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Programmable Logic Controllers - PLC

Parts of a PLC

Input Section - devices wired to the "input" terminals
1) Discrete devices such as contacts or electric switches
2) Analog devices like potentiometers or thermistors
3) Pulse generating devices such as shaft encoders

CPU - Central Processing Unit - (brain of the PLC)
- Fed from a clean AC power source
- Only IC's are connected to this supply
- Directly connected to the I/O devices
- ROM (read only memory)
- RAM (random access memory)
- EPROM (erasable programmable read only memory)
- EEPROM (electrically erasable programmable read only memory - erased and reprogrammed a limited amount)

Output Devices - provides power to loads by switching an external AC or DC source.
1) Discrete devices such as contactor, pilot lights, and solenoid valve coils.
2) Analog devices such as servo-valve coils, and variable drive input references.
3) Pulse driven devices such as stepper motors.

Programming Devices
- Handheld Programmers
- Dedicated (Terminals)
- Laptop


Advantages of a PLC
- Maintainability (Quickly replace components)
- Ease of installation (No hard wiring)
- Accuracy (Tolerance +- .01)
- Expandaability (Reuse for another purpose)
- Flexibility (On the Fly)
- Reliability (Semi- Conductors)

Numbering Systems

Binary 2 (0-1)
Octal 8 (0-7)
Decimal 10 (0-9)
Hexidecimal 16 (0-15)

There are many different ways to convert the numbers, and we have supplied another one below. Go get a calculator that has the bin, hex, dec, and oct functions on it - and save yourself a big headache. We have enough calculations in this industry without pouring insignificant ones on our plate.


Memory in a PLC
- executive memory (factory)
- user memory
- 1 bit
- 8 bits = 1 byte
- 16 bits, 2 bytes = 1 word


Normally Open

This is a normally open contact. 1 = examine on.

Normally Closed

This is a normally closed contact. 0 = examine off.


Represents relays that are energized when power flows to them. That same output status bit may control normally open and normally closed contacts elsewhere in the program.


Represents various instructions or functions like timers, counters and math operations.

And Operation

Input 1 and 2 must be "true" for output to operate. Inputs are in series.

Or Operation

These inputs are in parallel. Either input can cause output 1 to be energized.

Wiring to a PLC Device

Wiring a device to a PLC is very simple. Unlike hard wiring when your instructions were planned out with wires and devices - it is simply a matter of wiring to an output, or an input.

Wire inputs such as contacts and switches to the inputs on your screws.

Wire outputs such as coils and lights to your output screws on your PLC. This is the main advantage of the PLC system. If there is a modification to be made to the system then it just requires reprogramming, rather than rewiring the entire project.