Modbus Wiring Help

I am ignorant. I have a chiller connected to a Micrologix 1400 for the sole purpose of conversion to OPC via RSLinx. This was installed before my time. I have a newly installed air compressor that also has a modbus gateway. The configuration for Channel 0 on the Micrologix matches the configuration on the air compressor gateway.

The question I have: Can I simply terminate the two-conductor communication cable from the air compressor in parallel with the cable from the chiller at the PLC? Will this work?

Thanks in advance.

I'm assuming everything is talking on RS-485 and the 1400 is acting as the Modbus Master. As long as you configure and program the 1400 correctly as well as assign the proper modbus slave address to the compressor than you should be good to go.

I'm skipping over a lot of detail, of course. Do you have a good understanding of how the existing setup with the chiller/1400 work?

Thanks so much for responding!
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Correct. RS-485 and the 1400 is the master. Channel 0.
The configuration of Channel 0 matches what I have been provided from the air compressor vendor.

There is a MSG instruction which is requesting the data. It's configuration screen identifies the slave node as 1. I assume from reading I need to assign a different number to the air compressor and insert another MSG instruction for the air compressor.

My fear is the wiring. If I connect the two together, I don't want to short anything out or interrupt the data coming from the chiller.

MrCalibrator said:

Thanks so much for responding!
\
Correct. RS-485 and the 1400 is the master. Channel 0.
The configuration of Channel 0 matches what I have been provided from the air compressor vendor.

There is a MSG instruction which is requesting the data. It's configuration screen identifies the slave node as 1. I assume from reading I need to assign a different number to the air compressor and insert another MSG instruction for the air compressor.

Click to expand...

That sounds correct. If you are only reading data, then you will also need to pick unused addressing for the data. If you are writing to the new node, then you need to be very careful about the source data you are going to send.

My fear is the wiring. If I connect the two together, I don't want to short anything out or interrupt the data coming from the chiller.

Click to expand...

Worst case scenario is that you break communications with the existing set up while you figure out the wiring. I have long ago decided not to spend too much time trying to figure out which is which for RS-485 wiring among various vendors. I hook them up one at a time, swap the Rx and Tx or A and B or S+ and S- at the slave until I get it working.

If you new hardware is from the same maker as the one you have working, you stand a real good chance of hooking it up without interrupting communications.

You won't fry anything, but there is the chance of briefly stopping communication that already works. The other thing to consider is termination resistor for the RS-485 end points. I have seen many situation where it works without them, but I always try to include 120ohm termination at the end points. Some devices provide this built in and selectable with dip switches or jumpers. If the original slave node has termination, it will need to be turned off or physically removed and relocated to the new end node.