qbus module ID

From: Jerome H. Fine <jhfinexgs2_at_compsys.to>
Date: Mon Feb 7 21:23:44 2005

Pete Turnbull wrote:

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

How did DEC manage to sell an FPU with those DCJ-11s?

Or were those DCJ-11s used with the M8190 boards when
the FPU socket was disabled?

>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 have a BA123 where I usually run with slot 1 empty, an
M8190 in slot 2 and normal memory in slot 3. On rare
occasions, I put PMI memory in the top slot and replace
the memory in slot 3 with an ESDI Qbus controller. I am
pleased that you qualified your statement that if the CPU
is in slot 2 or lower, there MUST still be a PMI board in
slot 1 to activate the PMI memory feature. I also tend to
assume that it is NOT a good idea to mix normal and PMI
memory in the same system.

As for micronote 30, I could not find the specific section
where it describes your information, but I spent only 5 minutes
attempting to understand. I was not surprised I was unsuccessful
since I really do not have enough of a hardware or electronics

>>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
As I stated, my benchmark tests showed about a 20% reduction in the
time between normal memory and PMI memory. Based on the information
in micronote 30 that memory access via PMI is about 2.5 times as fast, an
overall 20% reduction in benchmark time seems reasonable. When I added
a 13% reduction in benchmark time due to a change from the 15 MHz
crystal to the 18 MHz crystal, the overall reduction of 33% was great.
I think that a 4.5 minute benchmark reduced to about 3.0 minutes. Then
when I tried a PDP-11/93 board, the time reduced to about 2.7 minutes.

>You can build a 4MB 11/83 with the CPU and just one MSV11-Q, or two
Actually, I understand that Plessey made a 4 MByte PMI board, so it
would be possible to have just a single PMI memory with all 4 MBytes
above the CPU. Otherwise, I just use 2 * MSV11-JE boards from DEC.

>>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.
Even since I experienced a bad power supply on a
BA123, I always run my hard drives on a separate
spare power supply. Specifically, in this case,
4 Hitachi DK515-78 drives arrived in a separate
box with many cables and their own power supply
along with fans. I pulled the drives, fans and
power supply and tie everything together with a
number 10 ground wire (to the BA123 chassis and the
case of the DEC distribution box - those are GREAT).
The separate power supply provides power to 3 fans
(one per drive) and the 3 hard drives.

I should mention that this does not seem to work
for TK50 / TK70 tape drives. When they are used
(the TK70 was at one point my primary backup - 2 1/2
tapes per 600 MByte drive), I plug them in, as required,
into the internal power supply. Otherwise, one hard
drive is used for development (on RT-11, 600 MBytes
is almost too much) with the other two drives switched
on when a backup is being done or just a comparison
of all 20 RT-11 partitions. The RQD11-EC also allows
me to keep the hard drives in WRITE PROTECT mode
except when I want to copy any recent changes to the
backup drive. Since at one time I was making frequent
changes to the RT-11 operating system to fix bugs,
especially the Y2K stuff, I wanted to be sure that
none of my goofs could cause any changes to the hard
drives. Lately, that also serves me well even under
an emulator since I am still correcting bugs in RT-11,
but this time using a STAND-ALONE program which hobby
users can run rather than performing a SYSGEN. Since
the STAND-ALONE program changes code in the monitor
after the boot is complete, the possibility arises that
a goof on my part can cause corruption of the emulated
hard drives if they were not in emulated WRITE PROTECT.

Sincerely yours,

Jerome Fine
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Received on Mon Feb 07 2005 - 21:23:44 GMT

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