Massbus

The Massbus is a high-performance computer input/output bus designed in the 1970s by Digital Equipment Corporation (DEC).

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 was intended to allow a single controller design to handle multiple peripheral models,[1]:7.0 and allowed the PDP-10, PDP-11, and VAX computer families to share a common set of peripherals. An additional business objective was to provide a subsystem entry price well below that of IBM storage subsystems which used large and expensive controllers unique to each storage technology and optimized for connecting large numbers of storage devices.

Logical implementation

The bus is logically implemented as two separate sections:

  • An asynchronous control bus used to access memory-mapped I/O registers in the individual storage devices,[1]:8.0 and
  • A high-speed, synchronous data bus that is used to carry the actual data transfers between the storage devices and the host bus adapter.[1]:8.0 The data bus is 18 bits wide plus parity.[1]:34.0

Massbus peripherals

Disk
RP06 disk drive, with cover for removable pack atop its sliding glass door
  • RP05/RP06 100/200 MB Memorex 677-51/677-01 pack-loaded (removable) disk drive[2][3][4]

The RP04[5] and RP06[6] disks were comparable to the 100 Mb IBM 3330 Mod I and IBM's 200 Mb Model II thereof. The 100 Mb RP05 was designated as "high performance" compared to the RP04.[7]

  • RP20 - 1.2Gb (non-removable)[8]
  • RMxx disk drives:
    • RM02/03/05/80[9]

Tape
  • TU45 9 track, capable of 800 BPI and 1600 BPI; 120,000 bytes/second[7]
  • TU78/TA78 6250 GCR[10][11][12]

Massbus CPU interfaces

References
  1. "Massbus Specification" (PDF). DEC STD 159.
  2. "RP06 disk drive". CHwiki/Computer History wiki.
  3. "RP06 disk drive". Computer History Museum.
  4. "RP05/RP06 disk drive installation manual" (PDF). Digital Equipment Corporation. EK-RP056-IN-001.
  5. "The DEC RP04 Disk Drive".
  6. "The DEC RP06 Disk Drive".
  7. EK-DEC20-SP-002 DECSYSTEM-20 Site Preparation Guide.
  8. "DECsystem-10 Technical Summary".
  9. RM MASSBUS Adapter Technical Description Manual (PDF). Digital Equipment Corporation. October 1980. p. IV.
  10. top density
  11. "TA78 Magnetic Tape Drive Service Manual" (PDF). Digital Equipment Corporation. 1984. EK-0TA78-SV-001.
  12. "DIGITAL TU78 Reel-to-Reel Tape Drive: Overview".
  13. Bell, C. Gordon; Mudge, J. Craig; McNamara, John E. (August 1979). Computer Engineering: A DEC View of Hardware Systems Design. Digital Press. p. 278. ISBN 0-932376-00-2.
  14. "RH750 Massbus Adapter Technical Description".
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