Modbus comms via Point IO (Allen-Bradley)

AndrewY

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Join Date
Oct 2007
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Auckland
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13
I have a CompactLogix (L43) PLC with Point IO remote rack (comms via DeviceNet). I want to talk Modbus (either RTU or ASCII) to a device near the remote rack.

Has anyone ever tried communicating via Modbus (either RTU or ASCII) via a Point I/O serial comms card (1734-485ASC)?

Is this even possible?
 
While you could implement Modbus on one of the POINT serial modules, it's a whole lot of work to manage those strings of data and checksums.

Far better is a module from Western Reserve Controls (wrcakron.com) called 1782-JDM. It takes care of the Modbus checksum onboard and gives you a very simple way to exchange Modbus transactions with a serial slave. The A-B POINT serial modules and the 1782-xxx modules share a similar heritage.

Another more sophisticated device is the Communicator family of network/serial bridges from HMS Fieldbus.
 
Thanks for the info Ken,

I've just been browsing through the Logix sample code for Modbus Master and have to agree - it's pretty messy work managing the raw data. We may end up going down that route though as we're rolling this out on multiple machines so the development time may be pay off in the long run.

One question I had from this sample code (having not dealt with serial comms on Rockwell gear & a long time since using DeviceNet on Rockwell) is how to access the serial data from the PointIO rather than from the on-board serial port.
 
The serial mechanism of the POINT ASCII modules and the built-in serial port are totally different. Instead of having the ASCII serial port instructions (ASCII Read, ASCII Read Line, Clear Buffers, ASCII Write, ASCII Write with Append) you have an Output image buffer into which you place serial data and a length, then change the value of a transaction counter to trigger the writing of the data.

The Sample Code would be reference material only for implementing Modbus on a POINT serial module.

If it were my system I wouldn't hesitate to use the 1782-JDM. In volume I'm sure they're comparable in price to the 1734-485ASC. I wrote some widely-used technotes on the SLC-500 and 1782-JDC almost ten years ago and have a great deal of regard for Western Reserve Controls.

What is the Modbus device you need to connect to ? Do you need to both read and write Modbus data ? Is it a multidrop system with multiple slaves ? How many different Modbus transactions do you plan to use (holding register read, coil write, etc) ?
 
At this stage we're only connecting to a single slave (a generator controller). The amount of data transfer is pretty minimal - read 2 or 3 status words (at ~1Hz) and write 1 or 2 words on the odd occasion.

The more I look at it the more I agree with your leaning towards a hardware gateway rather than PLC driver.
 

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