pdp11/84 PMI memory: What is the problem with Q bus?

Mark Matlock mark at matlockfamily.com
Tue Apr 21 10:48:08 CDT 2020

> Date: Sun, 19 Apr 2020 15:29:44 -0400
> From: Chris Zach <cz at alembic.crystel.com>
> To: CCTalk mailing list <cctalk at classiccmp.org>
> Subject: pdp11/84 PMI memory: What is the problem with Q bus?
> Message-ID: <3efa4105-3d3d-bb98-0358-8c46fca0fdcf at alembic.crystel.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
> So I picked up an 11/84 CPU, 3mb of memory, and a 11/84 Unibus card on 
> Ebay. Goal is to speed up my fastest 11 here
> For boot time, the diagnostics run in 13 seconds (from when it starts to 
> prompt) on the 11/73 board I have and 13 seconds on the 11/84 board. 
> This is with a camintonn 2mb half height memory board.
> Put in the first PMI module above the 11/84 CPU and tried it out. This 
> is a CA rev board which apparently only works in a Unibus pdp11 and not 
> a Q-bus one. Apparently it does work.
> So what exactly was the bug with the older PMI memory? Block mode DMA, 
> I'm using an MTI ESDI controller which can do 16 word block DMA on Q 
> Bus. Something else?

  Congrats on the PDP-11/84! I also have a PDP-11/84 that uses one of Joerg’s Hoppe’s 
UniBone devices to simultaneously emulate MSCP disks, RL02 disks (including the 
RL02 Load/Ready/WriteProtect/ and fault lights) and a DL11.

  I have a question about something you said above, that you are using a MTI 
ESDI disk controller and then you mention Qbus block mode DMA. Are you able to use a 
Qbus MTI controller in the 11/84’s Qbus section of the backplane? This is something 
I’ve often wondered about but never tried. 

 Also, you mention putting the PMI memory above the 11/84 CPU. In the 11/83 
Qbus backplane this is of course determines whether the M8637 memory is accessed via 
Q22 or PMI. In the 11/84 System Maintenance Guide Figure 2-8 shows the 
CPU card above the memory which if you were to do this in the 11/83 would mean 
that the memory will be accessed via Q22 and essentially become an 11/73.
In my 11/83 I have run both configurations to understand and measure the benefit of PMI.

  As has been mentioned the 11/84 can use any of the M8637 memory boards but the
11/83 can only use the M8637-D or -E versions. For anyone who is curious about what
Happens when a M8637-C version board is used as PMI memory in an 11/83 I can speak
From experience. This was running RSX11M+ and it boots fine but after a few minutes if
The system is active, the console starts to report that various installed tasks are corrupted
And the system will XDT a bit later. After this the disk is corrupted and you will need to restore
the system disk from backups after you get the correct PMI memory boards.

  I’m not completely sure how the write DMA operations put bad data through the disk controller
(I was using an Emulex UC07 with a SCSI2SD) into the disks but that is what happens.

Mark Matlock

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