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

Jerry Weiss jsw at ieee.org
Sun Apr 19 16:20:27 CDT 2020

On 4/19/20 2:29 PM, Chris Zach via cctalk wrote:
> 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
> ....
EK-MSV1J-UG 001 documents the the versions that don't do Q-Bus Cycles.  
uNOTE # 028 indicates that MSV-11 JB/JC (M8637-B/C) doesn't do block 
mode. I suspect the MTI card  falls back to single cycle DMA. Also RT11 
simply may not challenge the configuration sufficiently to demonstrate 
the limitations of the -CA memory variant.

I have not seen any document that describes this as a "bug", so it must 
be a "feature"  /s


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