Probs w. M3119 CXY08 MUX on VAX VMS 7.3

Jon Elson elson at
Tue Aug 18 12:05:17 CDT 2015

On 08/18/2015 10:48 AM, Holm Tiffe wrote:
> Understand my quandary: I have changed not a single bit on 
> the hardware while cleaning and repairing it and besides 
> that DHU11-DHV11 Switch the board was in the published 
> factory setting!
DEC went to this floating address/vector scheme back in the 
PDP-11 days, when they started building large systems like 
the 11/70 and using PDP-11's as the console for DECsystem 
20's.  They'd put a huge number of I/O boards in such 
systems.  A choice of 3 pre-assigned CSR addresses and 
interrupt vectors just would not cut it in such systems.
They had a utility you could run (at least on VAX) where you 
input the number of instances of each device, and it would 
print out a table of all the CSR and vectors to set them to.

So, the situation is that changing the number of one kind of 
board could alter the addresses of many OTHER boards!
Most likely, some board was added or removed from the system 
before you got it, and it caused the vector to now be wrong.

In some cases, you had to force a device to be at a 
non-standard address, possibly because a 3rd party device 
could not be configured at the address the DEC enumeration 
scheme wanted to put it at.  This was pretty easy to do in 
later VMS systems.

Unfortunately, this type of misconfiguration is fairly hard 
to detect with software.  Later devices (MSCP)  had an 
autoconfiguration scheme where the OS would assign the CSR 
and vector at boot time.


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