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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 DEC; 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 also used in the small KS10-based model of the DECSYSTEM-20 mainframe, to furnish it with inexpensive peripherals.

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

The UNIBUS contained 16 data lines, and 18 address lines, as well as a number of control lines. The 18 address lines allowed the direct 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, while generally adequate in the days of core memory, was to prove a severe handicap in the later phases of the UNIBUS' operational life. On machines with more main memory than could be addressed directly with 18 bits, it was necessary to provide a UNIBUS map to allows DMA devices to 'see' all of main memory.

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.

In analog terms, most of the signals of the UNIBUS were broadcast on wired-OR transmission lines. Most were held at a low voltage by the lines' terminators when idle; they were then driven to ground by the transmitter to assert a signal.

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


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 perform 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 with 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 knows there was some sort of error.


The UNIBUS had provisions for parity, but the details changed over time.

In the final, and lasting version, a slave which stored data along with parity (usually main memory) could indicate to a master, during a DATI/DATIP operation, that there had been a parity error seen when retrieving the data. The master could then take whatever action it deemed appropriate; most PDP-11 CPU's would perform a trap.



The UNIBUS is usually described as containing 56 lines. In its initial BC11A UNIBUS 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

The values on the C0 and C1 lines specify the cycle type; this table shows them:

C1 C0 Cycle type
0 0 DATI
1 0 DATO


For many years, the cable used to carry the UNIBUS from one backplane to another was the BC11A UNIBUS cable, a pair of wide (3-3/4 inch) white Flexprint 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 Berg headers for 40-conductor flat cables (known as H854 cables in DEC parlance); a pair of these cards, 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. Pins are identified in the standard DEC manner. There are two connectors on the cable, 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 Pin
Initialization and Shutdown
DC LO L Slow BF2
AC LO L Slow BF1
NPR L Fast AS2
BR7 L Fast AT2
BR6 L Fast AU2
BR5 L Fast BC1
BR4 L Fast BD2
NPG H Grant AU1
BG7 H Grant AV1
BG6 H Grant BA1
BG5 H Grant BB1
BG4 H Grant BE2
A00 L Fast BH2
A01 L Fast BH1
A02 L Fast BJ2
A03 L Fast BJ1
A04 L Fast BK2
A05 L Fast BK1
A06 L Fast BL2
A07 L Fast BL1
A08 L Fast BM2
A09 L Fast BM1
A10 L Fast BN2
A11 L Fast BN1
A12 L Fast BP2
A13 L Fast BP1
A14 L Fast BR2
A15 L Fast BR1
A16 L Fast BS2
A17 L Fast BS1
D00 L Fast AC1
D01 L Fast AD2
D02 L Fast AD1
D03 L Fast AE2
D04 L Fast AE1
D05 L Fast AF2
D06 L Fast AF1
D07 L Fast AH2
D08 L Fast AH1
D09 L Fast AJ2
D10 L Fast AJ1
D11 L Fast AK2
D12 L Fast AK1
D13 L Fast AL2
D14 L Fast AL1
D15 L Fast AM2
C0 L Fast BU2
C1 L Fast BT2
PA L Fast AM1
PB L Fast AN2
Ground N/A N/A AB2
Ground N/A N/A AC2
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
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
+5 N/A N/A BA2


Early on in the life of the UNIBUS, at a time when DEC was making extensive use of small FLIP CHIPs to build computers and peripherals, DEC produced a series of FLIP CHIPs (most of them single height) to be used in connecting peripherals to the UNIBUS. They were:


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), all below.

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.").

See also

External links