Umm, I think you're mixing up i) grant timeouts and ii) master-slave timeouts. All PDP-11 CPUs have master-slave timeout handling; after a short delay (10usec or so) with no SSYN (UNIBUS) or BRPLY (QBUS), they resume processing, and take an immediate trap. Most OS's (UNIX, for sure) use this when they are sizing memory. Grant timeouts are less well-documented. I think most CPUs deal with this (not receiving a SACK 'fairly quickly' in response to a grant); I think they basically just ignore t, and keep processing. (That is because there are legitimate causes for not having a grant ackknowledged; e.g. if a device is requesting an interrupt, and then just as the CPU sends out a grant, the device is reset, the device won't respond to the grant, since it's no longer trying to do an interrupt.) The 'SACK turnaround' I think is only used with system health verification; the system wants to make sure that the grant lines aren't broken anywhere. (That's because _if_ a grant line is broken, devices downstream of the break can no longer do interrupts, which generally _will_ hang the overall system, when interrupts don't work as usual.) To do this, the CPU sends an _un-requested_ grant out (on startup), and the SACK turnaround circuitry on the terminator turns it around and sends it back to the CPU; when the CPU sees that, it knows the grant line has no break. It probably caused more problems than it caught, which is my guess as to why no QBUS machine has/uses it. The -11/34 (not the /34A) has something unusual for grant timeouts, but I forget the details. I'll look it up. Noel