history of programmable logic controller

PLC Origin

  • Developed to replace relays in the late 1960s
  • Costs dropped and became popular by 1980s

Problems faced during the 60’s

  • Now used in many industrial designs
  • Control systems implemented using relay controller








  • Lack of flexibility for process changes or expansion
  • Troubleshooting
  • “Five hours to find it and five minutes to fix it”

Historical Background

  • The Hydromantic Division of the General Motors Corporation specified the design criteria for the first programmable controller (‘standard machine controller’) in 1968

Their primary goals

  • To eliminate the high costs associated with scrapping of assembly lines during model changeovers
  • Replace unreliable and inflexible relay-controlled systems – reliable
  • Provide for future expansion – modular
  • Work in an industrial environment – rugged
  • Reduction of machine downtime related to controls problems – fast

Four Companies took the challenge to build a controller prototype

  • Allen-Bradley, by way of Michigan-based Information Instruments, Inc.
  • Digital Equipment Corporation (DEC)
  • Century Detroit
  • Bedford Associates

PLC Development – Timeline

  • 1968      Programmable concept developed
  • 1969      Hardware CPU controller, with logic instructions,   1 K of memory and 128 I/O points
  • 1974      Use of several (multi) processors within a PLC –   timers and counters; arithmetic operations; 12 K   of memory and 1024 I/O points
  • 1976      Remote input/output systems introduced
  • 1977      Microprocessors – based PLC introduced
  • 1980    Intelligent I/O modules developed   Enhanced communications facilities   Enhanced software features (e.g.   documentation) Use of personal   microcomputers as programming aids
  • 1983  Low – cost small PLC’s introduced
  • 1985   Networking of all levels of PLC,   computer and machine using SCADA   software

PLC Definition – NEMA standard ICS3-1978

  • A digitally operating electronic apparatus which uses a programming memory for the internal storage of instructions for implementing specific functions such as logic, sequencing, timing, counting and arithmetic to control through digital or analog modules, various types of machines or process

Different from Other Computers

  • Armored for severe conditions
  • Dust
  • Moisture
  • Temperature
  • Have the facility for extensive input/output (I/O) arrangements.

PLC – Size


  • it covers units with up to 128 I/O’s and memories up to 2 Kbytes.
  • these PLC’s are capable of providing simple to advance levels or machine controls.


  • have up to 2048 I/O’s and memories up to 32 Kbytes.


  • the most sophisticated units of the PLC family. They have up to 8192 I/O’s and memories up to 750 Kbytes.
  • can control individual production processes or entire plant.

Advantages of PLC

  • Cost effective for controlling complex systems.
  • Flexible and can be reapplied to control other systems quickly and easily.
  • Computational abilities allow more sophisticated control.
  • Trouble shooting aids make programming easier and reduce downtime.
  • Reliable components make these likely to operate for years before failure.

Disadvantages of PLC

  • Too much work required in connecting wires.

Difficulty with replacements.

  • Difficulty in finding errors; requiring skillful work force.

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