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A Historical Look At PLC And Modicon

By Nita McKinney

The initial introduction of PLCs took place in the late 1960s. Designing of such devices was done with the main aim of eliminating the high cost of replacing complicated relay based machine control systems. Something referred to as a Modular Digital Controller, shortened to Modicon was proposed by Bedford Associates to a major vehicle manufacturer in the US. During that time, other firms proposed computer based schemes, one of them being PDP-8 based. The worlds earliest PLC was produced commercially by Modicon084.

When requirements for production changed, the control system also underwent change. When such change occurs frequently, this can become very expensive. As relays are mechanical devices, they tend to also have a lifetime that is limited, requiring strict maintenance programs to be adhered to. With so many relays involved, troubleshooting was also becoming quite tedious.

Consider a machine control panel that has hundreds or probably thousands of relays. A size of that magnitude can be overwhelming due to the complex working of numerous individual devices. Such relays would have to be connected individually together using wires in a way that facilitates the achievement of a certain desired outcomes. During that particular time, so many problems had to be solved.

These modern controllers also had to be programmed easily by plant and maintenance engineers. The programming changes had to be easily performed and the lifetime prolonged. In addition, they had to survive the seemingly harsh industrial environmental conditions. The answers to these problems lay in using a programmed technique that most people were already familiar with as well as the replacement of mechanical parts with ones in solid state.

The technologies for PLC that were dominant in the 1970s were bit-slice based CPUs and sequencer state machinery. When considering AB PLCs and Modicon ones, the most popular ones were AMD 2901 and the 2903 versions. Convectional microprocessors did not possess the power needed to rapidly solve PLC logic in all except the smallest PLCs. The microprocessors then went on to undergo evolution, with the manufacture of better PLC based on the microprocessors. In these times they are still being manufactured with reference to 2903. However, a faster PLC is yet to be made by Modicon to better the 984A/B/X.

Abilities of communications began to appear in about 1973. The first of similar system was Modicons Modbus. It enables PLC to communicate with other PLCs as well as being used to machines at a distance from them. Whats more, varying voltages could be sent and received using them to introduce them to the digital world. It is unfortunate that lack of standardization and rapidly changing technology has reduced PLC communications to merely incompatible protocols and physical networks.

The 1980s witnessed General Motors developing manufacturing automation protocol (MAP) in an attempt to standardize communications. The time was ideal for decreasing the size of PLC and making them software compatible via symbolic programming to personal computers and not portable programmers or programming terminals.

The introduction of new protocols occurred in the 1990s, as well as salvage of the 1980s popular protocols that were made up to date. A switch to PC based control system was done by the company that was responsible for the initial introduction of Modicon 084.

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Article Citation
MLA Style Citation:
McKinney, Nita "A Historical Look At PLC And Modicon." A Historical Look At PLC And Modicon. 22 Apr. 2014. 25 Sep 2014 <>.

APA Style Citation:
McKinney, N (2014, April 22). A Historical Look At PLC And Modicon. Retrieved September 25, 2014, from

Chicago Style Citation:
McKinney, Nita "A Historical Look At PLC And Modicon"

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