A lot of the early unibus boards which were 3 digit and replaced by a 4 digit were duel height boards that required a M7821 and M105 (not sure about the numbers), and DEC built those into a quad height board and added a "0".
On Wed, Jul 29, 2015 at 1:56 PM, william degnan <billdeg...@gmail.com> wrote: > 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.slyngs...@frontier.com> 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 > > > http://svn.so-much-stuff.com/svn/trunk/Eagle/projects/DEC/Mxxx/M7050/M705vsM7050.txt > > > > 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 > > > > -- > Bill > vintagecomputer.net >
