Plug compatible refers to " hardware that is designed to perform exactly like another vendor's product." The term PCM was originally applied to manufacturers who made replacements for IBM

peripherals A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...

. Later this term was used to refer to IBM-compatible computers.

PCM and peripherals

Before the rise of the PCM peripheral industry, computing systems were either configured with peripherals designed and built by the CPU vendor, or designed to use vendor-selected rebadged devices. The first example of plug compatible IBM subsystems were tape drives and controls offered by

Telex The telex network is a station-to-station switched network of teleprinters similar to a telephone network, using telegraph-grade connecting circuits for two-way text-based messages. Telex was a major method of sending written messages electroni ...

beginning 1965.

Memorex Memorex Corp. began as a computer tape producer and expanded to become both a consumer media supplier and a major IBM plug compatible peripheral supplier. It was broken up and ceased to exist after 1996 other than as a consumer electronics bran ...

in 1968 was first to enter the IBM plug-compatible disk followed shortly thereafter by a number of suppliers such as

CDC The Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It is a United States federal agency, under the Department of Health and Human Services, and is headquartered in Atlanta, Georgi ...

, Itel, and Storage Technology Corporation. This was boosted by the world's largest user of computing equipment in both directions. Ultimately plug-compatible products were offered for most peripherals and system main memory.

PCM and computer systems

A plug-compatible machine is one that has been designed to be

backward compatible Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especially in ...

with a prior machine. In particular, a new

computer system A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...

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. 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.).

PCM and IBM mainframes

The original example of PCM mainframes was the

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470 mainframe computer which was plug-compatible with the IBM System 360 and

370 __NOTOC__ Year 370 ( CCCLXX) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Augustus and Valens (or, less frequently, year 1123 ''Ab ...

, costing millions of dollars to develop. Similar systems were available from Comparex, Fujitsu, and Hitachi. Not all were large systems. Most of these system vendors eventually left the PCM market. In late 1981, there were eight PCM companies, and collectively they had 36 IBM-compatible models.

Non-computer usage of the term

The term may also be used to define replacement criteria for other components"A universal four-contact plug and jack assembly ..." 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 A bill of materials or product structure (sometimes bill of material, BOM or associated list) is a list of the raw materials, sub-assemblies, intermediate assemblies, sub-components, parts, and the quantities of each needed to manufacture an end ...

for such components. Similar issues arise for computer system interfaces when competitors wish to offer an easy upgrade path. In general, plug-compatible systems are designed where industry or

de facto standard A ''de facto'' standard is a custom or convention that has achieved a dominant position by public acceptance or market forces (for example, by early entrance to the market). is a Latin phrase (literally " in fact"), here meaning "in practice b ...

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 backward-compatible with its own early products.

References

{{Reflist Classes of computers Computer hardware Interoperability

PCM and peripherals

PCM and computer systems

PCM and IBM mainframes

Non-computer usage of the term

See also

References