Introduction to PLCs

Industrial Management Consulting
Industrial Management Consulting

What does ‘PLC’ mean?
A PLC (Programmable Logic Controllers)  is an industrial computer used to monitor inputs, and depending upon their state make decisions based on its program or logic, to control (turn on/off) its outputs to automate a machine or a process.

“A digitally operating electronic apparatus which uses a programmable memory for the internal storage of instructions by implementing specific functions such as logic sequencing, timing, counting, and arithmetic to control, through digital or analog input/output modules, various types of machines or processes”.

Traditional PLC Applications
In automated system, PLC controller is usually the central part of a process control system.
*To run more complex processes it is possible to connect more PLC controllers to a central computer.

Disadvantages of PLC control
– Too much work required in connecting wires.
– Difficulty with changes or replacements.
– Difficulty in finding errors; requiring skillful work force.
– When a problem occurs, hold-up time is indefinite, usually long.

Advantages of PLC control
* Rugged and designed to withstand vibrations, temperature, humidity, and noise.
* Have interfacing for inputs and outputs already inside the controller.
* Easily programmed and have an easily understood programming language.

Major Types of Industrial Control Systems
Industrial control system or ICS comprise of different types of control systems that are currently in operation in various industries. These control systems include PLC, SCADA and DCS and various others:

They are based on the Boolean logic operations whereas some models use timers and some have continuous control. These devices are computer based and are used to control various process and equipments within a facility. PLCs control the components in the DCS and SCADA systems but they are primary components in smaller control configurations.

Distributed Control Systems consists of decentralized elements and all the processes are controlled by these elements. Human interaction is minimized so the labor costs and injuries can be reduced.

Embedded Control

In this control system, small components are attached to the industrial computer system with the help of a network and control is exercised.


Supervisory Control And Data Acquisition refers to a centralized system and this system is composed of various subsystems like Remote Telemetry Units, Human Machine Interface, Programmable Logic Controller or PLC and Communications.

PLC Instructions

Functions and Instructions
Relay-type (Basic) instructions: I, O, OSR, SET, RES, T, C
Data Handling Instructions:
 Data move Instructions: MOV, COP, FLL, TOD, FRD, DEG, RAD (degrees to radian).
 Comparison instructions: EQU (equal), NEQ (not equal), GEQ (greater than or equal), GRT (greater than).
 Mathematical instructions.
 Continuous Control Instructions ( PID instructions ).
Program flow control instructions: MCR (master control reset), JMP, LBL, JSR, SBR, RET, SUS, REF
Specific instructions:
BSL, BSR (bit shift left/right), SQO (sequencer output), SQC (sequencer compare), SQL (sequencer load).
High speed counter instructions: HSC, HSL, RES, HSE
Communication instructions: MSQ, SVC
ASCII instructions: ABL, ACB, ACI, ACL, CAN

Internal Relays
Auxiliary relays, markers, flags, coils, bit storage.
Used to hold data, and behave like relays, being able to be switched on or off and switch other devices on or off. They do not exist as real-world switching devices but are merely bits in the storage memory.

Internal Relays Use
In programs with multiple input conditions or arrangements. For latching a circuit and for resetting a latch circuit. Giving special built-in functions with PLCs.

Retentive relays (battery-backed relays)
Such relays retain their state of activation, even when the power supply is off. They can be used in circuits to ensure a safe shutdown of plant in the event of a power failure and so enable it to restart in an appropriate manner.

Latch Instructions (Set and Reset)
 The set instruction causes the relay to self-hold,, i.e. latch. It then remains in that condition until the reset instruction is received.
 The latch instruction is often called a SET or OTL (output latch).
 The unlatch instruction is often called a RES (reset), OTU (output unlatch) or RST (reset).

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