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rammin48

Concept 2.6 Modbus

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Well I am working on a Modicon Quantum with redundant cpu set up. I admit I don't know much about modicon but have worked with many other PLC's. What is the best way to do modbus communications. I have read and tried to use several of the instructions. XMIT XXMIT. I am set up using IEC ladder and function blocks. The CPu is 140CPU11302. I need to read from 36 devices. I am frustrated by the documentation and the software. Which instruction do I use for Modbus RTU for reading only from multiple slaves? Does anyone have an example I could look at? I was using the XXMIT but the manual requires that for Modbus messaging that the MsgOut be WordArr9 but the block only allows ByteArr9? Any help would be appreciated.

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See example: http://forums.mrplc.com/index.php?autocom=downloads&showfile=271

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Yeah well the the CRNTL_XM FFB does not exist in 2.6. This is exactly why I am having trouble. The manuals do not match the instructions available. So I can't even figure out how to do something. The Processor is a 140CPU11302 which I have loaded the upgrade to allow IEC FFB's. The allowable IEC COMMS are Xmit and XXMIT. However the manuals do not match the blocks and the variables required to make this work.

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You must copy folder "URET_LIB" from site to folder "Concept\lib". And delete all ~*.* files from folder "Concept\lib". Block CRNTL_XM FFB will accessible after it.

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Now I see what you are talking about. I go it in the program and I am setting it up. I noticed you say it can do up to 10 devices. Is there a limit. I currently have to read from 36 devices. It is not alot of information but alot of devices? I will add to this thread if I have any more questions. Thank you for your help

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RS485 serial is typically limited to 32 devices. I would assume the same limit in the Modbus serial commands.

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The example have limit 10 devices. But you can have greater limit. Notes: 1. The XXMIT read from 1 device during minimum 5 scan time period . 2. RS485 serial is typically limited to 32 devices. You must use repiter. 3. See Bridge Multiplexer for better.

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Interesting that modbus is limited to 32 devices. I have done more using the prolinx modules. They also allow node addresses up to 254. Must be an impedance thing.

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It's not Modbus that is limited to 32 devices. The limitation is RS485 topology. You should not have more than 32 devices on an RS485 network without repeaters. It's a signal strength issue.

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I have done modbus on alot of plc's and devices and I have to say that Modicon really sucks. Considering that they were the main ones to start Modbus you would think that it would be a lot easier to set up an instruction and execute it. The instructions are cryptic and leave much to interpretation if you can find the help file that applies to the instruction you are using. Even though I have just about complete the code I would not be surprised If I don't have issues when I start to test.

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I don't miss working on Modicon equipment either. They make the simplest things to darn difficult. Like not even bothering to include simple SET and RESET commands in their 984 ladder logic instruction set. I'm much happier not working on their outdated stuff. Don't delete the segment, just remove the rungs. There must be at least as many segments as you have racks. So if you have 2 remote racks, make sure the program is set up to have at least 3 segments. Don't ask me why, but that's what Modicon told me years ago. Something to do with the IO refresh.

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