The Program Coordinator for Rockville Science Center invited me to exhibit at Rockville Science Day This was supposed to be a repeat of the demo I did at the Rockville Science Center. See link for more information on demo. I made a few changes. Trying to figure out something people could identify with for my signal processing Fast Fourier Transform (FFT) demo. I had wired up a touch tone phone so they could see how the Dual Tone Multi Frequency (DTMF) encoding of the keys worked. The 4k EduSystem 10 BASIC was recently recovered so I also added a BASIC demo to the disk.
Did a final check of the demo the day before. The Teletype interface started acting up. Found four bad diodes on a S203 card and replaced them. Then the Teletype keys started getting stuck down. Found the spring plate on the bottom of the keyboard had shifted. Fixed that and everything was running fine.
Got everything set up at the show and computer wouldn't run at all. Could single step through short code sequences but running at full speed corrupted the program. Tried wiggling cards in case one of the edge finger contacts wasn't good but it didn't seem to change anything. Checked the voltages at each row of cards in case one of the margin switches was acting up. All looked good. I did have my scope but with PDP-8 only failing in run, using it to troubleshoot would be time consuming. Since show was starting I tried wiggling the switches anyway. That made things much worse but after checking voltages again and more wiggling till all looked good it could run simple programs. It still wouldn't boot from the disk. I tried loading my punch block letters on Teletype tape program over console port and it ran so that was the demo I could do.
I let the public type what they wanted and get a souvenir tape. At the end of the tape it also punched the binary ASCII of what they typed. That sometimes was shorter than what they typed so the computer still had problems but was working well enough. Also tried to explain that this wasn't just a Rube Goldberg Dymo labeler and tape was used to store programs and data. Shortly I had a line that lasted until slightly after closing time. Not the most sciency demo but suspect I wouldn't have had as long a line for the FFT demo. They estimate they get more than 3500 people attending. With the line not sure my original plan of switching demos would have worked that well. Still a little disappointing after having similar problems the week before at VCF. Expect some difficulties with 50 year old computers but twice in a row is annoying. This show is going to need a little different approach than a VCF.
Haven't had time yet to find the true failure with the PDP-8.
If you have interest in the programs' source used for the demo contact me. There are a couple things I wanted to fix before I put them online and image has been added to over several years so need to track down all the source. Image of DF32 disk
The following picture links also have descriptions of what is shown in the pictures.
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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