PLCs vs. Microcontrollers

Microcontrollers (MCU) are complete computer systems on a chip. They combine an arithmetic logic unit (ALU), memory, timer/counters, serial port, input/output (I/O) ports and a clock oscillator.

Programmable logic controllers (PLC) are the control hubs for automated systems nad processes. They contain multiple inputs and outputs that use transistors and other circuitry to simulate switches and relays to control equipment. They're also programmable via standard computer interfaces and proprietary languages and network options.

Microcontroller - It is a Integrated circuit similar to microprocessor with limited resources. This can be programmed to work for a specific purpose like washing machine, timers, display units and other similar equipments. PLC - It is a programmable logic controller used to program for batch control. This is developed using a microcontroller.

A PLC HAS IN-BUILT MICRO-CONTROLLER. PLC IS USED FOR POWER SYSTEMS OR INDUSTRIAL CONTROL. A MICRO-CONTROLLER IS USED FOR EMBEDDED SYSTEMS.

PLC or Programmable Logic Controller has a Microprocessor in it. However, Microprocessors by themselves are not PLCs.

While computers can be used for process control but they are also not PLC.

PLC is a scheme for simultanious sampling of process input and simultanious loading of process control outputs under PLC cycle lime and within this time entire PLC logic is operated on input parameters at a time. There is no sequential process in PLC.

PLCs are designed to be with highest reliability and often work without problem for years, unattended. If you want your Microprocessor to be a PLC then you need to know, how to code the program that it will show a PLC like behaviour.

One effective difference is that PLC's are open architecture, meaning that the end user of the equipment can easily get into the code for troubleshooting and expansion. Most PLC's have programming that is IEC 1131-3 compliant.

Proprietary controllers are microprocessor based, and the MTBF (Mean Time Between Failure) doesn't match anything a PLC can do. Industrial process lines typically operate 24/7 and when they go down it's never at a convenient time to find a processor technician or source replacement parts only available from the original manufacturer. PLC hardware is commonly available from local suppliers.

My crusade in life is to replace proprietary controllers with PLC's in the material handling industry (powder handling equipment, especially gravimetric /LIW feeders). So my opinion will be very biased.

Regards, Delmar Schmidt
Melfi Technologies Houston
dschmidt@melfitechnologies.com

Open architecture alone does not say much about PLC. Main idea is to remove hard logic with software with lots of flexibility of wiring. No need to change wires even if they were wired to wrong place as one can change the wiring by software. Inputs and outputs are software wired and with added software logic.

Now a days PLCs are more like computers and have even Internet connectivity. More features also make it less reliable in some cases. Idea is to have highest reliability of hardwired logic in programmable form.

PLC make good target control machines at low cost and high flexibility. Custom designs require person to handle each talk while PLCs can be programmed quickly using simple ladder diagram language. This graphic language makes PLCs very special device. Unfortunately, each PLC has different sets of LOGIC in Ladder language. However, basic logic remains more or less intact.

One thing I didn't see mentioned yet is the extreamly robust isolation on inputs/outputs of PLCs. IMO, this is the main reason for the percieved reduced ammount of down time in PLCs compared to uC.

To the OP, what is the point of your question? If your just sparking up a thread, OK cool. But if you have a problem that you might like some insight on, this is a poor question. PLCs and uC have almost nothing in common. The originated from different roots, based on different needs. They have followed completely different paths in their evolution. Lastly they are used, for the most part, in completely different applications.

Not all PLC inputs and outputs are isolated. Some outputs are NPN or PNP transistors. Inputs are also of dirrent types. NPN, PNP, Relay, 4-20mA, 0-10V, Namur etc.

It is like selecting your hotel menu to make right dish. Eat what you like the most. Heere you use what you need or can afford.

You often get into problem if you do not know the type of output sensor may have and type of input your PLC has. Like PNP is a current source and NPN is a current sink. 4020mA can be source or sink. Namur, 4-20mA are typical voltage current level type, and 0-10V is purely voltage. Relay input may be NC or NO which is much of a problem but needs correction in the code. If your input is 10kHz pulse then it is impossible to use relay at input. Optical isolators also have similar problems.

Often people get sensors and proper PLC in one package. If they get into problem then they only refer to catalog part number and replace then blindly.

However, if you are entering this forest for the first time then be careful. Not so easy to play with everything around.

I really did not like to scare any one but this is a reality.

more infomation on Microcontrolller:

http://ee.jobmsn.com