QBUS

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The QBUS, also called the LSI-11 Bus (from its introduction in the LSI-11), was intended as a cheaper alternative to the UNIBUS general system bus from DEC. It was widely used in later PDP-11s and smaller VAXen.

While similar to the UNIBUS, both at a high level, in that it supported both main memory and peripheral controllers through read-write cycles, DMA, and interrupts, as well as in much of the low-level detail, such as being entirely asynchronous, and using the same driver chips for its wired-OR transmission lines, it differed in a number of ways.

The biggest difference 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 also supported 4 levels of interrupt priority, that support was optional, and used a more complex signalling mechanism, with only a single shared bus grant line, to do so. Later in its life, the QBUS added support for block transfers, to increase the transfer rate of the bus.

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 an identical read/write cycle); and interrupt cycles, in which a device causes the CPU to perform an interrupt.

Also like the UNIBUS, all QBUS transactions are asynchronous, and use interlocked request/response signals for control and timing; also, most QBUS signal lines are electrically bi-directional transmission lines (even though some of these are logically uni-directional); only the grant lines (BDMG and BIAK) are physically uni-directional, and are wired in a daisy-chain fashion.

Read/write cycles come in the same basic forms as on the UNIBUS: DATI for word reads, DATO for word writes, DATIO for word read-modify-write cycles, and DATOB and DATIOB for byte write/R-M-W cycles. On the UNIBUS, however, two control lines coded the cycle type; on the QBUS, discrete control lines exist for each type of cycle (BDIN and BDOUT, along with BWTBT).

Like the UNIBUS, bus lines are normally held at a high voltage by the terminator, and driven low to assert them; unlike the UNIBUS, on which bus grants are positive-going pulses, QBUS grants are also asserted negative.

Device addressing

To reduce the number of gates needed on devices to decode their addresses, the QBUS includes a special signal, BBS7, to indicate a reference to the device registers in the 'I/O page' of the bus address space. Devices need to look at only this signal and address lines 0 through 12 to recognize their address(es). (Some QBUS CPUs do not even drive the high address lines during references to the I/O page.)

Block transfers

The QBUS later added block transfer modes, DATBI and DATBO; in block mode, the address is sent only once in a group of cycles, thereby nearly doubling the transfer rate of the bus. Only later models of main memory (slaves) and devices (DMA masters) support this mode. The bus line BBS7 is 'recycled' (at a later portion in the cycle than its normal use) to request a block transfer; the bus line BREF (used for external memory refresh of MOS memory), which had by that time fallen into desuetude, was re-purposed to allow a memory to signal that it supported block mode.

Interrupts

On the QBUS, multi-level priority interrupts share a single grant line; to do this, interrupt-requesting devices must monitor the higher-priority request lines, and refrain from intercepting a grant if there is a higher-priority request pending. Early QBUS devices did not implement this multi-level priority scheme.

Parity

The bus BDAL17 line is driven during the data read phase of a read cycle to indicate that the addressed entity (usually main memory) implements parity or some other error detection system (e.g. ECC). If BDAL16 is asserted, that indicates that an error has occurred.

Backplanes

QBUS backplanes come mainly in two physical types, dual and quad. The QBUS itself is fully carried in a dual slot, and the quads are further sub-divided into two types, the so-called Q/Q and Q/CD.

In quad Q/Q backplanes, both sides of each quad slot are fully wired for QBUS, and so a single slot can hold two separate dual-width QBUS devices. The device locations are usually arranged for grant priority in so-called 'serpentine' order, i.e. one with the devices in the following kind of order (facing the backplane from the board side):

1-2
4-3
5-6
8-7
9-10

etc.

In a quad Q/CD backplane, the CD connectors form a private bus, sometimes called the CD interconnect, used to connect together board pairs. (The CD connectors run down the right-hand side, when facing the side of the backplane where the boards plug in, with the CPU at the top.)

NOTE WELL: For reasons which seem utterly incomprehensible, many boards designed for Q/CD slots (such as PMI cards) do not avoid the QBUS pins on the CD connectors which contain 'hazardous' (to TTL circuitry) voltages. [NOTE: The exact failure mode here is still not understood; the PMI spec was examined, but no clash of pin assignments was found. The warning is accurate, though: MicroNote 28 says "MSV11-J MODULES CAN[NOT] BE PLACED IN A Q/Q BACKPLANE SLOT. IF THIS IS ATTEMPTED PERMANENT DAMAGE WILL BE DONE TO THE BOARDS".] So, plugging such a card into a Q/Q backplane will generally destroy the card.

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 limitations - 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 01000000.

It is possible to manually upgrade 18-bit backplanes to 22-bit; see Upgrading QBUS backplanes.

Pinout

QBUS pins are identified by the scheme used for the UNIBUS; there are two connectors, A and B; pins on the component side of the board are 1, those on the solder side are 2. Pins are identified by the 'DEC alphabet' (i.e. by 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" or "Dxy" refer to a quad-wide slot (e.g. as used by the original LSI-11 CPU board).

See also

Further reading

  • LSI-11 Bus Theory of Operation, MicroNote #012 - good overview
  • LSI-11/23 Four-Level Interrupts, MicroNote #59
  • Block Mode DMA, MicroNote #113
  • Removing Modules from "Live" Backplanes, MicroNote #042

External links

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