What is PLC?

• A Programmable controller is a solid state user programmable control system with functions to control logic, sequencing, timing, arithmetic data manipulation and counting capabilities.
• It can be viewed as an industrial computer that has a central processor unit, memory, input output interface and a programming device
• It accepts data, status information from various sensing devices like limit switches, proximity switches, executes the user control program stored in the memory and gives appropriate output commands to devices such as solenoid valves, switches etc.
• Input output interface is the communication link between field devices and the controllers.
• Through these interfaces the processor can sense and measure physical quantities regarding a machine or process, such as, proximity, position, motion, level, temperature, pressure, etc.
• Based on status sensed, the CPU issues command to output devices such as valves, motors, alarms, etc.
• The programmer unit provides the man machine interface. It is used to enter the application program, which often uses a simple user-friendly logic.

​

Components of PLC

• Input Relays (contacts) - These are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.
• Counters - These are simulated counters and they can be programmed to count pulses. Typically these counters can count up, down or both up and down.
• Timers -A timer is an instruction that waits a set amount of time before doing something. As usual in industry, different types of timers are available with different manufacturers. The most common type of timer is an On-Delay Timer. This type of timer simply delays turning on its respective output. In other words, after our sensor (input) turns on we wait “x” number of seconds before activating a solenoid valve (output).
• Output Relays (coils) - These are connected to the outside world. They physically exist and send on/off signals to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen
• Data Storage - Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC.

PLC Operation

• A PLC works by continually scanning a program

Ladder Programming

• Ladder logic or ladder diagrams are the most common programming language used to program a PLC.
• The symbols used in relay ladder logic consist of a power rail to the left, a second power rail to the right, and individual circuits that connect the left power rail to the right. The logic of each circuit (or rung) is solved from left to right
• The symbols of these diagrams look like a ladder - with two side rails and circuits that resemble rungs on a ladder.

​

The logic of the rung above is such:

​

• If Input1 is ON (or true) - power (logic) completes the circuit from the left rail to the right rail - and Output1 turns ON (or true).
• If Input1 is OFF (or false) - then the circuit is not completed and logic does not flow to the right - and Output 1 is OFF (or false).

Ladder Programming

• There are many logic symbols available in Ladder Logic - including timers, counters, math, and data moves such that any logical condition or control loop can be represented in ladder logic.
• Conditions for normally open contact, normally closed contact, normally open coil, normally closed coil, timer and counter can be represented as symbols

​

Basic AND & OR Gates

Basic AND gate in Ladder

Simplified OR gate