COMM: Modbus Connection AB CompactLogix - Mitsubishi A2ACPUcomm

Mon Jun 8 18:29:46 EDT 2009

==> Automation List post by scadametrics...
To view or reply to this thread: http://www.control.com/thread/1243525982#1244488331

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Re: COMM: Modbus Connection AB CompactLogix - Mitsubishi A2ACPUcomm
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To answer this question fully, I'm pretty sure we need to know which of these PLC's is to be the master, and which is to be the slave.

If the CompactLogix is the master, then I'd recommend using a DIGI ONE IAP to map DF1 requests from the CompactLogix over to Modbus/RTU requests that will be sent to the Mitsubishi PLC.

The DIGI ONE IAP will also convert RS-232 to RS485, so you could eliminate that RS-232/RS-485 converter that you mentioned.

I've used the DIGI ONE IAP with success in the past for similar applications.

If the roles are reversed (Mitsubishi is the master), you may be able to do a Modbus/RTU-to-DF1 mapping, as well.

Also, there is an external Modbus/RTU library available for the CompactLogix.  At first glance, its setup looked complex, but I didn't delve into it in detail, so you may want to look into this method, as well.

FWIW, the less-expensive Micrologix series PLCs from AB offer native Modbus/RTU support on their serial ports.

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