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

Mark Matlock mark at matlockfamily.com
Thu Apr 23 00:17:56 CDT 2020

> Date: Tue, 21 Apr 2020 16:17:07 -0400
> From: Chris Zach <cz at alembic.crystel.com>
> To: cctalk at classiccmp.org
> Subject: Re: pdp11/84 PMI memory: What is the problem with Q bus?
> Message-ID: <27647f0e-19d4-b484-d288-e9f3bb715363 at alembic.crystel.com>
> Content-Type: text/plain; charset=utf-8; format=flowed
> Thanks Mark! Actually this was just the boards from the 11/84 (no idea 
> what happened to the chassis, drat) so it's an 11/84 CPU (18mhz, FPP 
> chip installed), 2 PMI boards (one old 2mb, one new 1mb) a console board 
> of some sort and the Unibus map.
> I popped this into my BA23 to speed things up a bit in place of my quad 
> height 11/73 CPU with 2mb memory. So far it seems to work, and with the 
> CA memory in the PMI slot managed to boot RSX11M 4.2 and compile up EMPIRE.

   I understand now. I also have a PDP-11/83 in a BA123 box and the CPU and 
PMI memory (M8637-E) can move from the Qbus box to the 11/84 box and work fine
In either. I spent a good bit of time tinkering with the 11/84 power supply which did not
Work until I put a M7556 minimum load module in it. Basically the +/- 12 Volts needs a
Load or you get a hung bus error. Also, with the Unibus you have to pay attention to
The NPR/NPG grant jumpers. 

   For disks on the QBus systems I used the UC07 and SCSI2SD. The UC07 can be
 can be configured for block mode transfers and I think it is doing it. I guess I’d need
 to hook up a logic analyzer to know for sure.

    The 11/84 uses a UniBone and when it is emulating a MSCP disk, it can do 150 
I/Os a second (using IOX on RSX11M+) The 11/83 using the UC07 and a SCSI2SD
Does about 60 I/Os per second. I think the UC07 could go faster but is limited by the
SDcard interface. I need to put a fast real SCSI disk on it sometime and see what it can do.

> My guess is the 11/84's Unibus talks directly to the PMI bus and 
> orchestrates the data transfers, but there is something wrong when the 
> PMI memory is accessed on the true Q bus. That would not happen on an 
> 11/84 (CPU and Map use PMI only) but when you have a Q bus DMA device it 
> probably manifests at random. It's possible the MTI card is throttling 
> the DMA to single mode instead of hog mode, wonder if I want to screw up 
> my disk to verify this….

What is the model # for your MTI disk controller?

> Drat. On the positive side it's chock full of 256k chips, which I could 
> pull off and put on the EA board to bring it up to 2mb memory. I have 
> air heat tools and a pre-heater so getting the chips off should be 
> pretty basic. Getting them on the new board though could be a pain since 
> all the holes are soldered over….

   Moving the RAM chips sounds a bit tedious but would certainly be possible. 

> No way to reprogram or fix it I assume?

    Are you asking about making the M8637-B or -C into the -D or -E? If so I’ve
 never seen any reference to doing that. I was very fortunate that the person
 Who sold me the M8637-C was willing to trade it for a -E.


More information about the cctech mailing list