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The PDP-8/E Minicomputer

The PDP-8/E is perhaps DEC's most successful PDP-8, and is pretty much identical to the PDP-8/F and PDP-8/M (notable differences are the power supply and the length of the cabinet; the E has a linear supply and a longer cabinet).

When most people think of a PDP-8, they are usually thinking of the 8/E.

The Front Panel

Front Panel of the PDP-8/E S M L XL

This is the front panel for the PDP-8/E. This particular PDP-8/E was used by the Canadian Coast Guard as part of the LORAN-C navigation system (read more). The normal "DIGITAL" logo is missing, replaced with the code letter "CDDP". This is the "vendor designation" for DEC (Digital Equipment Corporation) used by the suppliers of the LORAN-C system. Also notice the other non-standard artwork on the front panel -- detailed step-by-step instructions on how to bootstrap the machine.

There are currently three PDP-8/E's in the collection, the one discussed above that was used for LORAN-C (system #1), and two others (system #2 and #3) obtained from a generous donor in Ottawa.

System #2 is S/N M40 00000 467, and has the following options and modules:
KL8E M865 Teletype Interface
KA8E M835 External Positive I/O Bus Interface

System #3 is S/N 2871, dated 1972 02 24, and has the following options and modules:
KE8E M8340, M8341 EAE -- Extended Arithmetic Element
KD8E M8360 Data Break Interface
KA8E M8350 External Positive I/O Bus Interface
TD8E M868 DECTape controller (simple)

2006 12 31

I started in on the restoration of the TU56 tape drive. Mine came with an H716B power supply for the +5V and -15V, so the first thing was to get that running. I used a variac setup with 5VAC increments each hour. As of noon today, I'm up to 55VAC and everything looks good.

David Gesswein has a fabulous page detailing the restoration of his TU56, so I'll omit most of the details of mine because they are bound to be similar.

2007 05 10

Made some good progress on the TU56 restoration. The H716B power supply has been reformed, and the two big caps in the builtin power supply were also reformed (these were done with a DC power supply and a current limiting resistor).

All of the cards were removed, cleaned, and replaced.

Power up was done in the usual "let's try it for 10 seconds and see if anything gets hot" kind of way. This was then boosted to 30 seconds, 2 minutes, and finally 10 minutes. Nothing smoked :-)

Long story short, idiot boy (that's me!) put the front panel connectors back into the backplane upside down. No real excuse, just sloppy :-( Anyway, nothing bad happened, and, when the cards were put in the right way up, most LOCAL front panel operations worked. All four front panel lights are operational, and everything works in LOCAL except rewind on the left unit. Probably a flip-flop that doesn't flop, or a bad connection on the switch.

2007 05 11

No worries, turns out it was the switch. It just needed to be threatened with a soldering iron. I started disassembling the switch, and as I was warming up the iron, it started working again. So, I sprayed it with some contact cleaner and it's as good as new.

Next, I hooked it up in REMOTE mode, and verified that the STOP and REVERSE signals had the desired effects and that the unit selection worked (this was from the TD8E connector). Everything as expected. Finally, I connected a scope to all the head output lines (Timing, Mark, Data 0, 1, and 2) and verified that some form of square wave-like waveform appeared. Success!

This evening, I found an PDP-8/M and installed the M868 TD8E card, and connected the two together. After re-aquainting myself with the joys of toggling in the RIM, then loading the BIN, and then the dumptd8e program, I encountered a problem. The dumptd8e program rewinds the selected tape until it flies off the takeup reel and keeps rewinding until I terminate the program and put the TU56 back to LOCAL or OFF.

Well, that's the fun in antique computer restoration.

[Added 20080921 -- I got a winch off eBay a while ago, and finally got a chance to use it]

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