PDP 11/03

Noel Chiappa jnc at mercury.lcs.mit.edu
Mon Nov 9 21:31:01 CST 2015

    > From: Jim Stephensn

    > The 9400-YE has the cables that formerly went to the 11-780.  I'm not 
    > sure what it does. ... I'm not sure if the M9400 needs to be pulled or
    > not to run the system.

This one I actually can answer, since I've been looking for /780
documentation: found some, but nothing on the hardware config of the console
front end, alas. But I did discover about how the LSI-11 is interfaced to the

The M9400-YE is a standard QBUS card for doing QBUS extension boxes; the QBUS
leaves the M9400-YE (at the end of one box) on a couple of cables, which
normally go to a M9401 (the sister card to the M9400) in the first slot of
the next box.

In the /780, the cables from the M9400-YE instead go to a card in the VAX
CPU, an M8236 (the CIB - CPU Console Interface Board, I think). That board
contains i) a bunch of ROM for the LSI-11, and ii) registers which allow the
LSI-11 access to the inside of the /780 CPU - all of which are actually on
the LSI-11's QBUS, logically/electrically.

So, as to the M9400 - you can pull it, or leave it - there are no active
components on it (although it does have termination pull-ups). Do pull the
cables, though - they'd be un-terminated antennas...

    > The M9400 seems to possibly be the Floppy boot and some other logic.

There are some versions of the M9400 which contain ROM chips with a
bootstrap. The M9400-YE, which you have, does not have these; it only has the
headers for the cables, and the pull-up resistors.

    > The line below that I clipped out, about the M7940-YA is for the
    > adjacent card, and has no cable, just the open blue one waiting for an
    > IDE cable to the back for serial connections.

Err, it's not an IDE cable, although it is a 40-pin connector; it is used
with special round cables, the wiring of which I described in an earlier

    > A post by Tony Duell some years back states that the M7940-YA may have 
    > both a current loop and RS232 drivers for the port. That ... might
    > explain the puzzling extra bit in the Dec module description about
    > extra wires.

No, the stock M7940 _already_ has support for both EIA and 20mA; see the
DLV11 prints (MP-00055), pg. 6; the EIA stuff is in the top right-hand
corner, the 20mA in the bottom right-hand.

On that same page, lower left, note the baud rate clock generation; note
that's entirely set by jumpers, so if there's some off-board way to set
it in the M9740-YA, they must have modified this area of the board.


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