Plug compatible

Plug compatible refers to "hardware that is designed to perform exactly like another vendor's product."[1]

The term PCM can refer to

  • Plug-Compatible Manufacturer
  • Plug-Compatible Machine.

The term PCM was originally applied to manufacturers who made replacements for IBM peripherals.[2] Later this term was used to refer to IBM-compatible computers.[3]

PCM and peripherals

Before the rise of the PCM peripheral industry, computing systems were either

  • configured with peripherals designed and built by the CPU vendor or
  • designed to use rebadged devices selected by such vendors

Many peripherals were originally designed to be used with a specific central processing unit (CPU)[4]

The term PCM was originally applied to manufacturers who made replacements for IBM peripherals.

The first example of plug compatible IBM subsystems were tape drives and controls offered by Telex beginning 1965.[5] Memorex in 1968 was first to enter the IBM plug-compatible disk followed shortly thereafter by a number of suppliers such as CDC,[6] Itel, and Storage Technology Corporation. This was boosted by the world's largest user of computing equipment[7] in both directions.[8]

Ultimately plug-compatible products were offered for most peripherals and system main memory.[9]

PCM and computer systems

A plug-compatible machine is one that has been designed to be backward compatible with a prior machine. In particular, a new computer system that is plug-compatible has not only the same connectors and protocol interfaces to peripherals, but also binary-code compatibility—it runs the same software as the old system. A plug compatible manufacturer or PCM is a company that makes such products.

One recurring theme in plug-compatible systems is the ability to be bug compatible[10] as well. That is, if the forerunner system had software or interface problems, then the successor must have (or simulate) the same problems. Otherwise, the new system may generate unpredictable results, defeating the full compatibility objective. Thus, it is important for customers to understand the difference between a "bug" and a "feature", where the latter is defined as an intentional modification to the previous system (e.g. higher speed, lighter weight, smaller package, better operator controls, etc.).

PCM and IBM mainframes

The original example of PCM mainframes was the Amdahl 470 mainframe computer which was plug-compatible with the IBM System 360 and 370, costing millions of dollars to develop. An IBM customer could literally remove the 360 or 370 on Friday, install the Amdahl 470, attach the same connectors from the peripherals to the channel interfaces, and have the new mainframe up and running the same software on Sunday night. Unfortunately, system status indicators for operators of the new system were very different, which introduced a learning curve for operators and service technicians.

Similar systems were available from Comparex, Fujitsu,[11] and Hitachi. Not all were large systems.[12][13]

Most of these system vendors eventually left the PCM market.[14][15][16][17]

Non-computer usage of the term

The term may also be used to define replacement criteria for other components[18] available from multiple sources. For example, a plug-compatible cooling fan may need to have not only the same physical size and shape, but also similar capability, run from the same voltage, use similar power, attach with a standard electrical connector, and have similar mounting arrangements. Some non-conforming units may be re-packaged or modified to meet plug-compatible requirements, as where an adapter plate is provided for mounting, or a different tool and instructions are supplied for installation, and these modifications would be reflected in the bill of materials for such components. Similar issues arise for computer system interfaces when competitors wish to offer an easy upgrade path.

In general, plug-compatible systems are designed where industry or de facto standards have rigorously defined the environment, and there is a large installed population of machines that can benefit from third-party enhancements. Plug compatible does not mean identical replacement. However, nothing prevents a company from developing follow-on products that are backward-compatible with its own early products.

See also


  1. "plug-compatible". PC Magazine Encyclopedia. Ziff Davis.
  2. "Making the move into IBM-compatible peripheral products was a natural adjunct to products being developed for OEMs.""Moving into IBM-compatible peripheral products". Computerworld. August 18, 1980. p. 7.
  3. "plug-compatible mainframe (PCM).""direct-mail company to replace IBM with PCM". Computerworld. March 8, 1982. p. 69.
  4. Herbert Hovenkamp (2017). Principles of Antitrust. ISBN 978-1640200821.
  5. Pugh; et al. (1991). IBM's 360 and Early 370 Systems. p. 233.
  6. "Expected to produce $1 billion in revenues during fiscal 1980, CDC's peripherals business, advancing at 33% annually, is the fastest growing revenue producer within the company's diverse product line.""CDC PCM Peripherals - $1 Billion market". Computerworld. August 18, 1980. p. 7.
  7. "GSA has initiated a Government-wide replace existing leased peripheral devices with lower cost plug-to-plug compatible equipment offered by independent suppliers. This program was aimed at permitting competitive offers of peripherals by independent suppliers."The Creative Partnership: Government and the Professional Services. 1973.
  8. "... to allow the use of IBM plug-compatible peripherals on the CDC 6400, 6600 and 7600 systems installed at the LBL Computer Center. This has given the ability to replace unreliable CDC tape drives and controllers and overpriced CDC disk drives and controllers with their IBM plug-compatible counterparts.""For Reference" (PDF).
  9. "Historical Narrative Statement of Richard B. Mancke, Franklin M. Fisher and James W. McKie, Exhibit 14971, US vs. IBM, Part 2" (PDF). July 1980. pp. 750–796.
  10. "bug-for-bug compatible". The Jargon File. Eric S. Raymond. Same as bug-compatible, with the additional implication that much tedious effort went into ensuring that each (known) bug was replicated.
  11. "LEAD: Beating I.B.M. to the punch by one day, Fujitsu Ltd. announced a series of computers today that ...""Fujitsu Announces Mainframe". September 5, 1990.
  12. "A 3200 system can include up to 16M bytes, with virtual memory freeing programmers from artificial memory constraints. It can handle all major programming languages, such as Cobol, Fortran, PL/I, APL, Basic, and Assembler. The NCSS 3200 series will range in price from $200,00 to $600,000.""NCSS 3200" (PDF).
  13. Trilogy Systems Corporation was started by Gene Amdahl together with his son Carl Amdahl and Clifford Madden."ACSYS - new Amdahl startup". Computerworld. June 15, 1981. p. 11.
  14. Greenwald, John (1983-07-11). "The Colossus That Works". TIME. Archived from the original on 2008-05-14. Retrieved 2019-05-18.
  15. "Hitachi has been in the mainframe business for 50 years and currently its AP series of systems are sold to major organisations across Japan. Hitachi Data Systems used to sell Hitachi-made IBM plug-compatible mainframes outside Japan but stopped doing so in 2000.""Hitachi exits mainframe hardware business". The Register. May 24, 2017.
  16. "A notable PCM failure was Storage Technology (StorageTek), which was for many years one of the more successful of the plug-compatible peripheral suppliers. StorageTek's attempt to make its own processor and become another Amdahl or HDS almost drove it out of business. It took years to recover ...""ACS Heritage Project: Chapter 30".
  17. "Amdahl ...pulling out of the plug-compatible market in 2000 following IBM's launch of 64-bit systems.""Amdahl pulling out of the plug-compatible market in 2000". Computerworld.
  18. "A universal four-contact plug and jack assembly ...""Patent US20040175993 - Universal audio jack and plug". September 9, 2004.
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