Question

(TCO 2) Describe how PLCs detect whether a switch is open or closed.

Answer 1

The purpose of a PROGRAM LOGIC CONTROLLER ( PLC ) is to directly replace electromechanical relays and switches as logic elements, including a digital computer with a stored program, and able to provide the interconnection of many relays to perform certain logical tasks.

A PLC has many “input” terminals, by which it indicates “high” and “low” logical states from sensors and switches.

It also has many output terminals, through which it outputs “high” and “low” signals to the output connected devices.

Signal connection and programming standards vary for different models of PLC, but the programming concept is same that is used in it.

The actual logic of the control system is established inside the PLC by means of a computer program. This program decides which output gets energized under which input conditions.

For the easy understanding purpose the program that used in PLC is usually written in the form of a ladder logic diagram, with switch and relay symbols, there are no actual switch contacts or relay coils operating inside the PLC to create the logical relationships between input and output. These are imaginary contacts and coils.

Consider an example as shown in below image.

The image shows how a PLC operates to control switching ON or OFF of light. Actually the switching action is controlled by the program that is uploaded in the PLC.

When the pushbutton switch is not pressed, no power is sent to input of the PLC. Following the program, which shows a normally-open contact in series with a , no “power” will be sent to the light. Thus, the PLC’s output remains de-energized, and the light will not glow.

If the pushbutton switch is pressed the power will be sent to the PLC’s input and the PLC starts providing internal logical connection between input and output with the help of logical program and the light at the output starts glowing.

It must be understood that the physical connection that shows the input is applied to the output device when the push button is pressed and the electrical connections are not exist physically, They exist as commands in a computer program only.

Once a program has been loaded to the PLC from the personal computer, the personal computer may be unplugged from the PLC, and the PLC will continue to follow the programmed commands and with the help of this logical program it always detects and collects data about the devices present in the system whether they are working or not and make a control on it for proper operation.