qbus module ID

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Mon Feb 7 02:31:23 2005

On Feb 6 2005, 22:17, Jerome H. Fine wrote:

> After attempting to check the KDJ11-BF board, I now
> agree.
> From what I heard, when DEC originally attempted to
> use the 18 MHz crystal, the FPU bug was not initially
> apparent and it took a while before DEC realized that
> the higher speed required a change to the micro code
> for both the CPU and the FPU. But it was possible
> to reliably run the older versions of the CPU chip with
> the FPU using the 15 MHz crystal.

Mostly. There's more than one bug; there are DCJ-11s which won't work
with the FPU, at any speed.

> >Indeed. Diagnostics and operating systems distinguish 11/73 from
> >by looking at a bit that relates to PMI memory.
> >
> >
> I think you are referring to what the software sees when the PMI
> memory is either active (above the CPU) or not (below the CPU).
> However, I have no understanding about what the hardware does to
> activate the PMI stuff when the memory is above the CPU.

Not much. AFAIR there's a bit in a CPU status register which is
connected to the upper side of the board, that is, to the PMI bus.
 When the CPU is in slot 1, there's nothing above it and that bit
floats high (logic zero, since the bus is negative logic). All PMI
memory boards ground that bus connection, so when the CPU is in slot 2
or 3 (or 4, in a BA123) with a PMI memory board above it, the bit is
pulled low (logic 1). That also causes memory accesses to use the PMI
bus instead of the QBus. The timing on the PMI bus is different, and
faster. It uses strobes, not handshakes. See Micronote 30 if you want
the details.

> I concluded from
> the 5 different tests that the PDP-11/93 was vastly over-
> priced and over-rated relative to a PDP-11/83 EXCEPT for
> the fact that a 4 MByte PDP-11/93 uses a single quad slot
> whereas the equivalent 4 MByte PDP-11/83 needs 4 quad slots.

The relative speeds will depend on which PMI/non-PMI memory you use, as
you saw from the comparison of your DEC board and he Plessey one. It
will also depend on other factors, like what's on the bus and how it's

You can build a 4MB 11/83 with the CPU and just one MSV11-Q, or two

> BUT, I do have a problem which I have never been able to
> solve. I use the PDP-11/83 with 2 PMI memory boards inside
> a BA123 box. I can't seem to remember having a problem
> when the EDSI controller (Sigma RQD11-EC quad ESDI controller
> with 3 Hitachi DK515-78 600 MByte ESDI hard disk drives)
> was initially used.

Maybe you're drawing too much power from one of the supplies, and the
voltage is drooping. The box is rated for two hard drives.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Mon Feb 07 2005 - 02:31:23 GMT

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