> On Apr 9, 2018, at 9:28 PM, Mark Pizzolato <m...@infocomm.com> wrote:
> 
> On Monday, April 9, 2018 at 4:56 PM, Paul Koning wrote:
>>> ...
>> 
>> I don't remember controllers with more than 4 units per device, but MSCP unit
>> numbers are arbitrary 8 bit values.  So a limit of 4 units is proper, but a
>> limitation on those unit numbers being 0..3 is not accurate.  It matters in 
>> some
>> OS.  For example, VMS has type/controller/unit addressing (DUA3 etc.) but
>> RSTS does not.  So on RSTS, if you have more than one MSCP controller, they
>> must have disjoint unit numbers.  Since the unit numbers are 8 bits, that's 
>> easy
>> to do.  But I don't see how I can do that in SIMH.
> 
> I see 2 approaches to adding support for different unit numbers:
> 1) A unit number is settable uniquely for each of the 4 drives that can be 
> attached to any of these controllers.  This would map to a conceptual Drive 
> Unit 
> Plug.  Did these exist on DEC MSCP drives?
> 2) A 'base' unit number can be set on a controller and each drive on that 
> controller 
> would then have unit numbers equal to the base plus their existing internal 
> unit 
> numbers.  This may have been how third party MSCP controllers worked.
> 
> Which of these best reflects how the real hardware worked?

Option 1.  Drives like the RA80 had a unit number cap covering the select lamp. 
 It looked similar to the unit number light on older drives like the RL01, but 
it was built differently.  If you pulled off the cap, you'd see 8 plastic 
prongs extending from the back rim, which you could cut to make the binary 
encoding of the desired unit number.  You'd then mark the plug with the number 
(with stick-on numbers perhaps).  I think some pre-made number plugs for the 
small numbers were available, but you could make your own and that's how you'd 
get a unit number 142 or the like.

        paul


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