M9047 and bus grant (was Re: Industrial VAX 630 booting saga)

--- David Woyciesjes <dwoyciesjes(a)comcast.net> wrote: Bus Grant. Unlike ISA and PCI-type machines, the Unibus and Qbus peripherals generate requests by breaking a chain of signals that go from slot to slot. If you look closely (or not so closely ;-) at your M9047, you'll see that there are no active or passive components on it - just a few backplane pins jumpered together. A peripheral card that wants to make an interrupt (or DMA) request will normally have those pins electrically "jumpered" and will "disconnect" them to initiate the request. This has the intentional side-effect that cards further away from the CPU can each make their own requests on the same line, but will never be noticed until the closer card has reconnected the signal. Unibus and Qbus machines do not run out of IRQs like ISA machines. They run out of physical slots to stuff boards into, or, in rare cases, the bus gets electrically overloaded with too many cards for the signals to propagate without too much noise (look up DC and AC Bus Loading in the literature). For a real-world example, we had our primary VAX-11/750 (.8 VUPS, *roughly* equivalent to the same integer horsepower as a Mac SE, an Amiga 1000 or a Sega Genesis) with MASSBUS disk (SI9900), MASSBUS tape (TU78) and a single Unibus with 3 Emulex 16-port serial boards, a 56Kbps DDCMP link (DMR-11?), a UDA-50 with one or two RA81s, an RX50 (RUX50), an RL02 (RL11), a TU80 (TS11), 6 COMBOARDs (68000-based DMA protocol engines with a sync serial port and line printer port), and a DMF-32 (8 serial, a sync serial (DDCMP) and a line printer). If I've remembered everthing that was on there, that makes at least 20 interrupt vectors in use. Plenty to spare. Very nice cards to have. I wish when we were making grant cards, that we had put pins in both Qbus and Unibus locations on the fingers so we could move jumpers to select what kind of bus to grant. We supplied a GC747 (our number) grant card with every Unibus COMBOARD, but never did with any Qbus COMBOARD. :-( -ethan