What is a PLC?


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i am doing a project about PLC..but i don't know what the hack is a PLC. please help me...

some questions

1) what is a PLC?
2) what are the programming languages used?
3) what is a soft PLC?
4) what are the advantages of a PLC compared to a PC

thanks first.



PLC : A controller which is doing logical operations and that logic can be programable is called Porgrammable Logic Controller - PLC
>PLC Programmable Logic Controller.
Most started off using ladder logic. The software is in a for which resembles a wiring diagram, so it is easy for electrically minded persons to follow.
Used in industry for automatically controlling moving machinery.
They have been aroung for a long time and have been more reliable than PCs.

anantha krishnan

Also, there are three way of programming. LADDER , STATEMENTLINE , SEQUENTIAL FLOW CHART.
lADDER is more familiar one.
> i am doing a project about PLC..but i don't know what the hack is a PLC. pleasee help me...

> 1) what is a PLC?

A Programmable Logic Controller is a device that, when programmed from an outside source, sends required control signals to, and receives required signals from, external devices (sensors, motors, etc.) to facilitate the control of a plant.

> 2) what are the programming languages used?

Ladder logic, STL and function block diagrams are the main methods of programming a PLC. I love ladder logic, but most people seem to prefer STL nowadays.

> 3) what is a soft PLC?

Not too sure, but it looks like some sort of quasi-PLC that requires the use of a PC. PLCs are independent of PCs, except for initial programming.

> 4) what are the advantages of a PLC compared to a PC

PLCs are fully contained units: they have DAC and ADC built right in, so no special card is needed. Analog and digital inputs (number varies) are also built in. I also believe that a PLC has the potential to control far more devices than a PC could, although I could be wrong on that.

Lee Rehorn
UWO Electrical and Computer Engineering Dept.
PLC or Programmable Logic Controller was an intelligent system of modules which were introduced in the Control & instrumentation industry for replacing relay based logic. Exact history is not known to me but this is an accepted belief.
Over a period of time, better I/O handling capabilities and more programming elements have been added along with improvements in communication and thus this product (PLC) is capable of doing almost anything in I&C.

There are 5 or 6 languages used
Ladder Logic.
Statement List.
Sequential function charts (SFC).
Instruction list (IL)
Function Block Diagram (FBD)
3GL and 4GL languages are also supported by many.

Soft PLC is a product. Soft PLC if you mean Software PLC is a PLC implementation on a PC. The only difference here should be the use of commercial PC's instead of prop. hardware designs. The second design difference (on theory) should be that in a Software PLC the OS is commercial and there is a user program, while PLC has an embedded firmware. So any SOFTWARE PLC which embeds its software into the PLC and it becomes a firmware should then become a PLC.

PLC has proven itself in the market in very critical applications. Please note that here critical does not mean the same critical as in PC systems and servers. PC automation is still in its infancy stage and reliability is an issue. This has been overcome by many using Linux, raid disks etc. and we hope that it will pick up as the lead product very soon. PC automation is already established in several machine automation and in non critical data acquisition systems.

For more indepth knowledge visis ISA.org and get books, manuals and training programs on PLC.

A PLC generally looks at "inputs" wired or communicated to the PLC, and depending on your "programming", turns off or on "outputs" wired to the PLC. This is a very simplistic description of what PLCs are capable of nowadays....many new PLCs offer indirect/indexed addressing, array management for data moving, and connection to DeviceNet, ControlNet, EtherNet, Profibus (pick a flavor), and other networks. Lots of possiblities.....many debate a computer is better, but hey, the PLC has a microprocessor in it anyway, so there isn't THAT much difference to start off with. This is also why we haven't seen computers run rough-shod over PLCs: ultimately they both have the ability to be enhanced in similar ways -- memory, speed, hardware, etc.

Bouchard, James

Look at chapter 9 - Programmable controllers in "Fundmentals of Industrial
Control" by Charles Albert published by ISA. Other books on PLC's will also
provide answers to your questions too.

James Bouchard
thanks alot. i have found the answers to the questions using the site that some of you gave me. thanks alot
Here are some words to answer question no 4.
PLCs are made of the same components as PCs (ICs, memory, etc.) except that PLCs include inputs & outputs (ON/OFF, analog) specially designed and protected for industrial environment.

PLCs also include what we generally call watch dog devices. PLCs are designed and tested (software & hardware) to auto-verify themselves in case of user program bugs. This is an important feature compare to PCs because PLCs are used to control industrial process that could be potentially dangerous for workers. We are used to applications failure in PC machine! But for PLCs, this is something that must be avoided.