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.
The bus had multiplexed address and data lines, and was available in 16-, 18-, and 22-address-bit configurations (data width remained 16 bits in all three versions).
Important note: The 16-bit and 18/22-bit backplanes are electrically incompatible and mixing the two may damage cards on the bus.
It is possible to upgrade 18-bit backplanes to 22-bit; see Upgrading QBUS backplanes.
Contents
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.
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 |
|---|---|
| Ground | AJ1 |
Signals marked with a "*" show cases where two signals use the same pin (not at the same time, obviously).
By pin
BIRQ5 AA1 (old BSpare1) BIRQ6 AB1 (old BSpare2) BDAL16 AC1 (old BSpare3) BDAL17 AD1 (old BSpare4) SSpare1 AE1 (alt +5B) SSpare2 AF1 (alt SRUN/SMENBL on CF1) SSpare3 AH1 (alt SRUN on CH1) Ground AJ1 MSpareA AK1 MSpareB AL1 Ground AM1 BDMR AN1 BHALT AP1 BREF AR1 +5B/+12B AS1 (old PSpare3) Ground AT1 PSpare1 AU1 +5B AV1 +5 AA2 -12/-5 AB2 Ground AC2 +12 AD2 BDOUT AE2 BRPLY AF2 BDIN AH2 BSYNC AJ2 BWTBT AK2 BIRQ4 AL2 (was BIRQ) BIAKI AM2 BIAKO AN2 BBS7 AP2 BDMGI AR2 BDMGO AS2 BINIT AT2 BDAL00 AU2 BDAL01 AV2 BDCOK BA1 BPOK BB1 BDAL18 BC1 (old SSpare4) BDAL19 BD1 (old SSpare5) BDAL20 BE1 (old SSpare6) BDAL21 BF1 (old SSpare7) SSpare8 BH1 Ground BJ1 MSpareB BK1 MSpareB BL1 Ground BM1 BSACK BN1 BIRQ7 BP1 (old PSpare6) BEVNT BR1 PSpare4/+12B BS1 Ground BT1 PSpare2 BU1 +5 BV1 +5 BA2 -12/-5 BB2 Ground BC2 +12 BD2 BDAL02 BE2 BDAL03 BF2 BDAL04 BH2 BDAL05 BJ2 BDAL06 BK2 BDAL07 BL2 BDAL08 BM2 BDAL09 BN2 BDAL10 BP2 BDAL11 BR2 BDAL12 BS2 BDAL13 BT2 BDAL14 BU2 BDAL15 BV2
