What is a "PLC"?

Updated: Jul 8

PLC stands for "programmable controller" and is sometimes known as a "sequence controller" or SC. These are defined as electronic devices that control various devices via I/O sections and have built-in memory for storing programmable instructions

Warning: The content of this article is cited from the Learn PLC Programming by Mitsubishi Electric


What do PLCs do?


The devices that are operated by personnel or that detect machine states in sequence control are called "instruction signals" or "condition signals." Devices that notify personnel of machine states or that move machines are called "loads."

PLCs are responsible for carrying out "sequence control." The diagram above illustrates the control of these devices. Where the condition signals and loads in the devices are connected to the PLC.

How PLCs Work


How does sequence control work?

Devices connected to the input side of the PLC are called "input devices," while devices connected to the output side of the PLC are called "output devices." One device is connected to each terminal block.

One input device and one output device may merely be connected to a PLC in this way, and the connection for performing sequence control is performed electronically inside the PLC.

PLC internal connections are programmed using dedicated sequence language (instructions), and a combination of these instructions is called a "sequence program."

Sequence control is performed in accordance with this program, so there is no need to wire externally.

Actually consider a PLC as a group of relays and timers

The figure above shows how the input devices, output devices and the sequence program are configured. The input devices is connected to the PLC's input relays, and the output device is controlled via contacts for external outputs.

Input relays: The input relays convert the signals from an external device to signals for the PLC. In the above figure, the input device is designed to operate merely by connecting across the input terminal and the COM terminal. However, it should be noted that PLCs have a limited number of contacts, even though, an infinite number of contacts is provided on the sequence program.

The PLC has a built-in power supply for the input relays. When the contact of the input device is conducting, current flows along the dotted line to drive the input relay.

When the output relays on the sequence program are driven, the contacts for external output close. Provide a power supply for driving the external device outside the PLC.



Regardless of whether the external input device is a N.O. contact or a N.C. contact, the contact on the PLC that closes when the COM and input terminals are conducting is an N.O. contact, and the contact that opens is a N.C. contact.

Output contacts: Output contacts are otherwise known as just outputs. These are coutrolled by the sequence program, and are necessary for driving an external load. Devices having a different power supply (AC or DC) can be connected to the contacts for external output.



When the output relays on the sequence program are driven, the contacts for external output close. Provide a power supply for driving the external device outside the PLC

An I/O number is assigned to each terminal to make the I/O devices connected to the input terminals and output terminals correspond to the input relays and output relays of the sequence program. Timers and counters held internally by the PLC are also assigned to devices numbers in addition to each terminal number.

* Element numbers ⋅⋅⋅⋅⋅⋅ Element numbers comprise a symbol that shows what the devices is and a number that categorizes individual devices

The number of inputs relays, output relays, timers, counters, and other devices varies according to the model of PLC

Expressions used in sequence diagrams and sequence programs

The sequence program on a PLC replaces the input device and output device connected to each of the external terminals with a circuit for performing sequence control.


The following describes the various differences when a relay sequence program is replaced with a sequence program.

1- How N.O. contacts and N.C. contacts are expressed:

2- The power supply circuit is not expressed.

3- The sequence program is assigned devices numbers explained in the previous page.

4- Limit switch 2 in the sequence diagram is a N.C. contact but becomes a N.O. contact in a sequence program.

This is because the role of limit switch 2 in the above sequence diagram is to break the self-hold circuit of the MC and stop the MC. So, we should consider that it is in a conducting state at all times on the circuit.

Next, remember the operation of the input relay on page 2-4. In operation of the N.O. contacts and N.C. contacts in the sequence program, N.O. contacts turn ON and N.C. contacts turn OFF when the input relay is electrically conducting due to a change in state of the contact on the external wiring. Due to this fact, a N.O. contact must be used on the sequence program to obtain the same operating state.

In this way, when a PLC is used, N.O. contacts and N.C. contacts can be used for each of the contacts on the sequence program.

Wiring and Programs


What are PLC wiring and programs like?


The wiring of a PLC wiring can be divided up into I/O wiring and internal wiring

The connections between input terminals and input relay coils and between output contacts of output relays and output terminals were already established at the delivery from the factory.

Advantages of Using PLCs


Effective use of PLCs


Comparison with relay control

REFERENCE: Learn PLC Programming - Mitsubishi Electric

#PLC #programmablecontroller #sequencecontroller #instructionsignals #conditionsignals #loads #relay #timer #NO #NC #IOdevices #MC #PLCwiring

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