Question about UNIBUS terminators, M9300

Noel Chiappa jnc at
Thu May 26 05:39:32 CDT 2016

    > From: Bill Degnan

    > I have an M9300 bus terminator which I read is the same as a M930 with
    > the NPR logic (so you don't also need an NPR terminator in slot 3/4).

Err, the M9300 would go in the same place as a M930, i.e. the UNIBUS in/out
dual connector group, usually at the top (A/B connectors) of a slot in a
backplane, in either the first or last slot _of the entire UNIBUS_.

    > I am thinking I can replace the M930 and G7273 in the last slot of my
    > backplane with a W2-open M9300.

As a UNIBUS in/out dual-width device, the M9300 does not have separate 'grant
in' and 'grant out' pins - just _one_ pin for the grant; the pin will
function as 'in' _or_ 'out', depending on whether the card in question (of
whatever type) is placed in the first or last slot of the UNIBUS.

The dual-width G7273 goes in the middle connectors (C/D) of an SPC/MUD slot,
to jumper both bus grants (BG4-BG7) and also NPG, all of which have both an
'in' and an 'out' pin in SPC/MUD slots (look at the G7273, you'll see 5 pairs
of pins jumpered together - 1 set on one side, NPG; 4 sets on the other,
BR4-BR7). So an M9300 cannot replace a G7273: it's intended for use in an
entirely different kind of connector group.

You might want to read the UNIBUS description in one of the earlier versions
of the "PDP-11 Peripherals Handbook", which explains how the grants work:
basically, they are daisy-chained through every device, so if a UNIBUS
SPC/MUD backplane (which can hold a UNIBUS device in every slot) has a slot
which does not contain a device, you have to put something with grant jumpers
in instead.

Whether the jumper need to be BG4-BG7 _only_ (the little small grant jumper
cards), or a G7273 (which _also_ jumpers NPG) depends on whether _that
particular slot_ has had its NPG jumper (wirewrap on the backplane) pulled,
or not - most backplanes come with jumpers on NPG on all slots, and you have
to remove the jumper if a device uses DMA. (In the early days, most did not,
which is why that was the default.)

    > There are jumpers on this card. W1, W2, W. I did not find any specific
    > examples online of scenarios for the jumpers
    > ...
    > I think I get why one would remove the W2 jumper but if W1 is removed
    > (open) instead can someone give me an example scenario for when you'd
    > want to use this card "for beginning of non processor bus termination".
    > Can someone give me an example of when you'd do this?

The device the M9300 was invented for was probably the RH11-AB, which is
where one most often finds them. The RH11 is an UNIBUS device which is a
MASSBUS controller; the RH11-AB has connectors for _two_ UNIBI (so one
RH11-AB can be 'in' two PDP-11's at the same time; i.e. all the devices
connected to that controller can be accessed from either machine).

If it's only connected to a single CPU, though, what does one do with the
second UNIBUS? That's where the M9300 comes in. It simulated the NPG-granting
section of a CPU, and when jumpered to do that, it goes at the _start_ of a
UNIBUS - e.g. the second UNIBUS in the RH11-AB. (Leave all the jumpers in,
and it functions like an M930, and can go at either end).

You can find a description of its use in the RH11-AB, as well as a
description of how the M9300 works, in the "RH11-AB Option Description"
document (available online), starting on page 4-32.

I can't conceive of any use for one in most PDP-11's, though (outside an
RH11-AB, of course).


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