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1976 Digital PDP-8/A Minicomputer


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  1976 Digital PDP-8/A Minicomputer by Bill Degnan - 10/06/2022 09:58
Partially Assembled. Click image for larger view.

This system came nearly complete, but was missing the cable that connects the power console to the backplane. I got one from the parts bin. I also had to move one of the molex connectors from the main transformer to a different location on the power backplane. Some of the cards were in the wrong slots. I wonder if this is why whomever had the computer originally gave up on it.

A edge-on view of the controller cards installed. The original module assignment sheet on the back of the chassis indicates a M8350 - KA8E posibus interface was part of the original configuration but it was removed at some point. As stated above, some cards as they are in this photo are in the wrong positions. Click image for larger view.

The jumpers on the system's DKC8AA (M8316), used for front panel, serial, and parallel port control is configured for Teletype (Rev D: S1-1,2,3,5,7 OFF). Click image for larger view.

More Photos

Next - "Testing PDP-8/A Without Paper Tape Diagnostics" from the Operator's Handbook.

First, I moved the M8316 from slot 4 to slot 3 per the manual Table 3-2. The manual says that the serial/console board (M8316) and extended option board (M8317) can be in either slots 2 or 3 interchangably. The M8315 (CPU KK8-A) has to be in slot 1 .

CPU switch 7 of 8 is the only jumper in the ON position of the M8515 CPU card. Switches 1-6 in the OFF position = 0000. This will cause the system to jump to 0000 and wait for console input.

The MM8-AA 8K Core memory card that came with the system was installed in a 4-hole backplane slot (!) so I don't think it's the right place for it. The jumpers indicate Fields 0 and 1 (W1-3 and W1-2 jumpers are connected). For now I removed the DATARAM card until I can learn more about it.

Powered on the system and ran the AC counter program from the manual. Works fine. Ran a few other front panel diagnostics and they checked out.

So I guess this works. Next, either find a Teletype to attach to this or configure for a serial terminal.

I posted a 14 second video to my Twitter account of the AC counter program running, if you want to check it out. My Twitter handle is @billdeg



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