PDP-11/10 repair started

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. 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. -tony