Communicating between a 1771-DB basic module and an RS-485 module.

[mattman814]

I am using a PLC5(RS5 ladder logic software) and trying to figure out the proper protocol to get the Allen Bradley 1771-DB basic module communicating with an RS-485 module( which happens to be an Emhart weld controller). Based on the literature received with the 1771 card I have configured a block transfer to send data to the RS-485 module. The 1771 module has a Basic program in it and the indicator lights appear to be sending but never receiving. The RS-485 module shows no response at all...

Thanks for any input.

 

[akreel]

If the module is sending but not receiving, there are a few "likely suspects."

1) The RS485 circuit is wired incorrectly. It won't hurt you to try reversing the polarity. This is a VERY common problem because manufacturers don't seem to follow any specific labelling convention.

2) The message string is incorrect. If you can use a 485/232 converter to listen in, that would help. Also, recheck the documentation on the slave device.

3) Your baud rate, flow control, or addressing is screwy. Double-check your settings to make sure they are compatible.

AK