Plug-compatible

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. One notable example is the Amdahl 470 mainframe computer which was plug-compatible with the IBM System 360 and 370. An IBM customer could literally tear out 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.

Related Topics:
Backwards compatible - Computer system - CPU - Amdahl - 470 - Mainframe computer - IBM - System 360 - 370

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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. It should be noted that 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.

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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).

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