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BenJones

N : N Networking

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Hello all, Just a quick question. We are about to start a new project here and there are many ways we could approach it. I was trying to think of the best way to lay out the network and how I would program the PLC's. Please refer to attachment. We have a main building and 2 containers. Once a year we have to do maintenance on a container and only have half a day, so we will have a third container and swap it with either container 1 or 2 (which ever needs swapping). Because of the short time we have to swap the container I was thinking of the easiest method of connecting the Main Master PLC to the two slave PLC's (as little cables as possible). I thought the best way would be to use 3 PLC's in total and N : N networking between the PLC's, so there is just one RS485 per container. I could then send all the data via the container PLC to the Master PLC in the main building. Can I ask, does this seem to be a sensible way to do it ? And, is N : N networking really hard and complex to program, or should it be quite straight forward ? Many thanks, Ben Edited by BenJones

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Sounds like a good plan, however how much data should be transferred between the slave PLCs and the master PLC?? If I recall, N:N cannot exchange a lot of data..... An alternative would be to run Modbus Master in the master PLC, and Modbus slaves in the slave PLC. All the wiring will be the same, it's just a matter of what protocol you will use. By the way; what programming software do you use??

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If I understand what you are trying to do, I can see a convoluted path ahead. If you are using any kind of networking, you'll have to deal with node addresses, missing nodes, etc. Say your master is node 0, and the containers are nodes 1,2, and 3. If you swap containers 1 and 3, suddenly all the traffic that was going to node 1 will have to be re-routed to node 3, and node 1 no longer exists. Then next down day you use container 1 to swap out for container 2. So now you have node 1 where node 2 used to be and node 3 where node 1 used to be. If your I/O count is low enough, I'd just do it all with one PLC and use quick-disconnect cables to swap out containers. That way it doesn't matter which container is where. If you have a lot of data, then go with your networking idea, but you'll have to write the code to handle enabling and disabling communications and routing data to the right location depending on which container is where. I'd put a screen on an HMI for the operators to select with container is in which location.

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3 FX3U or FX3G with Ethernet would work. N:N is going to limit the amount of data you can move. How much information you plan to share would have to be the first thing to determine. N:N is as simple as it gets for setup. Basically in each slave you just need to set a node number. The master has like 7 words to set up the network.

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Hi ! Thanks for the advice. Kaare - I'm using GX IEC Developer v7.03. I think I understand what you mean. I've attached a picture to try and decripbe this. The only problem with this set up is that I'd have to change the slave numbers when replacing a container to suit either container 1 or 2. However, if I can do it this way it'd be easier to program I think. I have not yet set up a channel as a slave, but I have looked at the function block to use in the 'solution pack'. It looks like the function block only sets the slave number (station number) and the comm's setup (baud rate etc), so I was wondering how I read the information from the container PLC's so that I can then read it into the main PLC. Is the the modbus addressing for the slave PLC's just the slave PLC registers or is special mapping required to certain registers ? Ben

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Hi ! Yes, I already thought of this ! After I've decided the best way of setting up the network I shall think of this further. I want to get to a point where the program for all 3 containers is the same, and then I just hard wire a digital input which then switches the slave numbers around so the PLC slave number in the third container can either be 1 or 2 depending on which is being swapped out. Thanks for the link on the quick disconnect cables. This was also something I was thinking about. It may save costs as well as extra programming with the slave PLC's. If I were to go down this route, may be a more powerful main PLC would be a good idea, the Q - series for example. Cheers, Ben Edited by BenJones

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Ok, thanks Crossbow ! . I shall think further about the amount of information I need to transfer

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CC-link anyone? The programs could be identical - just set the dial on the hardware. For: Relatively low cost. Bulletproof auto-detection of slaves. Simple to program. Against: Some cost. If you already have Ethernet/RS 485 its an added cost. Not as simple as the plug and socket.

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To cook a compote of different networks on the same level it is very, very bad way! I support the first sentence of JRoss, and do not see the slightest difficulty in realization. The address of slave could be substituted in the programming sequence of master to the request-response of slaves, depending on which container has been excluded of process. And it's very very easy!

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I like those disconnect couplers. That particular line has a modular version that can be configured based on your needs. Say with higher gauge wires for motor control and lower gauge for signalling. They even have pneumatic modules! Double-check your needs and processor specs before jumping from FX3U to Q. The FX3U is a very powerful brick, and overlaps the Q in power and functionality. Unless you have a lot of I/O, significant motion or communication needs, or need major expansion capability, you're probably alright with the FX3U.

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Ok, sure, we'll probably be sticking with the FX then I think. Seems to do what is says on the tin and should be ok for the next project too.

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