PDP-11/10 repair started

Johnny Billquist bqt at update.uu.se
Mon Oct 5 09:53:24 CDT 2015

On 2015-10-05 13:50, tony duell wrote:
>>> The specific issue of an open grant chain locking the Unibus is a quirk
>>> of the M9302 terminator (which asserts SACK under such conditions). This
>>> is unlikely to be a problem on an 11/10.
>> Why? The 11/10 also have a Unibus, and also needs the terminator as well
>> as the SACK and other signals in order.
> The M9302 includes logic to assert SACK if a grant (any BG or NPG) gets to it,
> meaning no device on the bus as intercepted the grant. This causes problems
> with an open grant chain in that the CPU sees the SACK, tries to deassert
> the grant (which it hasn't asserted in the first place) and the bus is locked with
> SACK asserted and no grants. The M930 terminator does not include said logic.
> As a result an open grant chain will cause problems on a machine where the
> terminator is an M9302. On a machine where it's an M930, an open grant chain
> is of course a bad idea (interrupts and DMA will not work properly) but it will not
> lock the bus and prevent the front panel from working.
> The 11/10 generally uses an M930 terminator.

Right. However, the SACK signal exists, even on the 11/10.
And if that signal is floating, or non-working, you can still have 
problems. So, termination is still important.
The additional feature of of the M9302 is just to catch if BG signals 
are sent out when noone was actually requesting an interrupt.

So your comment about open grant chain locking is relevant, but do not 
negate the claim that you need to make sure you have terminators in 
place, and that signals are carried through the whole backplane.

Note that my original comment was not about BG/BR continuity as such, 
but termination and signals running through the whole bus. BG/BR is just 
one part of that.

>>> But you do need some kind of termination/pullups on the Unibus. At least
>>> fit the CPU-end terminator (M930 in the right slot near the CPU boards) if
>>> you haven't done so already.
>> You most likely want to terminate the other end as well.
> It may not be a perfect electrical match, but if all you have is the CPU backplane,
> or even a BA11-K full of backplanes, I am certain a terminator at the CPU end only
> will get the machine doing something, even running programs correctly. Totally
> ignoring the front panel is not caused by a missing far-end terminator on such a
> small machine.

The 11/10 do not have a built-in terminator at the CPU end. You are 
supposed to have two M930 on the backplane.


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