DEC M705 vs M7050 in PC05

william degnan billdegnan at
Wed Jul 29 13:56:16 CDT 2015

It could also be the extra "0" is for some reason intended to be
installed in an expansion chassis for the system.  It is a stretch to
make this assumption, the thought occured because I know the power
supply in an expansion chassis I have is called h7420a, whereas the
main unit part number is h742a..

On Wed, Jul 29, 2015 at 1:51 PM, Vincent Slyngstad
<v.slyngstad at> wrote:
> 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!
>    Vince


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