When polling a Rosemount 3095FB (Modbus/RS485) device with a Modicon Momentum PLC (78-010), I get response code 128 from the XXMIT block ("unexpected response").

When using a Modbus master simulation program on my PC with a 485/232 converter, I can talk to the Rosemount just fine, and also to the Modicon (when configured for slave mode of course). Also, the Modicon will communication with another modicon. I'm confident that all hardware is correct, but I'm stumped.

I did notice that on the o'scope, the Rosemount adds essentially an extra stop bit. The modicon ONLY supports 1 stop bit, and the Rosemount is fixed at 8-N-1. However, after receiving the stop bit (I'll call the bus high), it clearly has one more full and exact bit cell high, before the next start bit (low). I've decoded all data from the reply on the o'scope, and the reply is perfect except for the extra "stop" bit.

I'm wondering if this is forcing the XXMit block to claim "unexpected response". In my opinion, when a master receives a stop bit, it should wait until it sees the next start bit, without regard to how many stop bits there are (i.e., 1 stop bits means "at least one").

My Modicon distributor/support-extraordinaire is stumped, as is Schneider tech support. I'm at a loss.

Anyone have any ideas? Anybody ever connect a Momentum to a 3095FB using Concept 2.5 w/XXMIT?

Thank you!

 

Try to use in your Rosemount 8,e,1, it should solve your issue.

Concept (and Momentum PLC modbus port) can manage only 1 stop bit, not 2!! But even, odd or none parity as well.

XXMIT settings needs to be the same as set in the Concept MB port settings tab.

Make sure that modbus function code is used to read your device (normally 3) supported in your Rosemount device.

Important: Make sure, which registers you want to read from the device: 3x or 4x range, as they have different MB function code to send!!

Good luck!