DEC M705 vs M7050 in PC05

Vincent Slyngstad v.slyngstad at
Wed Jul 29 12:51:39 CDT 2015

From: Jack Rubin: Wednesday, July 29, 2015 10:08 AM
> 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?

I don't know where to find backplane documentation, but I did at one
time diff the net-lists for some versions of M705 vs M7050.  Those
results are paraphrased in

The gist of it seems to be that to use an M7050 in an M705 backplane,
BU1 must be high (will probably float high), and BC1 must not be grounded,
New output pins BL2, BT2, and test point AC1 should be unconnected.
A fair chance it will "just work".

Going the other way is probably more difficult, as you'd have to fake
BL2 and BT2 somehow.

Hope that helps!


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