Programmable Logic Controllers - Research Paper Example

PLC supports standard I/O connections and is compatible with standard signal levels. It is considered a central part of any process control system.

Keywords: Input sensing devices, communication module, CPU, memory, output load devices, power supply.

fig.: Internal structure of a typical PLC

1) Input/output module: The input module consists of sensing devices such as switches and various types of sensors as per requirement. Output generally consists of load devices such as motors, valves, pumps, etc.

2) Input/output communication module: The communication module is typically termed a bus. It consists of the data bus, address bus, control bus. It is used to carry signals between the CPU-input module and the CPU-output module.

3) CPU: The CPU is the central processing unit. It gathers signals, does arithmetic computations, and works the stored logic upon signals to give output. It is also used to store data and retrieve it whenever needed. CPU is selected based upon 16/32/64 bit system requirement. The logic or program in the form of a ladder diagram or likewise is stored in CPU memory. CPU memory consists of ROM (for storing fixed data), and RAM (which stores temporary variables). RAM is also known as operating memory. ROM can be erasable and/or programmable. PLC can have memory sizes ranging from 1kb to 20kb.

4) Power supply: Most PLC controllers work either at 24 VDC or 220 VAC. ( PLC manual Web site, copyright 2008, October 2, 2008, <http://www.plcmanual.com/plc-hardware>)

Processes in a PLC:

1. Read the inputs: Inputs are continuously monitored and any changes whatsoever are sensed by the input sensing devices and then recorded in CPU memory.
2. Execute the program: RAM memory inside the CPU is divided into input memory and output memory. The stored variables are fetched as and when required to run the logic stored in the ladder diagram. The output generated is stored in output memory from where it is transported to the output module through communication devices.
3. Update the outputs: The state of output depends on the output variables generated as a result of program execution. Continuous monitoring of inputs facilitates updating outputs after sensing any change in the input parameters.