the kids so let them play the tank game and spacewar while I answered questions.
Other than the talk preparation it was supposed to be only a simple equipment check for getting ready. Reality corrected me on that. Bumped one of the margin switches and it didn't reclose properly when I set it back so computer stopped running. Had to use contact cleaner on those switches to get them to behave again. Also had to do the same to some of the front panel switches. A piece of plastic for the teletype type area cover broke so I glued that. The teletype interface was occasionally acting up. It finally failed enough for me to track down and replace a bad diode.
Also had some software problems. The tank game generated two shots with one in the wrong location with each button push. Tracked that to it was using MQL MQA instruction to swap the MQ and AC register. That isn't valid on the Straight 8 since it didn't operate reliably and DEC didn't fix until later machines. Don't know if it worked at the previous demos or generated points off screen so wasn't noticeable. Put in a patch to replace with valid instructions and it was mostly fixed. The first update or two still sometimes generates an extra dot. For some reason they generate two points for each shot update which seem to be identical. Only have disassembled source so difficult to understand the code.
The FFT demo only worked with single sine wave. If I switched to square wave the output was all messed up. Looks like I started fixing an issue I saw with the second pass code overwriting some of the first pass code but got sidetracked before finishing. More details near end of my writeup of restoring DECUS-8-250 source as .odt file or as .pdf file.
I also brought various related ephemera. No pictures were taken of it.
The following picture links also have descriptions of what is shown in the pictures.
Giving talk ( 91K)
Discussing what's in the machine (103K)
Watching hello world load (107K)
Feel free to contact me, David Gesswein firstname.lastname@example.org with any questions, comments on the web site, or if you have related equipment, documentation, software etc. you are willing to part with. I am interested in anything PDP-8 related, computers, peripherals used with them, DEC or third party, or documentation.
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