PDP-11/34a hung bus problem.

Noel Chiappa jnc at mercury.lcs.mit.edu
Sun May 15 07:29:22 CDT 2016

    > From: Don North

    > If the bootstrap card is an M9312 with the standard console PROM, it
    > does NOT require any memory to be present/accessible to get to the ODT
    > prompt that prints out the registers and waits for a command.

Ah, right you are; I'm not familiar with the M9312 codes (you seem to have
that all well in hand :-), I've only studied the M873 and M9301 codes.

But the M9301 'console without testing' code does in fact look like it would
run without any memory in the machine. (Which explains some odd features in
the code - I'd always wondered about the 'unusual' subroutine caling
sequence, but now I see it allows it to work without any memory.)

    > From: Ed Groenenberg

    > ISTR that there is a jumper which selects for +12 or +15 volt.

Not that I am aware of - see the power circuitry in the drawings, MP-00742,
pg. 25. Maybe you're thinking of the M7891 (MS11-L), which does have such
a jumper?

    >> The M7891 uses +/-15V as well as +5V?

    > Yes, both +5 and +15 is used and measure ok on the backplane.

Actually, having looked at the prints, it also uses either -5V or -12V/-15V
(there's a jumper). So you might want to figure out i) which your system has
(in a BA11-K box, if you have an H745 'brick' you will have -15V, if an H754
-5V; if a BA11-L box, different versions of the H777 provide different
voltages, but I'm too lazy to check :-), and ii) check to make sure the -
voltage is good too.

Although I doubt the - voltage is causing this problem; I'm pretty sure only
the memory chips use it, so it's it's not right, probably the memory would
return bad data, is all.


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