DEC M705 vs M7050 in PC05

Jörg Hoppe j_hoppe at
Thu Jul 30 06:35:11 CDT 2015

> If you were following Joerg Hoppe's recent PC05 auction on eBay, you might have noticed that his system had an M705 in the backplane where I would have expected an M7050. This is the way he received it and the restored unit works as it should.
> Clearly the cards are similar but different but are they interchangeable? Would the backplane wiring be different and if so where would this be recorded?
> My copy of the PC04/05 maintenance manual (DEC-00-PCOA-D(1)) makes no mention of the M705 in this location. The PC04/05 print set on Bitsavers has the M705 on the module utilization list (p45) but crossed out with no reference notes. There is an ECO list on the sheet, showing PC05 ECO's 3, 16 and 30 but no specifics. Is there an available ECO log that would cover these notes?
> Then there's the K303, but that's another story...

While repairing I found these functional differences between M705- and 
M7050-based PC05's:

Differences are the generation of the reader's data clock signal 
(Stepper motor position vs. feed hole pulse) and the "end of tape" 
condition" (tape photo cell vs. feed hole timeout).


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