PDP-11/10 repair started

tony duell ard at p850ug1.demon.co.uk
Mon Oct 5 06:50:12 CDT 2015

> > 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.  

> > 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.


More information about the cctalk mailing list