The Massbus was a high-performance computer input/output bus designed in the 1970s by the Digital Equipment Corporation of Maynard, Massachusetts.

The bus was used by Digital to interconnect its highest-performance computers with magnetic disk and magnetic tape storage equipment. The use of a common bus allowed the PDP-10, PDP-11, and VAX computer families to share a common set of peripherals.

Logical implementation

The bus was logically implemented as two separate sections:

• An asynchronous control bus used to access memory-mapped I/O registers in the individual storage devices, and
• A high-speed, synchronous data channel that was used to carry the actual data transfers between the storage devices and the host bus adapter.

Massbus storage devices each contained their own autonomous controller units, allowing fully-overlapped operation of multiple storage units connected to a single Massbus. The interface between the computer and the Massbus was basically a pass-through device that allowed connection of the common Massbus to the individual computer's internal bus (whether PDP-10 memory bus, Unibus, PDP-11/70 cache bus, or VAX Synchronous Backplane Interconnect). Whenever a storage controller had a data transfer ready, it arbitrated for the use of the Massbus's synchronous data channel.

Physical implementation

The bus was physically implemented in two forms:

• Shielded, controlled-impedance, flat, grey BC06R cables with Berg-styled IDC connectors at each end. Three cables operating in parallel were required to carry all of the Massbus signals.
• Single large, round, heavily shielded cables with ZIF connectors at each end.

The less-expensive flat grey cables were used within shielded equipment enclosures while the round cables were used to connect the enclosures. Transition headers allowed switching freely between the two types of cables and a single Massbus could be daisy-chained between the controller and up to eight mass storage devices. A very heavy ground conductor (wire) also usually joined the equipment.

Massbus peripherals

Disk:

• RP04 100 MB ISS pack-loaded disk drive
• RP05/RP05 100/200 MB Memorex pack-loaded disk drive
• RP07

Tape:

• TU16
• TU45
• TU77
• TU78

Massbus CPU interfaces

• RH10 -- To the PDP-10's memory bus
• RH11 -- To the PDP-11's Unibus
• RH70 -- To the PDP-11/70's Cache Bus
• RH780 -- To the VAX-11/780's Synchronous Backplane Interconnect

Computer buses | DEC hardware