PLC Programming

A brief history of the PLC

Programmable Logic Controllers

Bedford Associates, founded by Richard Morley introduced the first Programmable Logic Controller in 1968.  This PLC was known as the Modular Digital Controller from which the MODICON company derived its name.  The History of the PLC as told to Howard Hendricks by Dick Morley provides an interesting insight into the early development of the PLC.

Schnieder Quantum PLC

Programmable Logic Controllers were developed to provide a replacement for large relay based control panels.  These systems were inflexible requiring major rewiring or replacement whenever the control sequence was to be changed.

The development of the micro processor from the mid 1970's have allowed Programmable Logic Controllers to take on more complex tasks and larger functions as the speed of the processor increased.

Ladder Logic

PLC had to be maintainable by technicians and electrical personnel.  To support this the programming language of Ladder Logic was developed.  Ladder Logic is based on the relay and contact symbols technicians were used to through wiring diagrams of electrical control panels.

Until recently there has been no formal programming standard for PLC's.  The introduction of the IEC 61131 Standard in 1998 provides a more formal approach to coding.  PLC Manufacturers have so far been slow on the uptake of the standard with partial implementation.  The SearchEng article IEC 61131-3, a Standard for PLC Software by R.W. Lewis provides an introduction to the standard.

The documentation for early PLC Programs was either non existent or very poor, just providing simple addressing and basic comments, making large programs difficult to follow.  This has been greatly improved with the development of PLC Programming Packages.

SCADA and HMI

The early programmable logic controllers interfaced with the operator in much the same way as the relay control panel, via push-buttons and switches for control and lamps for indication.

The introduction of the Personal Computer (PC) in the 1980's allowed for the development of a computer based interface to the operator, these where initially via simple Supervisory Control and Data Acquisition (SCADA) systems and more recently via Dedicated Operator Control Panels, known as Human Machine Interfaces (HMI).