QBUS
The QBUS, previously known as the "LSI-11 bus", was intended as a cheaper alternative to the UNIBUS system by Digital Equipment Corporation. It was widely used in PDP-11s and VAXen.
While similar in overall terms to the UNIBUS, in that it supported both memory and devices through read-write cycles, DMA, and interrupts, and was entirely asynchronous, it differed in a number of ways.
One was that it used multiplexed address and data lines (to reduce the pin count), as opposed to the separate address and data lines of the UNIBUS. Another was that although it supported 4 levels of interrupt priotity, that support was optional, and used a more complex signalling mechanism, with only a single grant line, to do so.
Signalling
Like the UNIBUS, there are three basic kinds of cycles on the QBUS: data read/write cycles (in which a 'master' reads or writes data to/from a 'slave', which is usually, but not always, memory); DMA cycles (in which a device gains control of the bus so that it can do a read/write cycle); and interrupt cycles, in which a device causes the CPU to perform an interrupt.
All QBUS transactions are asynchronous, and use interlocked request/response signals for control and timing.
Variable address size
The QBUS was available in 16-, 18-, and 22-address-bit configurations (data width remained 16 bits in all three versions). The three versions are often referred to as Q16, Q18 and Q22.
CPUs, devices and backplanes all are one of the three alternatives; for instance, the earliest CPU, the LSI-11, is a Q16 device. Mixing cards and backplanes of differing address widths may, or may not, work; or may work, but with limitataions - and may sometimes initially appear to work, but, when examined carefully, not work.
Important note: The 16-bit and 18/22-bit backplanes are electrically incompatible and mixing the two may damage cards on the bus.
One example of the kind of limitation that may occur happens when using a Q18 DMA device in a Q22 system. The device will function correctly, but can only do transfers to the lower 256KB of memory; software that uses this device will have to work around that limitation.
An example of something that looks like it might work, but does not in fact work, is mixing Q18 and Q22 memory cards in a Q22 system, with more than 256Kbytes of memory in total. The problem is that the Q18 memory card will respond at multiple places in the 22-bit address space; e.g. if a Q18 card is configured at address 0, it will also respond at 1000000.
It is possible to upgrade 18-bit backplanes to 22-bit; see Upgrading QBUS backplanes.
Pinout
QBUS pins are identified in the standard UNIBUS manner; there are two connectors, A and B; pins on the component side are 1, those on the solder side are 2. Pins are identified by the 'DEC alphabet', A-V, with G, I, O and Q dropped.
By signal
| Signal | Pin | Signal | Pin |
|---|---|---|---|
| Ground | AJ1 | BDAL00 | AU2 |
| Ground | AM1 | BDAL01 | AV2 |
| Ground | AT1 | BDAL02 | BE2 |
| Ground | AC2 | BDAL03 | BF2 |
| Ground | BJ1 | BDAL04 | BH2 |
| Ground | BM1 | BDAL05 | BJ2 |
| Ground | BT1 | BDAL06 | BK2 |
| Ground | BC2 | BDAL07 | BL2 |
| +5 | AA2 | BDAL08 | BM2 |
| +5 | BV1 | BDAL09 | BN2 |
| +5 | BA2 | BDAL10 | BP2 |
| +5B | AE1 * | BDAL11 | BR2 |
| +5B | AS1 * | BDAL12 | BS2 |
| +5B | AV1 | BDAL13 | BT2 |
| +12 | AD2 | BDAL14 | BU2 |
| +12 | BD2 | BDAL15 | BV2 |
| +12B | AS1 * | BDAL16 | AC1 |
| +12B | BS1 | BDAL17 | AD1 |
| -12 | AB2 | BDAL18 | BC1 |
| -12 | BB2 | BDAL19 | BD1 |
| BDAL20 | BE1 | ||
| ASpare2 | BU1 | BDAL21 | BF1 |
| MSpareA | AK1 | ||
| MSpareB | AL1 | BBS7 | AP2 |
| MSpareB | BK1 | BDIN | AH2 |
| MSpareB | BL1 | BDOUT | AE2 |
| PSpare1 | AU1 | BREF | AR1 |
| PSpare2 | BU1 | BRPLY | AF2 |
| PSpare4 | BS1 | BSACK | BN1 |
| SSpare1 | AE1 * | BSYNC | AJ2 |
| SSpare2 | AF1 | BWTBT | AK2 |
| SSpare3 | AH1 * | BDMGI | AR2 |
| SSpare8 | BH1 | BDMGO | AS2 |
| BDMR | AN1 | ||
| BDCOK | BA1 | BIAKI | AM2 |
| BEVNT | BR1 | BIAKO | AN2 |
| BHALT | AP1 | BIRQ4 | AL2 |
| BINIT | AT2 | BIRQ5 | AA1 |
| BPOK | BB1 | BIRQ6 | AB1 |
| SRUN | AH1 * | BIRQ7 | BP1 |
Signals marked with a "*" show cases where two signals use the same pin (not at the same time, obviously).
By pin
| Signal | Pin | Note | Signal | Pin | Note |
|---|---|---|---|---|---|
| BIRQ5 | AA1 | old BSpare1 | +5 | AA2 | |
| BIRQ6 | AB1 | old BSpare2 | -12/-5 | AB2 | |
| BDAL16 | AC1 | old BSpare3 | Ground | AC2 | |
| BDAL17 | AD1 | old BSpare4 | +12 | AD2 | |
| SSpare1 | AE1 | alt +5B | BDOUT | AE2 | |
| SSpare2 | AF1 | alt SRUN/SMENBL on CF1 | BRPLY | AF2 | |
| SSpare3 | AH1 | alt SRUN on CH1 | BDIN | AH2 | |
| Ground | AJ1 | BSYNC | AJ2 | ||
| MSpareA | AK1 | BWTBT | AK2 | ||
| MSpareB | AL1 | BIRQ4 | AL2 | was BIRQ | |
| Ground | AM1 | BIAKI | AM2 | ||
| BDMR | AN1 | BIAKO | AN2 | ||
| BHALT | AP1 | BBS7 | AP2 | ||
| BREF | AR1 | BDMGI | AR2 | ||
| +5B/+12B | AS1 | old PSpare3 | BDMGO | AS2 | |
| Ground | AT1 | BINIT | AT2 | ||
| PSpare1 | AU1 | BDAL00 | AU2 | ||
| +5B | AV1 | BDAL01 | AV2 | ||
| BDCOK | BA1 | +5 | BA2 | ||
| BPOK | BB1 | -12/-5 | BB2 | ||
| BDAL18 | BC1 | old SSpare4 | Ground | BC2 | |
| BDAL19 | BD1 | old SSpare5 | +12 | BD2 | |
| BDAL20 | BE1 | old SSpare6 | BDAL02 | BE2 | |
| BDAL21 | BF1 | old SSpare7 | BDAL03 | BF2 | |
| SSpare8 | BH1 | BDAL04 | BH2 | ||
| Ground | BJ1 | BDAL05 | BJ2 | ||
| MSpareB | BK1 | BDAL06 | BK2 | ||
| MSpareB | BL1 | BDAL07 | BL2 | ||
| Ground | BM1 | BDAL08 | BM2 | ||
| BSACK | BN1 | BDAL09 | BN2 | ||
| BIRQ7 | BP1 | old PSpare6 | BDAL10 | BP2 | |
| BEVNT | BR1 | BDAL11 | BR2 | ||
| PSpare4/+12B | BS1 | BDAL12 | BS2 | ||
| Ground | BT1 | BDAL13 | BT2 | ||
| PSpare2 | BU1 | BDAL14 | BU2 | ||
| +5 | BV1 | BDAL15 | BV2 |
References to pin "Cxy" refer to a quad-wide slot (e.g. as used by the original LSI-11 CPU board).
