Massbus
Encyclopedia
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. An additional business objective was to provide a subsystem entry price well below that of IBM storage subsystems which used large and expensive controllers that were unique to each storage technology and optimized for connecting large numbers of storage devices.
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.
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.
Note the RM02 uses a different drive pulley to slow the disk transfers to a level compatible with a Unibus based PDP11. The RM03 spun at full speed, and was supported on the pdp11/70 and larger systems. Curiously enough the DECSYSTEM 2020 could support an RM03 with its Unibus-based interface due to a highly buffered Unibus-to-memory interface.
Tape:
Analog I/O
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
input/output
Input/output
In computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
bus designed in the 1970s by the Digital Equipment Corporation
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
of Maynard
Maynard, Massachusetts
Maynard is a town in Middlesex County, Massachusetts, United States. As of the 2010 census, the town population was 10,106.- History :Maynard, located on the Assabet River, was incorporated as an independent municipality in 1871. Prior to that it was known as 'Assabet Village' but was legally...
, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
.
The bus was used by Digital to interconnect its highest-performance computers with magnetic disk and magnetic tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
storage equipment. The use of a common bus allowed the PDP-10
PDP-10
The PDP-10 was a mainframe computer family manufactured by Digital Equipment Corporation from the late 1960s on; the name stands for "Programmed Data Processor model 10". The first model was delivered in 1966...
, PDP-11
PDP-11
The PDP-11 was a series of 16-bit minicomputers sold by Digital Equipment Corporation from 1970 into the 1990s, one of a succession of products in the PDP series. The PDP-11 replaced the PDP-8 in many real-time applications, although both product lines lived in parallel for more than 10 years...
, 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 that were unique to each storage technology and optimized for connecting large numbers of storage devices.
Logical implementation
The bus was logically implemented as two separate sections:- An asynchronous control bus used to access memory-mapped I/OMemory-mapped I/OMemory-mapped I/O and port I/O are two complementary methods of performing input/output between the CPU and peripheral devices in a computer...
registers in the individual storage devices, and - A high-speed, synchronous data bus that was used to carry the actual data transfers between the storage devices and the host bus adapter. The data bus was 18 bits wide plus parity. 16 bits were used for PDP-11 and VAX systems, 18 bits for DEC-10s.
- Multiple devices of different types could transmit data over the shared data path. However, this was never supported by DEC operating systems.
- Static dual port was also provided to permit failover or manual switching of storage devices to another CPU.
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
Unibus
The Unibus was the earliest of several computer bus technologies used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts.-History:...
, PDP-11/70 cache bus, or VAX Synchronous Backplane Interconnect
Synchronous Backplane Interconnect
The Synchronous Backplane Interconnect was the internal processor-memory bus used by early VAX computers manufactured by the Digital Equipment Corporation of Maynard, Massachusetts....
). 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 BergBerg connectorA Berg connector is a brand of electrical connector used in computer hardware. Berg connectors are manufactured by Berg Electronics Corporation of St. Louis, Missouri, a division of Framatome Connectors International....
-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.
- All signals were full differential, and the data bus cycle was 1 microsecond.
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: (capacities noted are raw, not formatted)- RM02 80MB, CDC 9762 pack-loaded disk drives, CDC unique disks spin at 2400 RPM
- RM03 80MB, CDC 9762 pack-loaded disk drives, CDC unique disks spin at 3600 RPM
Note the RM02 uses a different drive pulley to slow the disk transfers to a level compatible with a Unibus based PDP11. The RM03 spun at full speed, and was supported on the pdp11/70 and larger systems. Curiously enough the DECSYSTEM 2020 could support an RM03 with its Unibus-based interface due to a highly buffered Unibus-to-memory interface.
- RM05 300MB CDC 9766 pack-loaded disk drives, CDC unique
- RP04 100 MB ISS/Sperry Univac pack-loaded disk drive, IBM 3330 type
- RP05/RP06 100/200 MB MemorexMemorexMemorex began as a computer tape producer and expanded to become a major IBM plug compatible peripheral supplier. It is now a consumer electronics brand of Imation specializing in disk recordable media for CD and DVD drives, flash memory, computer accessories and other electronics.Established in...
677-51/677-01 pack-loaded disk drive IBM 3330 type - RP07 500MB ISS/Sperry Univac non-removable disks IBM 3350 type
- RS03 1MB (formatted) very fast fixed-head disk drives, DEC proprietary plated disk
- RS04 2MB (formatted) very fast fixed-head disk drives, DEC proprietary plated disk
Tape:
- TU16 1600bpi
- TU45 1600bpi
- TU77 1600bpi
- TU78 6250 GCR
Analog I/O
- DR01 UnibusUnibusThe Unibus was the earliest of several computer bus technologies used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts.-History:...
size analog I/O board that connected to the RH20 Massbus controller on a PDP-10PDP-10The PDP-10 was a mainframe computer family manufactured by Digital Equipment Corporation from the late 1960s on; the name stands for "Programmed Data Processor model 10". The first model was delivered in 1966...
Massbus CPU interfaces
- RH10—To the PDP-10's memory bus
- RH11—To the PDP-11's UnibusUnibusThe Unibus was the earliest of several computer bus technologies used with PDP-11 and early VAX systems manufactured by the Digital Equipment Corporation of Maynard, Massachusetts.-History:...
- RH11C—to the KS10's (DECSYSTEM 2020) Unibus
- RH20—To the PDP-10 KL10 CPU (DECsystem 109x, DECSYSTEM-2040/2050/206x) channel bus
- RH70—To the PDP-11/70's Cache Bus
- RH750—To the VAX-11/75x bus
- RH780—To the VAX-11/78x, VAX 86xx SBI (Synchronous Backplane Interconnect) bus