Compatible plug refers to “hardware that is designed to perform exactly like another vendor’s product.” 
The term PCM can refer to
- P lug- C ompatible M anufacturer
- P lug- C ompatible M achine.
The term PCM was originally applied to manufacturers who made replacements for IBM peripherals.  IBM-compatible computers. 
PCM and Peripherals
Before the rise of the PCM peripheral industry, computing systems were
- configured with peripherals and built by the CPU vendor gold
- designed to use rebadged devices
Many peripherals were originally designed with a specific central processing unit (CPU) 
The term PCM was originally applied to manufacturers who made replacements for IBM peripherals.
The first example of IBM compatible plug subsystems were tape drives and controls offered by Telex beginning 1965.  Memorex in 1968 was first to enter the IBM plug-compatible disk, followed by CDC ,  Itel, and Storage Technology Corporation . This was boosted by the world’s largest user of computing equipment  in both directions ! 
Ultimately plug-compatible products have been offered for most devices and systems. 
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 compatible manufacturer or PCM plug is a company that makes such products.
One recurring theme in plug-compatible systems is the ability to be compatible bug  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 to understand the difference between a “bug” and a “feature”, where the latter is defined as an intentional modification to the previous system (eg 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, the system is very different from the new system, which has been introduced to the learning curve for operators and service technicians.
Similar systems were available from Comparex , Fujitsu  and Hitachi . Not all were large systems.  
Most of these system vendors evenly left the PCM market.   
Non-computer usage of the term
The term can also be used for other components  available from multiple sources. For example, a plug-compatible cooling fan may have the same physical size and shape, but also similar capabilities, and the same voltage, similar electrical power, and similar similar arrangements. Some non-conforming units may be re-packaged or modified to meet the requirements of this document. materials for such components. Similar issues arise for computer system interfaces.
In general, plug-compatible systems are designed where industry or de facto standards are rigorously defined, and there is a large population of machines that can benefit from third-party enhancements. Compatible plug does not mean identical replacement. However, nothing prevents a company from developing follow-on products that are backwards compatible with its own early products.
- Clone (computing)
- Computer compatibility
- Drop-in replacement
- Pin compatibility
- Proprietary hardware
- Vendor lock-in
- Hercules_ (emulator)
- Jump up^ “plug-compatible” . Ziff Davis .
- Jump up^ “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.
- Jump up^ “mainframe plug-compatible (PCM).” “direct-mail company to replace IBM with PCM” . Computerworld . March 8, 1982. p. 69.
- Jump up^ Herbert Hovenkamp (2017). Principles of Antitrust . ISBN 1640200827 .
- Jump up^ Pugh; et al. (1991). IBM’s 360 and Early 370 Systems . p. 233.
- Jump up^ “Expected to produce $ 1 trillion in revenue 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.
- Jump up^ “GSA has initiated a Government-wide program.to replace existing devices with lower cost plug-in-plug compatible equipment offered by independent suppliers. The Creative Partnership: Government and the Professional Services . 1973.
- Jump up^ “… to allow the use of IBM plug-compatible peripherals on the CDC 6400, 6600 and 7600 systems installed at the LBL Computer Center. drives and controllers with their IBM plug-compatible counterparts. ” “For Reference”(PDF) .
- Jump up^ “Historical Narrative Statement of Richard B. Mancke, Franklin M. Fisher and James W. McKie,” Exhibit 14971, US vs. IBM, Section 50, p. 750-796, July 1980
- Jump up^ “bug-for-bug compatible.” Same as bug-compatible, with the additional implication that it’s a bit of a bug ( “bug-for-bug compatible”) .
- Jump up^ “LEAD: Beating IBM to the punch by one day, Fujitsu Ltd. announced a series of computers today that …” “Fujitsu Announces Mainframe” . NYTimes.com . September 5, 1990.
- Jump up^ “A 3200 system can include up to 16M bytes, such as Cobol, Fortran, PL / I, PLA, Basic, and Assembler.NCSS3200 series will range from $ 200.00 to $ 600,000. ” “NCSS 3200” (PDF) .
- Jump up^ Trilogy Systems Corporation was started byGene Amdahltogether with his son Carl Amdahl and Clifford Madden. “ACSYS – new Amdahl startup” . Computerworld . June 15, 1981. p. 11.
- Jump up^ “Hitachi has been in the mainframe business for 50 years and is currently operating in Japan. Hitachi Data Systems used to sell Hitachi-made IBM plug-compatible mainframe outside Japan but stopped doing so in 2000. ” “Hitachi exits mainframe hardware business” . The Register . May 24, 2017.
- Jump up^ “A notable PCM failure was Storage Technology (StorageTek), which was for many years one of the most successful of the plug-compatible peripheral vendors.StorageTek’s attempt to make its own processor and another Amdahl or HDS almost drove it out It took years to recover … “ ” ACS Heritage Project: Chapter 30 ” .
- Jump up^ “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 .
- Jump up^ “A Universal Four-way Plug and Jack Assembly …” “US20040175993 Patent – Universal audio jack and plug” . September 9, 2004.