[TUHS] PDP-11/23 questions ...

Pete Turnbull pete at dunnington.plus.com
Fri Jun 9 08:44:15 AEST 2006

On Jun 8 2006, 17:15, asbesto wrote:
> 1) we lack the power supply of the 11/23 cpu. From the
>    schematics, we see that only +5V, +12V and -12V are required,
>    so we will try to use a normal PC power supply for the QBUS
>    backplane; does somebody know about problems in doing this?

Is it missing or just not working?  It's hard to imagine a BA11-N box
like the one in your picture without the PSU, since the screws that
hold the front panel onto the backplane go through the PSU cover.  If
it's simply not working, it's not usually hard to repair.

You will need to ensure that the BDCOK H (Bus DC OK, active high)
signal is held high, also the BPOK H (Bus Power OK, active high, from
the AC input) signal or the CPU won't run -- the normal PSU does this.
 "High" means tied to no less than 3.5V DC.  The PSU also provides a
mains-frequency square-wave at about 3.5V-4V which drives the BEVENT L
line for a real-time clock interrupt, which Unix needs.  One of the
switches on the front panel can be configured to control this (there
are times when you might want to switch it off).  Note that devices
that turn off BEVENT, including the switch on the front panel, or the
DIP switch on the CPU card, do it by shorting that line to ground!  The
same switch that can be configured to stop the BEVENT signal, is also
often used to control the rack's power controller via a 3-wire cable
with a 3-pin AMP Mate-N-Lok connector on each end.

The front panel with the three switches also has a flip-flop controlled
by one of the switches, connected to the BHALT L line, and another
connected by a flip-flop to BINIT L.  The first halts the CPU when
enabled (active low), the other provides a pulse to start it.

The RUN light on the panel is driven by the SRUN L signal on the first
slot in the backplane.

Most of the signals I've mentioned are carried between the backplane
and the panel by a narrow ribbon cable.  The backplane pinout is shown
in a PostScript file called QBusConnsBig.ps on my web page at
QBusConns.ps is the same file, but actual size, if you want to hold it
up against the backplane.

> 2) what kind of UNIX can be run on an 11/23 using a RL02 disk
>    drive? (just one, unfortunately :!)

Nothing later than about 7th Edition, because BSD needs separate I&D
space, which an 11/23 doesn't have (2.9 BSD might work, I can't
remember).  BSD (any version) is much too big for a single RL02 anyway.
 7th Edition works; my original PDP-11 Unix system is my second 11/23,
still in its original condition, which looks rather like yours, except
it has two RL02s and a slightly earlier front panel.  Be aware that the
RL11/RLV11/RLV12 driver was not a standard feature of 7th Edition,

You ought to do an inventory of the cards.  7th Edition wants at least
256K of memory.  You might also want to see what version of the CPU you

Pete						Peter Turnbull
						Network Manager
						University of York

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