Modbus RTU Comms to Rockwell Powerflex 525

[Jieve]

I am trying to control a Rockwell Powerflex 525 via Modbus RTU (RS485) using an ABB AC800M DCS controller, and am having some issues getting connected. The ABB controller has RS232C Modbus RTU comms, so I'm using an Automation Direct FA-ISOCON (RS232 - RS485 converter) to interface them. The PLC keeps reporting a lack of response from the slave when trying to connect.



After troubleshooting for a while, I ended up using the ModbusSlave software tool and a USB-RS485 converter to read the communication messages coming from the PLC, and it seems that the PLC is continuously sending modbus function code 08 to the drive, which the drive does not support. I was able to control the drive using the same USB-RS485 converter via PC, so I'm guessing this function code is the issue.



Anyone know of a way around this? Am I stuck using a different interface device or different communication card? Was hoping to use Modbus RTU since both devices have it built-in.


Thanks

 

[5618]

I have never used an ABB controller, so I don't know what serial port instructions or control are available. When I first saw this post, I thought about looking at CCW sample code for PF525 control over Modbus RTU. In a small bit of research, I came across this document, AC_800M_5.1_Communication_Protocols. The statement "Some slaves do not understand FC8. To avoid problems, set Poll Time to zero" on pages 85-86 leads me to think maybe that will stop the automatic polling for diagnostics, then it's up to you to control the communication in your code.

https://library.e.abb.com/public/6a203d3016146ec6c12578570041d62a/3BSE035982-510_-_en_AC_800M_5.1_Communication_Protocols.pdf

 

[Jieve]

@5618: Super helpful, you may have just saved me from swapping to an ABB VFD. And I probably already read that document at least 5 times prior to having this issue


Will give this a shot when on-site next and report back on the results.



Thanks!