Help with rx3i Modbus

[spacehitchhiker42 0]

I'm trying to get an rx3i system with a IC695CMM002 card to talk to a Moxa Io-logik module over rs485 modbus. I can get my computer to talk to it over a usb adapter, but i can't get the PLC to talk to it. I've tried setting up the other port as a modbus slave, but I can't get my computer to talk to it either. I've checked my serial settings, and they all match. I've got the data exchanges all set up. I've tried swapping the polarity of my sr485 lines. I don't have any faults in the fault table. When I send a request from the computer to the PLC with qmodmaster, I see the status light on the port flash, but qmodmaster keeps timing out. I don't think it's getting any responses back from the PLC.

Anybody else run into this kind of problem before? Are there any weird quirks i need to know about?

[pturmel 159]

I'm not familiar with that card, but it sounds like reasonable pattern on the face.  I'd probably go straight to an oscilloscope to compare PC versus PLC waveforms.

[Steve Bailey 118]

I posted a few questions for the OP at Phil Melore's site. An occasional blink of the LED on the Modbus slave port, suggests that the slave doesn't understand the command being sent by the master. It could be due to a mismatch of port settings. It could also be due to the master sending a command to a slave with a different Modbus ID than what you've set up the CMM002 for. 

[pturmel 159]

What are your serial parameters?  I ask because Modbus RTU defaults to E-8-1 but requires two stop bits when parity is turned off.  A device that strictly follows the Modbus spec will reject N-8-1 transmissions.