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.
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.
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.)
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 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.
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.
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.
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):
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.
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).
Signals marked with a "*" show cases where two signals use the same pin (not at the same time, obviously).
|SSpare2||AF1||alt SRUN/SMENBL on CF1||BRPLY||AF2|
|SSpare3||AH1||alt SRUN on CH1||BDIN||AH2|
References to pin "Cxy" or "Dxy" refer to a quad-wide slot (e.g. as used by the original LSI-11 CPU board).
- DMA Request and Grant
- CD interconnect
- Private Memory Interconnect
- LSI-11 Bus interface chips
- Bus Arbitration on the Unibus and QBUS
- UNIBUS and QBUS termination
- QBUS backplanes
- QBUS processors
- QBUS memories
- QBUS devices
- pdp11 bus handbook (EB-17525-20/79-070-14-55)
- Microsystems Handbook - documents block mode in the very detailed and comprehensive Appendix A, "Q-bus" (pp. A-17–A-20; pp. 177-180 of the PDF)
- Supermicrosystems Handbook - the QBUS is documented in the very detailed and comprehensive Appendix A, "Q-bus" (pp. 204-255 of the PDF)
- MICRO/PDP-ll Handbook - Appendix E: LSI-11 BUS Technical Specifications (pp. 255-301)