PLC Overview

PLC  Overview

What do the individual work mean ?

IT may make more sense to look at the work in reverse order.

PLC Controller - This is the keyword A PLC monitors various Conditions, and based on these conditions, it determines an outcome. in other word it has the ability to control the outcome based on the status of different input such as sensors, switches and numeric values from analog signals etc.

PLC Logiic - How the PLC determines an outcome is based on the logical rules it has been taught A Simple example. it is night time and door is open then turn on the Light.

Programmable - The PLC is tough the rules to How it should use the input Conditions to create an autcome though its Programming software. being Programmable makes it versatile, so if needs or conditions change. the PLC can Be reprogrammed to meet these changes.


PLC Overview - 

Four internal areas -

1. CPU
2. Memory
3. communications Ports
4. Input/Output  

CPU - Decision making Controls other areas.

Memory - User's Control Program stored in non volatile section of memory. also I/O status and data is kept in memory.

Communication Ports - Load user's Program from PC, also exchange data with external devices, including  other PLC.

Input/Output - Handles Interface of signals to real world devices.

PLC I/O - The Input and Output Connections interface to the real world device. as seen in the schematic example. conditions. and status of the Input are monitored, decisions made by the user's control Program exexuting via the CPU and output's activated Based on the Outcome.

PLC Ladder Logic - A PLC is Programmed using application software running on a PC. The most Widely used Programming method, carried over from relay based control systems is ladder logic made up of contacts coil and other function that are arranged as rungs on a ladder. shown here is a simple example of three ladder logic rungs. externally wired Input X001 or internal control relay C1 activate Timer T1 the Timer signals Completion By its T1 contact and activates output Y001 which is wired to a real world device to a real world device The end coil tells the program scan to start over.

Program Scan - It is Helpful when Programming a PLC to understand How the Ladder Logic Program is Scanned. once the PLC is in Run mode the CPU executed in the order shown in the flow diagram.

• Status of the Input device are read and stored in data Registers
• Housekeeping of any peripheral devices.
• Scan the user's ladder logic left to right sequencing through the rungs.
• Computer the results and write updates to the outputs.
• Do diagnostics and if all is well repeat the scan.