Probs w. M3119 CXY08 MUX on VAX VMS 7.3

Jon Elson elson at
Tue Aug 18 21:27:27 CDT 2015

On 08/18/2015 01:00 PM, Johnny Billquist wrote:
> The floating address space was pretty much there from the 
> start for the Unibus, even before you had "large" systems. 
> For most controllers, only the first one has a fixed 
> address, and all others were assigned from the floating 
> space. Makes sense, as it was just too costly to 
> statically assign space in the I/O page for all possibly 
> configurations you could imagine.
The CAPABILITY to do it was always there, that is quite 
true.  But, most of the OLD PDP-11 devices had a VERY 
limited selection of addresses.  Now, some DID have 
something like 6 address bits settable in a jumper or 
wire-wrap field, but I know a number of devices had just a 
couple DIP switches that limited you to 4 or 8 possible 
addresses.  I know on some old stuff I actually cut traces 
and re-patched the address select bits to make them run at 
non-standard addresses.
My PDP-11 experience started in 1975, and was with stuff 
that was probably almost a decade old even then.
>> 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.
> Very easy to do in RSX-11M-PLUS as well. A simple one line 
> command, which can be done on the running system.
OK, I ran RSX-11M, but never M-PLUS.  I remember doing 
sysgens late at night to reconfigure the I/O addresses.
>> 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.
> Well, half correct anyway.
> The CSR is never autoassigned. It always is configured by 
> switches or jumpers on the board. Some of the more modern 
> controllers, like MSCP controllers, setup the vector 
> through software.
OK, what I USED to know is just fading away!  Yes, what you 
say makes perfect sense.


More information about the cctech mailing list