Plug compatibility

Lore & Background

Before the rise of the plug-compatible 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 examples of plug-compatible IBM subsystems were tape drives and controls offered by Telex beginning 1965. Memorex in 1968 was first to enter the IBM plug-compatible disk market, followed shortly thereafter by a number of suppliers such as CDC, Itel, and Storage Technology Corporation. This was boosted by the world's largest user of computing equipment, the US General Services Administration, buying plug-compatible equipment. Eventually there were third-party plug-compatible alternatives to most first-party peripherals and first-party system main memory.

A plug-compatible machine is one that is backward 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. 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 objective of full compatibility. 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.).

The original example of plug-compatible mainframes was the Amdahl 470 mainframe computer which was plug-compatible with the IBM System 360 and 370, 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.

Reader's Guide

Plug compatibility's significance lies in its role in opening up the computer hardware market to third-party competition. By allowing peripherals and entire systems to be interchangeable with those of dominant vendors like IBM, it gave customers more choices and often lower prices. The US General Services Administration, the world's largest user of computing equipment at the time, boosted the market by buying plug-compatible equipment. The concept extended beyond peripherals to mainframe computers, with the Amdahl 470 as the original example, followed by offerings from Comparex, Fujitsu, and Hitachi. However, most of these system vendors eventually left the PCM market. Plug compatibility also required bug compatibility—the successor system had to replicate or simulate the predecessor's software or interface problems to avoid unpredictable results. This distinction between a bug and a feature became important for customers. The concept also applies to non-computer components, such as cooling fans, where physical size, voltage, power, connector, and mounting must match. Plug-compatible systems are typically 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. Plug compatible does not mean identical, but nothing prevents a company from developing follow-on products that are backward-compatible with its own early products.

Did You Know?

• Memorex in 1968 was the first to enter the IBM plug-compatible disk market.
• The original plug-compatible mainframe was the Amdahl 470, compatible with IBM System 360 and 370.
• In late 1981, there were eight PCM companies with 36 IBM-compatible models collectively.