m (clean up, References using AWB)
A '''plug-compatible''' machine is one that has been designed to be [[backwards 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]]
The term may also be used to define replacement criteria for other components 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.
The term PCM was originally applied to manufacturers who made replacements for [[IBM]] peripherals and later IBM mainframes.
The first example of a plug compatible IBM subsystems were tape drives and controls offered by [[
| author=Pugh et al
| title=IBM's 360 and Early 370 Systems
}}</ref> [[Memorex]] in 1968 was first to enter the IBM plug-compatible disk followed shortly thereafter by a number of suppliers such as CDC, Itel, etc. Ultimately plug-compatible products were offered for most peripherals and system main memory.<ref>"HISTORICAL NARRATIVE STATEMENTOF RICHARD B. MANCKE, FRANKLIN M. FISHER AND JAMESW. McKIE," Exhibit 14971, US vs. IBM, Section 50, p. 750-796, July 1980</ref>
The original example of PCM mainframes was the [[Amdahl Corporation|Amdahl]] [[Amdahl 470|470]] [[mainframe computer]] which was plug-compatible with the [[IBM]] [[System 360]] and [[System 370|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]] and [[Hitachi, Ltd.|Hitachi]], and there were a wide variety of companies providing disk subsystems, including [[Memorex]] and [[Storage Technology Corporation]].
[[Category:Classes of computers]]