UNIBUS and QBUS termination

UNIBUS and QBUS termination are very similar, since in analog terms the two buses are very similar (e.g. their use of the same transceiver chips, such as DS8641's).

Their terminators do a number of things:

• they provide analog termination the otherwise un-terminated transmission lines of the bus (i.e. a resistance at the end of a transmission line that prevents reflections of signals passing down it, 'bouncing' off its end);

• provide pullups (so those wired-OR transmission lines normally float at roughly 3 volts, unless actively driven low by one of the boards plugged into the bus);

• in some cases, they do 'SACK turnaround' (an 'unused' bus grant from the CPU, theoretically for a device's interrupt, is 'turned around' by the terminator, and retuned to the CPU as the SACK signal); this prevents the CPU from freezing up.

The first is not absolutely necessary; this can be seen in small QBUS systems which can get away without it in small configurations. Whether it's needed on particular system is hard to predict, theoretically; the easiest thing is to just try it and see. Note: it may 'work' without it, but not be as reliable as with it.

The second is necessary, but can be provided anywhere on the bus; most UNIBUS/QBUS CPUs do so, with the notable exception of the KD11-D of the PDP-11/04, and the KD11-E/-EA of the PDP-11/34-34A. (In the PDP-11/40, there's an oddball exception: the M981 UNIBUS jumper does it - but that's 'sort of' part of the CPU.)

(Note that the first and second are usually both done by a single set of components.)

The third is required by some UNIBUS CPUs (ISTR that the PDP-11/04 won't run without it), but most (including all QBUS CPUs, such as the KDF11 CPUs) don't need it; they will timeout unused grants. (SACK turnaround could potentially be used for a start-up 'safety check' where an un-requested - and thus 'un-grabbed' by any device - bus grant from the CPU on start-up is 'turned around' by the terminator; this verifies that the bus grant lines are un-broken between the CPU and the terminator - e.g. by someone forgetting to plug in a grant continuity card.)