Networking PLCs

Dear all,

First of all, let me tell you that I'm new to anything that has to do with PLCs. I just started to get familiarised with the concept and I would like to know more.

The reason I'm contacting you is to collect some information on if and how is it possible to make a PLC network over the ethernet. The idea is to have a LAN of PLCs, each one with their own IP address, on which I can configure and control my PLCs, using a single PC. This would replace an existing network of PLCs, individually wired to a PC, which is bulky and messy.

So I can think of a couple of questions to start with..
1. How can I interface a PLC to an ethernet?
2. What kind of network topology would suit such a configuration?
3. How can I make my network vendor independent, i.e. using different PLCs manufacturers on the same network?
4. Do I need to develop a custom control software, or could this be an off-the-shelf item?

Well, that would do for now. Any suggestions are welcome and I mean any!!!

Regards,

Dimitris

1. i think every PLC manufacturer is offering some interface for ethernet, but the differecens are in inner communication protocol format

2. ethernet network has 'only one topology'.... just have a network spider with swtiches/hubs/servers and nodes connected anywhere - so it's same than you build network from PC's - everything is just IP adress

3. this may be less or more difficult... in depency od what PLC manufacturer(s) you want to choose... but imho there is way to use a PC as transmission station between different PLC's. with proper software

4. i think where are plenty of OPC and other comm server drivers available on the market, so when proper software with multiple platform driver selected... there would not be reason to write your own


...maybe if you would be more hardware specific, you will get better answer...

marius said:

1. i think every PLC manufacturer is offering some interface for ethernet, but the differecens are in inner communication protocol format

2. ethernet network has 'only one topology'.... just have a network spider with swtiches/hubs/servers and nodes connected anywhere - so it's same than you build network from PC's - everything is just IP adress

3. this may be less or more difficult... in depency od what PLC manufacturer(s) you want to choose... but imho there is way to use a PC as transmission station between different PLC's. with proper software

4. i think where are plenty of OPC and other comm server drivers available on the market, so when proper software with multiple platform driver selected... there would not be reason to write your own


...maybe if you would be more hardware specific, you will get better answer...

Click to expand...

Hi Marius,

Thanks very much for the quick reply!
You're right of course, that I need to be more specific about the technology I intend to use, but frankly I'm not at that stage yet!
I'm only beginning to understand PLCs and investigating the possibility of creating a network. I will however have more information soon.
In any case thank you for your comments - you've been very helpful already!!

Dimitris

Dimitris

This should be very easy. As Marius already mentioned, they're just IP addresses. Anything that you may already know about networking PCs on ethernet applies to networking PLCs on ethernet. Addressing, subnets, etc.

You can certainly have multiple brands of PLCs on a network, just like you can have different brands of PCs -- they're "just IP addresses". However, if you wish to do anything with those different brands, you will obviously have to have their specific communication/programming software available. Another thing to remember here will be that, though they may all be on the same network, different brands of PLCs may or may not (probably not...) be able to communicate with each other.

Regarding your question 4... what do you mean by "custom control software"? Do you mean software to program the PLCs or programs to view process information from the PLCs? If you mean programming software, this will be determined by the brand of PLCs you select.

If you mean you want to view process information, then there are many, many options. You can use programs like InTouch or RSView to communicate with the PLCs - via either standard or custom communication links. Or you can write your own viewing program. Depends on what you need and what your skills are.

You may be able to add modules to your existing PLC's to get them on Ethernet or some may communicate over a serial to Ethernet converter, this could save many many thousands

dkontod1 said:

1. How can I interface a PLC to an ethernet?

Click to expand...

By installing an Ethernet module for that PLC family. Some PLCs already come with an Ethernet port on them; some (usually small or really old ones) don't have an Ethernet capability at all. In that case one of those "Etherenet-to-serial" converters might work (try before you buy!).

2. What kind of network topology would suit such a configuration?

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Most PLCs Ethernet modules are 10BaseT. So a standard bus topology with inexpensive swithces will suffice if you are planning just to collect production data and to do remote monitoring/programming.

3. How can I make my network vendor independent, i.e. using different PLCs manufacturers on the same network?

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Yes, you can have PLCs from different vendors on the same network: they all run some vendor-specific protocols on top of the old and tried TCP or UDP. Be aware that although some newer PLC brands may support 100 Mbit/sec, most of them are only 10. Another important thing to remember is that PLCs usually require fixed IP addresses. A separate subnet might be a good idea.

You can be vendor-independent if you are planning to collect production data, i.e. read and write PLC data memory. There is a middleware standard called OPC and you can buy an OPC server with a set of communication drivers for each PLC brand. Check KEPWare, for example, for more information.

Presenting that data in some usable format would require some programming though. As simple as DDE links to Excel or as advanced as a custom C program, whatever you choose. Most OPC servers currently present themselves as COM objects; .NET is to follow soon.

There is no vendor independency if you mean to deal with PLC programs: programming is strictly vendor-specific and programming protocols are usually not disclosed.

4. Do I need to develop a custom control software, or could this be an off-the-shelf item?

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See the above. You may have to, in order to present the collected data in useful for other people form.

I do not like OPC - have used it several times and had nothing but problems.

May I suggest a SCADA system that can use OPC but, generally speaking has drivers for virtually all brands of PLC - Citect. www.citect.com - the whole exercise then becomes very easy to implement.

BobB said:

I do not like OPC - have used it several times and had nothing but problems.

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It took me a while to get it right the first time in VB. Can't tell about the others, but KEPServer's COM interface documentation was not exactly what I would like to see.

Once I got through the initial troubles though, it's a breeze. It's not awfully fast (especially if you want to do synchronous writes), but it never meant to be.