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Homer_BL

Wirelss Modbus

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Hello! Suppose that I must realise wired Modbus RTU communication between AB and Siemens PLCs. It should be like this (A). Has anyone did it? Any comments? Should it work? Any suggestions regarding communication settings? Now, if it can work, suppopse I must convert it to a wireless solution (B). Has anyone did it? Any comments? Should it work? Any suggestions regarding communication settings? Thanks and best regards. P.S. I put this post in Siemens forum too.

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Modbus is a very simple serial protocol that uses RS-485 as the media. It is very simple to get it to work with wireless. Look at www.modbus.org for complete specs (can't say that about Allen Bradley!) I would just use Aerocomm radios though @ about $150 a piece compared to what Elpro charges for theirs. You can order Aerocomm radios through Mouser (www.mouser.com). If you are using Micrologix PLC's, they handle Modbus natively and life is simple. If you are using any other Allen Bradley PLC family, you have essentially 3 choices. You can use the code that AB publishes in their software database to implement Modbus in the PLC for ControlLogix (not available for PLC-5 or SLC but it's not too hard to write your own if you are good at this stuff). You will still have to get an RS-232/RS-485 converter (www.bb-elec.com). Second, you can use an external DF1-to-Modbus converter and connect it to the serial port of any Allen Bradley PLC. Digi makes the Digi One IAP. Again, B&B Electronics sells those, too. Finally, you can put a Modbus scanner/slave directly in the PLC rack. Prosoft Technologies makes these. I highly recommend you use the STANDARD communication settings for Modbus (9600 baud, 2 stop bits, etc.). These are the defaults. See the standard (www.modbus.org) for details on the basic, minimum, REQUIRED implementation for Modbus/RTU. Try to use this one first and only get creative after that if you aren't getting results. Please recognize that for whatever reason, every single PLC vendor out there takes it upon themselves to put some sort of screwy non-standard twists on the Modbus standard and it always takes a while to figure out how they managed to screw up the simplest PLC protocol out there. Honeywell and Modicon are the prime suspects when it comes to this sort of thing, but like I said, almost all of them are guilty.

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If you are using Micrologix PLC's, they handle Modbus natively and life is simple.... I will use MicroLogix. I highly recommend you use the STANDARD communication settings for Modbus (9600 baud, 2 stop bits, etc.). OK, I will use this settings. Honeywell and Modicon are the prime suspects when it comes to this sort of thing, but like I said, almost all of them are guilty. I believe Siemens will be quilty too . Thanks, paulengr.

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>Suppose that I must realise wired Modbus RTU communication between AB and Siemens PLCs. It should be like this (A). Correct, with each Modbus slave having its unique slave node ID number. >Has anyone did it? Not with these models, but other Modbus devices. >Any comments? Should it work? In principle, there is no reason why it will not work, as well as Modbus does. However, the Modbus master controls all the communication. Slaves are silent unless spoken too. Modbus slaves cannot talk directly to other Modbus slaves. If Slave 2 needs data from slave 3, the master must read from Slave 3 and then write the data to slave 2. Making it all work depends in some part on the ability of the master to set the time-out response interval. Delays in getting a response cannot be allowed to 'hang' the system, so the master times the response. Hopefully that timer is settable. Wiring the serial to the radio and dealing with the wired side can be a challenge. the A/B +/- lines are not standardized. I find Siemens' implementation Modbus absolutely obtuse (but I'm not a Siemens guy, either). One forum participant had a thread concerning Siemens Modbus. He entioned that he got reasonably good help from Siemens technical support though.

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Homer, it is perfectly reasonable to convert this from a wired Modbus RTU to a wireless solution. To further Dan’s point it is important to consider two questions when exploring wireless: 1. How fast must the pulling rates be for Modbus (wireless will be slower than a wired configuration)? 2. What is the distance between the PLC’s (as this would determine what wireless technology to use)? Each wireless technology is suited for different data rates and distances. For example, WLAN is for a fast speed covering shorter distances. Trusted wireless is a 900MHz solution that is ideal for scada speeds at longer distances (3-20 miles).

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