Modbus connection to Trane Fan&Coil Unit

I'm trying to automate the AC in my building. I've found that the indoor units have a Modbus (4 pin) connector (see attachment). I assume all pins are signal pins (full duplex TX+, TX-, RX+, RX-) since there is no voltage on the pins. Under the connector it says COM4. I've done some digging and Trane supposedly uses a proprietary protocol called COMM4 (note two Ms).

I found an online manual for an identical unit here:
https://www.manualslib.com/manual/1948585/CooperAndhunter-Ch-Irt05nm.html?page=24#manual

It says it is using Modbus 9600 baud (I'm assuming 8N1). I'm using an Arduino with a RS485 board (5V). However I can't get any response from my Modbus commands. I've tried every combination of the pins (full/half duplex). I've set the unit's slave address and configured it to long distance control which should enable Modbus...

I don't know if it actually use Modbus or it's own COMM4 protocol or if I'm using the wrong voltage (maybe 10V?) or something else.

Any help appreciated.
 

Attachments

1. The Modbus spec says that the required parity is to be EVEN parity, although ODD or NONE can be optionally offered (that's not necessarily implemented by every vendor). But just in the case that this device implement EVEN parity and only EVEN parity, try 8E1 instead of 8N1.

2. A quick read of that document indicates that Modbus only works in "Long-Distance Controller address mode", whatever that is, and that "Centrallized controller address" mode and "Long-Distance Controller address" modes are mutually exclusive, which makes sense since there is only one hardware bus that can support only one protocol at a time.

3. 4 pin (presumably 4-wire) RS-485 gets jumpered to connect to 2-wire RS-485 by jumpering the (+)'s together and the (-)'s together, for instance Rx- jumpered to Tx-, RX+ jumpered to Tx+. Try swapping at the 2-wire end because some vendors label Plus and minus the opposite of other vendors.
 
1. The Modbus spec says that the required parity is to be EVEN parity, although ODD or NONE can be optionally offered (that's not necessarily implemented by every vendor). But just in the case that this device implement EVEN parity and only EVEN parity, try 8E1 instead of 8N1.

2. A quick read of that document indicates that Modbus only works in "Long-Distance Controller address mode", whatever that is, and that "Centrallized controller address" mode and "Long-Distance Controller address" modes are mutually exclusive, which makes sense since there is only one hardware bus that can support only one protocol at a time.

3. 4 pin (presumably 4-wire) RS-485 gets jumpered to connect to 2-wire RS-485 by jumpering the (+)'s together and the (-)'s together, for instance Rx- jumpered to Tx-, RX+ jumpered to Tx+. Try swapping at the 2-wire end because some vendors label Plus and minus the opposite of other vendors.
4 pin (presumably 4-wire) RS-485 gets jumpered to connect to 2-wire RS-485 by jumpering the (+)'s together and the (-)'s together, for instance Rx- jumpered to Tx-, RX+ jumpered to Tx+. Try swapping at the 2-wire end because some vendors label Plus and minus the opposite of other vendors.
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1. I’ll try that. However I came across another manual froma different unit where it said 8N1…

2.When you use centralized connection it hooks up to a centralized controller in one of the two 2 pin connector instead of the long distance 4 pin mod bus.

3. I tried all combos. I even hooked up an oscilloscope to see if something was returned but all I saw was the packets I sent.

Do you know if all standard modbus work at 5V? It’s actually -2.5 to +2.5 that Im sending since it is a 5V device. Perhaps the cut off is at +-5V?
 
1. Do you know if all standard modbus work at 5V? It’s actually -2.5 to +2.5 that Im sending since it is a 5V device. Perhaps the cut off is at +-5V?

Answer: The plus/minus 2.5V is part of the RS-485 hardware bus spec. Modbus is the protocol, the set of rules (who talks when, how com is established, error handling) that runs on RS-485.

2. I even hooked up an oscilloscope

Comment: RS-485 is a differential signal, viewed with on a 2 channel scope as ch 1 plus 'inverted' ch2, effectively subtracting ch 2 from ch 1.
 
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