PLC learning series 9: PLC selection criteria.

PLC selection is generally based on the controller’s necessary inputs, outputs and features. The first choice is the controller sort; rack, mini, micro, or based on software.

This decision will depend upon the basic criteria listed below.

System requirements:

  • To determine any solution, the starting point must be to know what to achieve.
  • The program design begins by breaking down the task into a number of simple comprehensible elements, each of which can be described easily.

Application requirements:

Requirements for input and output devices. Determine which input and output devices the system needs after determining the operation of the system.

  • List the necessary feature and define a particular device type.
  • In relation to discrete (On / Off) logic, the need for special operations.
  • List the necessary sophisticated features in addition to a straight forward, discrete logic.

Electrical Requirements:

The electrical requirements for inputs, outputs and system energy ; consider three components when determining a system’s electrical requirements:

  • Incoming power (power for the control system);
  • Input device voltage; and
  • Output voltage and current

Speed of Operation:

How fast the control system must operate (speed of operation).

When determining speed of operation, consider these points:

  • How fast does the process occur or machine operate?
  • Are there “time critical” operations or events that must be detected?
  • In what time frame must the fastest action occur (input device detection to output device activation)?
  • Does the control system need to count pulses from an encoder or flow-meter and respond quickly?


If the request needs non-process information sharing, i.e. communication. Communication involves sharing data or status of applications with other electronic devices, such as a computer or monitor at the station of an operator.

Communication can occur locally via a twisted pair wire, or remotely via telephone or radio modem.

Operator Interface:

If the system requires control or interaction from the operator. Many apps involve operator interfaces to transmit machine or process status information or enable an operator to input data.

Traditional operator interfaces include pushbuttons, LED numeric display and pilot lights. In descriptive text, electronic operator interface systems show machine status emails, display part count and track alarms. They can also be used to input information.

Physical Environment:

The physical environment where the system of command will be situated. Consider the location of the control system setting. Housing the control scheme in a suitable IP-rated enclosure in hostile settings.

PLC Learning series:

PLC learning Series I: What is PLC? Functions of PLC
PLC learning series 2: PLC programming languages
PLC learning series 3: PLC Architecture and wiring of PLC
PLC learning series 5: 8 Rules for ladder diagram programming
PLC learning series 6: PLC process Scan basics
PLC learning series 8: Instruction List programming
PLC learning series 9: PLC selection criteria.
PLC learning Series 10: PLC timers
PLC learning series 11 : How to interface PLC with SCADA?
PLC learning series 12: Troubleshooting PLC
PLC learning Series 13: Counters in PLC


Instrumentation Engineer

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