digital equipment corporation · maynard, massachusetts
ASCII Art and Picture Capture

The Vintage Computer Federation hosted the Vintage Computer Festival East 2023. Other coverage of show

Probably of the most interest is page with pictures captured by my PDP-8/E, information on how this was done, and Video of my display. Another short video.

I brought my PDP-8/E computer with TU10 9-track tape drive for picture storage. RX02 8 inch floppy drive was used for running OS/8 operating system. This was a repeat of part of my 2017 display with some changes. I try to modify exhibits if I bring them back and thought the TU10 tape drive would be more interesting than the RK05 cartridge hard drive. The TU10 also hadn't been to VCF since 2009. Thought it wouldn't take too long to modify my software to use the TU10 but ended up taking a significant amount of time to modify the code and tune up the drive. I had purchased a new head off eBay to see if it reduced the read errors a while ago and put it in the drive but drive didn't like it. Further testing for this display showed the new head was bad so had to put the original back in. Head was new in box but looked to have been stored in a damp environment. Some of the read coils were open.

The RK05 only holds 800k words per partition so couldn't hold a single 640x480 color image which is 900k words. I had made the hardware capable of capturing high resolution images but didn't demonstrate it last time. The only period device I know of capable of printing that resolution was film recorder which I didn't have. May make a neat addition for next time. Using a PDP-8 as the most complicated way of taking a Polaroid photo. I was going to bring a modern printer to print from my laptop. Forgot to grab it when packing so emailed people their pictures.

I also switched to a more period camera than last time. Picked up a 70's and a 80's camera at hamfests. The 70's was the look I wanted but it was dead. Found Vidicon seal had failed so it was gassy. Put a new one in but picture quality wasn't great. No manual for how to adjust and didn't have time to play with it more. The 80's worked for a little while then died back in 2017. To get it working for this show I had to replace all the electrolytic capacitors. Most had green corrosion on the leads where they had been leaking. Only a little corrosion on the PCB. After that it had an OK picture. All this playing ran me out of time so I didn't get to make another attempt at finding a better set of characters to represent the intensity levels which probably should have had a higher priority.

Everything was working fine before loading it up to go to VCF. When I powered it on at VCF on Friday evening the PDP-8 run light was on and it wouldn't respond to the front panel. All of Saturday was spent troubleshooting. I first pulled all cards not needed for the demo. The behavior changed to not running. Found that was the power good signal was running through board I made for remote control of the PDP-8 when I had set it up so it could be run from my web site. Connecting the power good back to backplane went back to run light stuck on.

Tried board from VCF PDP-8/E. Showed problem was my M8330 timing board but their machine had an early M833 which didn't work properly with my boards. Borrowed a scope (after forgetting to bring mine) and found the main timing chain shift register was losing the 0 that should have been recirculating through the shift register. Found that if I manually shorted two points together to force a reset of the circuit it would start operating properly until the next power cycle. The power up reset isn't operating correctly. Haven't had time to find and fix the root cause.

Started putting boards back in but found the serial board wouldn't transmit. Tracked that down to a dip switch had gotten slightly shifted when the boards were stacked so it was open but wasn't obvious. Also had a side track on memory problems when I forgot to put back in the memory extension and timeshare board to allow the machine to address more than 4k. Do much better without time pressure.

Next tried to copy a file from the tape I made of ASCII art pictures. All I got was errors. After a bunch of wasted time found the problem was that I had removed the write enable ring to prevent accidental write to that tape. Write ring humor. The OS/8 MCPIP program gave errors when reading from a write protected tape. Suspect filling out the problem report form at the end of the manual isn't going to get it fixed. Putting write ring back in made it happy.

Now time to go get dinner and head to the hotel. I had brought some lighting to try to optimize the picture and hoped might be able to do some optimizing picture quality but time for playing was gone.

Sunday I was able to do my demo. Busy all day.

PDP-8 self portrait

The following picture links also have descriptions of what is shown in the pictures.

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Setup (101K)  Computer ( 86K)
Printer ( 87K)  Printer ribbon ( 83K)
ASCII art ( 74K)
Table ( 77K)
Computer full (125K)  Cards removed (102K)

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Feel free to contact me, David Gesswein djg@pdp8online.com 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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