m (clean up, typos fixed: etc) → etc.) using AWB)
(Minor restructure to make the text on IBM mainframe PCM a dedicated section)
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 runs the same [[CPU]] software as the old system. A '''plug compatible manufacturer''' or '''PCM''' is a company that makes such products.
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.
One notable example on a grand scale is 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 [[Fujitsu]] and [[Hitachi, Ltd.|Hitachi]], and there were a wide variety of companies providing disk subsystems, including [[Memorex]] and [[Storage Technology Corporation]].▼
In general, plug-compatible systems are designed where industry or [[de facto standard]]s 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 backwards compatible with its own early products.
One recurring theme in plug-compatible systems is the ability to be [[bug compatible]] 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.).
== See also ==