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I heard from a colleague that typical PLC serve as an interface between a the signals and, for want of a better word, a PC. The connection, being typically done with ethernet, and the PC runs some Codesys, or Labview applications.

This was the way the application could be changed while running, which is one of main advantage of PLC for debugging and maintenance.

However, looking at one example, like this one, I can see the Ethernet ports, but I can't find any precise information about that topic.

Looking around, I found that a remote PC can be used as a server and thus the PLC would communicate with it, typically via Ethernet. But it is not a must.

Can someone clarify that trivial point for me? Must or must not a PLC be connected with an external PC to run the application. And if it's not, can we still change the code on-line?

For the background, I have to decide on using a PLC or a µC-based solution, and up to a few days ago I ignored the existence of PLCs.

I should add that I have read the question: Why use PLC instead of microcontroller?, but unless I missed it, it does not answer that specific question.

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  • \$\begingroup\$ depending what you want to do, a PLC or a uC can both be used in the same situations. A PLC can run a SCADA style interface with a SCADA server running on a PC, allowing the PC to read/write/interface with the PLC during run-time. The scale-ability and ruggedness of PLCs make them a good choice for a large automation set-up which is mostly simple input/output/data acquisition. Can you tell us more about the application/situation you are in, and perhaps we can help you choose which one may be better in the long run. \$\endgroup\$
    – KyranF
    Jul 30 '15 at 6:03
  • \$\begingroup\$ @KyranF can or must? We produce power supplies (tens of kW) and and I need a master controller: which should be responsible for starting/stopping the system, security, and sharing the load between the different sub-units. \$\endgroup\$ Jul 30 '15 at 6:09
  • \$\begingroup\$ PLCs can be purchased with double-redundancy processors inside them, and are usually available with some kind of "safety" ratings or guarantees. For dangerous industrial control, I suppose a PLC is the safest bet (no pun intended) \$\endgroup\$
    – KyranF
    Jul 30 '15 at 6:13
  • \$\begingroup\$ Also in that question you linked at the bottom, some of the other answers (like the one from Alberto) give great reasons why in your situation a PLC may be better than a custom uC based solution \$\endgroup\$
    – KyranF
    Jul 30 '15 at 6:16
  • \$\begingroup\$ @KyranF, yes I also saw that. The thing is we already have a µC that will be included anyway for the subsystems. And I am more familiar with the µC, and being the one who's going to program it, I see advantages there. Nevertheless, I am still unclear about the need of a PC. Can the PLC run without an external PC, and if so at what cost? (apart from the PC price, obviously). \$\endgroup\$ Jul 30 '15 at 6:21
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Can the PLC run without an external PC, and if so at what cost? (apart from the PC price, obviously).

Absolutely a PLC will run without a PC. The costs vary greatly depending on features (# of I/O, Redundant processors, etc. etc.)

That being said, the only thing the PLC would be able to control is the attached I/O (Contactors, Relays, Alarm Lights, etc.).

You would not be able to do any programming of the PLC. You could have a lap top computer (for instance) with a programming cable (usually USB or Ethernet) that you could use to program as needed.

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A PLC does not need a computer to run the program. 99.9% of the time a computer is connected is for programming or debugging. For programming, some PLCs don't even need a computer - see this Allen Bradley handheld programmer. For simple or local monitoring and control, a programmable HMI is mostly used in most industrial settings.

In a more complex setup such as in a power plant or a manufacturing plant, a SCADA(supervisory control and data acquisition) system is used. A PLC interracts with the real world e.g controlling machines, responding to user inputs such as buttons being pressed, collecting readings from sensors such as thermocouples, pressure transmitters, encoders, etc. Applications running on servers such as OPC servers or historians collect data from various PLCs around the plant. This data can be consumed by client applications such as HMI operator stations in a control room or by trending software. The client applications can write back to the PLC through the OPC server allowing for remote control. A PLC can also collect data from other PLCs through the OPC server instead of having a direct link.

While with hard work you can achieve the same with a microcontroller, the question is why would you? You are going to be reinventing the wheel most of the time. Need a specialty function such as bar code reader, high speed counter, encorders, thermocouples, RTD, 4-20ma current or voltage input/output, Timers, AC/DC inputs outputs, relay outputs, VFD control, Modbus, Fieldbus, Profibus, HART communication, etc?? PLC is got you covered. Just plug the I/O for what you need, configure it and you are in business. Try implementing that with a microcontroller. Plus don't forget most industrial places won't allow your homebrew microcontroller. Is it underwriters laboratories or RoHS rated? No go for most places

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A PLC as its called a Programmable Logic Control, it can be programmed with the help of Computer initially (Advance Programmers for some PLCs computer isn't even necessary for programming). once the PLC is programmed and its in run mode it doesn't need computer to be attached to it as it has its own processor. you need pc attached to PLC just for debugging the program or monitoring the process if the processes are working according to plan.

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