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The UNIBUS (or Unibus - the capitalization style changed over time) was the earliest of two bus technologies used with PDP-11s manufactured by Digital Equipment Corporation; it was first seen in the PDP-11/20, in 1970. Later, early VAX systems from that company used the UNIBUS as an I/O bus.

It was the only bus in most PDP-11 systems, and thus supported several capabilities: the ability of the processor to read and write main memory, and device registers; and the ability for devices to do DMA transfers to memory, and interrupt the CPU.

It could exist on a cable (the original mode of operation), and later, within a backplane (in 'Small Peripheral Controller (SPC)' or 'Modified UNIBUS Device (MUD)' slots). Up to 20 nodes (devices) could be connected to a single UNIBUS segment; additional segments could be connected via a bus repeater.

The UNIBUS contained 16 data lines, and 18 address lines, as well as a number of control lines. The 18 address lines allowed the addressing of a maximum of 256 KBytes. Typically, the top 8 KBytes of address space was reserved for the registers of the memory mapped I/O devices used in the PDP-11 architecture; this block is often referred to as the I/O page. The limit of 18 address lines was to prove a severe handicap in the later phases of the UNIBUS' operational life.

The bus was completely asynchronous, allowing a mixture of fast and slow devices. It also allowed arbitration (selection of the next bus master) while the current bus master was still performing data transfers, overlapping the two functions.

The design deliberately minimized the amount of redundant logic required in the system. For example, a system always contained more slave devices than master devices, so most of the complex logic required to implement asynchronous data transfers was forced into the relatively few master devices. For interrupts, only the interrupt-fielding processor needed to contain the complicated timing logic.

The end result was that most I/O controllers could be implemented with very simple logic; most of the critical logic was later implemented as a custom MSI IC.


Two control lines (C0 and C1) allowed the selection of four different data transfer cycle types in normal master/slave cycles:

  • DATI (Data In, a read)
  • DATIP (Data In/Pause, the first portion of a Read-Modify-Write operation. A DATO or DATOB operation completes this.)
  • DATO (Data Out, a word write)
  • DATOB (Data Out/Byte, a byte write)

During these operations, several signals (MSYN - Master Sync; SSYN - Slave Sync; and BBSY - Bus Busy) are used to synchronize between the master and slave, to indicate when the data is ready to be read (during a DATI/DATIP), or has been written (during a DATO/DATOB).

During an interrupt cycle, a fifth style of transfer was used to convey an interrupt vector from the interrupting device to the interrupt-fielding processor.

DMA and Interrupts

Both DMA (in which a device must request control of the bus, so that it can peform a normal master/slave cycle, only with itself as the master), and interrupts, use the same mechanism for the device to communicate with the CPU to request something, and for the CPU to communicate with the device that it has granted the device's request.

There are bus request lines (NPR for DMA, and BR4-BR7 for interrupts), and bus grant lines (NPG for DMA, and BG4-7 for interrupts). The request lines are normal 'wired-OR' broadcast bus lines, but the grant lines are special; they are wired in series - one device's 'grant out' line is connected to the next device's 'grant in' line, starting the the CPU's 'grant out' line.

When a device receives a grant, it uses another bus line (SACK) to indicate that it received its grant; if the CPU sends a grant, and does not see a SACK in response, it know there was some sort of error.


Details of the operation of the UNIBUS may be found in the DEC publications "pdp11 peripherals handbook" (various editions from 1972 to 1977), and the "pdp11 bus handbook" (1979).

NOTE: There is a serious editing error in the latter volume, in the description of UNIBUS arbitration. On page 38, immediately after step 13 of the NPR Arbitration Sequence ("13. .... SACK must be negated before BBSY may be negated."), it says "A bus master may issue an interrupt command to the interrupt fielding processor."

Despite its location in the text, this does not apply to the "NPR arbitration sequence" being discussed above. There is an editing error - this text is (or should be) separate from the "NPR Arbitration Sequence" section just before it; it belongs with "BR Interrupt Arbitration Sequence" - that header (on pg. 39) was put in the wrong place.

The 1975 "peripherals handbook" has very similar text, but it does have a section header after the NPR details (line 13 is identical), and before the start of the (very similar) BR text ("A bus master that has gained control ... through a BRn/BGn arbitration transaction may issue an interrupt command to the processor.").


The UNIBUS is usually described as containing 56 lines. In its initial BC11A cable instantiation, the UNIBUS was composed of 72 wires (2 standard DEC board edge connectors, with 36 lines per connector); when not counting the power and ground lines, this was reduced to the canonical 56.

Among the UNIBUS signals are:

  • BR4-BR7 - Bus (Interrupt) Requests at priorities 4 (lowest) through 7 (highest)
  • BG4-BG7 - Bus (Interrupt) Grants at priorities 4 (lowest) through 7 (highest)
  • NPR - Non Processor (DMA) Request
  • NPG - Non Processor (DMA) Grant
  • MSYNC - Master Sync
  • SSYNC - Slave Sync
  • BBSY - Bus Busy
  • SACK - Selection Acknowledge
  • PA, PB - Parity control
  • C0, C1 - Cycle Control


For many years, the cable used to carry the UNIBUS from one backplane to another was the BC11A cable, a pair of wide (3-3/4 inch) white flexible printed circuit flat cables, separated by a thin foam layer, with small printed circuit boards with edge connector fingers on each end of the cable. The latter plugged into 'UNIBUS In' and 'UNIBUS out' slots in backplanes. The flat cables actually contained 64 connectors each; every other trace was grounded, to prevent cross-talk between the signal lines.

DEC later developed a series of cards (the M9014, an extended height dual card, and the M9042 short dual card) which plug into the same slots, and contain three 2x20 headers for 40-conductor flat cables (known as H854 cables in DEC parlance); a pair of these, and three cables, perform the same role as a BC11A cable. Both of these cards use the same header pinout, so they may be used interchangeably.


The following table gives the pinout for the flat cable form and SPC slot form. Pins are identified in the standard DEC 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.

Signal Assertion Termination Cable Pin SPC Pin
Initialization and Shutdown
DC LO L Slow BF2 CN1
AC LO L Slow BF1 CV1
NPR L Fast AS2 FJ1
BR7 L Fast AT2 DD2
BR6 L Fast AU2 DE2
BR5 L Fast BC1 DF2
BR4 L Fast BD2 DH2
NPG H Grant AU1 In-CA1; Out-CB1
BG7 H Grant AV1 In-DK2; Out-DL2
BG6 H Grant BA1 In-DM2; Out-DN2
BG5 H Grant BB1 In-DP2; Out-DR2
BG4 H Grant BE2 In-DS2; Out-DT2
A00 L Fast BH2 EH2
A01 L Fast BH1 EH1
A02 L Fast BJ2 EF1
A03 L Fast BJ1 EV2
A04 L Fast BK2 EU2
A05 L Fast BK1 EV1
A06 L Fast BL2 EU1
A07 L Fast BL1 EP2
A08 L Fast BM2 EN2
A09 L Fast BM1 ER1
A10 L Fast BN2 EP1
A11 L Fast BN1 EL1
A12 L Fast BP2 EC1
A13 L Fast BP1 EK2
A14 L Fast BR2 EK1
A15 L Fast BR1 ED2
A16 L Fast BS2 EE2
A17 L Fast BS1 ED1
D00 L Fast AC1 CS2
D01 L Fast AD2 CR2
D02 L Fast AD1 CU2, FE2%
D03 L Fast AE2 CT2, FL1%
D04 L Fast AE1 CN2, FN2%
D05 L Fast AF2 CP2, FF1%
D06 L Fast AF1 CV2, FF2%
D07 L Fast AH2 CM2, FH1%
D08 L Fast AH1 CL2, FK1%
D09 L Fast AJ2 CK2
D10 L Fast AJ1 CJ2
D11 L Fast AK2 CH1
D12 L Fast AK1 CH2
D13 L Fast AL2 CF2
D14 L Fast AL1 CE2
D15 L Fast AM2 CD2
C0 L Fast BU2 EJ2
C1 L Fast BT2 EF2
PA L Fast AM1 CC1
PB L Fast AN2 CS1
SSYN L Fast BU1 EJ1, FC1%
Ground N/A N/A AB2
Ground N/A N/A AC2 xC2
Ground N/A N/A AN1
Ground N/A N/A AP1
Ground N/A N/A AR1
Ground N/A N/A AS1
Ground N/A N/A AT1 xT1
Ground N/A N/A AV2
Ground N/A N/A BB2
Ground N/A N/A BC2
Ground N/A N/A BD1
Ground N/A N/A BE1
Ground N/A N/A BT1
Ground N/A N/A BV2
+5 N/A N/A AA2 xA2
+5 N/A N/A BA2
-15 N/A N/A N/A xB2 (except 1A, 1B, 4A, 4B)

Entries of the form 'xYN' mean that that is available on all 4 connectors (A, B, C and D) in each slot.

% For forward compatibility, use the first pin rather than the second

See also