Modbus and the 1763-NC01

Hi,

I'm tearing my hair out trying to connect a simple temperature controller to an ML1100 using a standard RS485 modbus network.

I've tried everything from swapping the A-B wires over to changing baud rates and handshaking settings. I can't get any comms at all.

I'm getting plenty of messages sent but no messages recieved. The only thing I can think of is the physical wiring of the NC01 breakout box. It has a term, A, B, COM, SHLD and CGND connections.

Obviously my data lines go to A and B but where does the sheild go? Does it connect to COM or SHLD? Other's have mentioned jumpering A to TERM and COM to CGND. I have tried all possibles but still get nothing.

Have also tried a teminating resistor but as I'm withing 300mm on a test bed I can't see this being the problem.

Any ideas?

Jake

Attached is the image from one possible setup for Modbus on an 1100. Obviously the upper part may vary in Baud, Parity, Stop Bits and Data Bits. But make sure that the 'Protocol Control' is set up as shown - 'No Handshaking (RS485 Network)'

yep tried that too. next step if I cant get this working will be a MOXA MODBUS gateway, with Tcip

Does the slave device have a data common pin or terminal ?

In RS-485, there is always a common point against which the two data voltages are measured. Many devices use the chassis ground or earth ground for this common point. Rockwell Automation uses an isolated common terminal. This is the one labeled COM.

Shield and Chassis Ground terminals on the 1761-NET-AIC or 1763-NC01 are exactly what they sound like: the Shield is not connected to anything internally but gives you a terminal to daisy-chain shield drain wires. The Chassis Ground pin is connected to the chassis ground of the controller.


Try connecting a wire from the COM terminal to your "2-wire" device's chassis ground.

Its a plastic temperature controller so don't have a chassis ground. Only have a 3 wire connector with + - and GND.

Have brought a RS232 - 485 converter so I can use a MODBUS sniffer. If I get that side working I'll then try again with the PLC. Other thing I'm considering is ditching the NC01 cable and buy a TIBBO serial to ethernet adaptor then I will be able to MSG with TCIP.