A PLC (Programmable Logic Controller) is a digitally powered electrical device. It employs digital or analogue I/O modules to control various machines or processes, carrying out logic, sequencing, and actions as necessary. The instructions are stored in its programmable memory.
Block diagram of the Programmable Logic Controller
Examples of input devices include pushbuttons, limit switches, level sensors, and proximity sensors. The input module converts the high power (220 V AC or DC) signals coming from these devices into low power signals that are compatible with the CPU's digital circuitry.
All input signals are converted by such an input module into accessible binary representations. The input data is subsequently stored in the input picture table in RAM memory.
The CPU is regarded to be the "brain" of the PLC. It manages and carries out each PLC function in accordance with the supplied programme instructions. Before executing a programme instruction, the processor first receives input data from the input image table, solves its logic instructionally, and saves the result in the output image table. Scanning is the term for this action.
The output module acts as a signal amplifier. It converts the low power signals from the output image table into high power signals that, among other output devices, can power relays, solenoids, contactors, and indicator lamps.
The information that the operating system needs to function is permanently stored in system memory (ROM). The two parts of application memory are user programme memory and variable data memory. User programme memory is where the user's programme instructions are kept.
The information file for the timer and counter, the status file for the input and output, the values of the physical variables used in the process being regulated, and all of these are kept in data memory.
Using a programming tool, the user programme is put into the PLC's memory. The two types of programming tools are as follows. A handheld device with an LCD screen is one of them.
These devices only display one line of the programme at a time. On the other hand, programming is carried out using a CRT 20 6001 SUULL system and a keyboard 6.
This will display the first four or six lines of the programme. On occasion, a computer is used to programme a PLC (PC).
As soon as the PLC obtains power and is set into RUN mode, the processor starts scanning the control programme in the programme memory.
The controller first identifies the status of each input from the input image table before executing the logic function in line with the control programmes.
The response is then recorded in binary format in the output image table. After that, the output modules get this output data. Output modules convert output data into binary representation to fit output devices.
A collection of one or more instructions for carrying out a certain action makes up a PLC programme. These programmes are stored in the PLC's memory using a programming tool.
There are three types of programming methods for programming a PLC
All actions that must be performed in this manner are described using statements. The required steps are mentioned on the left side of the statement. They are known as OPcodes. The right half of the statement contains all the information needed for the action. They are referred to as operands.
The Network 1 Operation was finished.
I : 0 LD AND
A I:0/1
= O:1/0
Network A2
LD I:0/2
O I:0/3 OR
= O:0/1
In this mode, a box represents each rung. The name of each activity is determined by its objective. The methods are also referred to as rectangle shapes. Both the left and right sides of the rectangle list inputs and outputs, respectively. The function block diagram in the image above represents the ladder diagram and the identical functions stated in the statement listing.