Probs w. M3119 CXY08 MUX on VAX VMS 7.3

Johnny Billquist bqt at
Wed Aug 19 07:23:26 CDT 2015

On 2015-08-19 04:27, Jon Elson wrote:
> 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.

1975 was the year the 11/70 was introduced. The Unibus only came into 
existence five years before this, in 1970. :-)
So, the gear you worked on was definitely no more than five years old.

Anyway, I can actually pinpoint the floating address space convention in 
time pretty good.
The PDP-11 Peripherials Handbook from 1972 have each device listed with 
specific addresses assigned for as many options as any customers were 
expected to ever have. There is no mention of floating address space for 
CSRs, but there is floating vector space.
The PDP-11 Peripherial Handbook frmo 1973 do define the floating address 
space, so this is the timeframe when it was realized that address space 
needed to be better utilized (was too small for reserved addresses for 
everything that was popping up).

So devices up until 1972/1973 might have a more limited number of 
jumpers, to just select from the reserved range for that specific 
device. Anything from 1973 onwards, I would expect to have full 
flexibility on address selection on the Unibus.

The Qbus is different in that DEC went back to a more limited 
flexibility there.

>>> 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.

Yep. And SYSGENs took time. What a chore... :-)


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