Multiple Modbus devices to PLC

Hi,

I have a Micrologix 1100 PLC with modbus/RS-232 port. I use the port for programming, but since I set up RSLinx with Ethernet, I now have the serial port free for Modbus devices. I have integrated a single modbus device before, but it had it's own small PC board with 9-pin cable to PLC.

This time, I need to add three modbus devices, which only have 6 wires each, 24v +/-/GND and DataA DataB and DataGND. It's obvious what to do on the power side.

My question is, do I just tie the data lines to one another A, B, Gnd at the panel end, and make a cable to go into the controller? Or do I need some extra interface? Perhaps, if I just tie lines together, I can use terminal blocks with commons to make it look neat...

I know that my 3 devices have different modbus addresses already factory-set. I have also set my micro's serial port to be a Modbus Master.

Thanks All!

Since the communication pins on your Modbus devices are labeled Data A, Data B, and DataGND, it is a safe bet that they use the RS-485 serial standard.

The MicroLogix 1100 has both RS-232 and RS-485 interfaces built into the Channel 0 plug.

The easiest and cleanest way to use the RS485 interface is to plug in a 1763-NC01 cable and select "No Handshaking (485 Network)" in the Control Line setting drop-down in the Channel 0 serial port configuration window in RSLogix 500.

The usual wiring configuration is to daisy-chain a cable from device to device. For short systems many folks use Cat5 cable, but for longer runs you should get a three or more conductor cable made for RS485.

RS485 needs three wires: A, B, and GND. Consider the "GND" to be a signal common only and keep it separate from the actual chassis or protective earth ground.

For a short system at a low data rate (like 19200 or 9600 bits/sec) try the system with no termination resistor between A and B. Ideally you'd use a scope to check the waveforms on the network and then decide whether or not to use termination or bias resistors.

B&B Electric and Robust DataComm are two very good vendors whose online resources about RS485 networking I trust.