m (→See also: added Drop-in replacement)
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]]—it runs the same software as the old system. A '''plug compatible manufacturer''' or '''PCM''' is a company that makes such products.
|display-authors=etal}}</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
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.