Troubleshooting BACNET MS/TP Communications

This guide provides steps to diagnose and resolve communication issues on a BACnet Master-Slave/Token-Passing (MS/TP) network.

✅ Troubleshooting Steps

Follow these steps to diagnose BACnet MS/TP communication issues:

1.  Software Update 
   1. First and foremost, update the software on the controller to the latest version found on the AAON website. The website address for the controls software is https://www.aaon.com/controls.

2. Communication Mode

   1. Ensure the controller is set up for the appropriate BACnet communication mode, specifically MS/TP or IP. This configuration is typically done on the onboard display of the VCCX controller.
3.  Physical Layer  
   1. Cable Requirements: BACnet specifications call for a 2-wire, twisted pair with shielding. The cable must have an impedance of less than 30pF/ft. If the wire does not specifically state its impedance, it is considered the wrong wire. A cable run of around 100 feet will work with almost any type of cable, but once you reach 1,000 feet, you’ll need proper communications cabling. For even longer runs, both the correct cable type and proper terminations are essential.
   2. Topology and Wiring: The connections must be in a daisy chain arrangement from device to device, connecting positive to positive (+ to +) and negative to negative (– to –) throughout the network. Star or branched arrangements are not permitted.
      • A device with the + and – connections swapped (a reverse connected device) will periodically interfere with normal communications, similar to a device running the wrong baud rate.
   3. Shielding and Grounding: The shield must be contiguous throughout the daisy chain but should not be connected (landed) at each device. The correct arrangement involves the shields being twisted together but not landed. The shield should connect to ground at one point only.
      • The shield may be landed at one controller only to provide the required ground connection since the AAON BACnet interface is a grounded common interface.
   4. Connecting to Isolated Devices: AAON devices (like the VCCX2) are non-isolated, ground referenced. When connecting to an isolated device, it is critical that the common of the isolated device be connected to the same ground that the VCCX2 is connected to. Without this common connection, charge differentials can build up, potentially preventing communication.
      • The shield may be connected to the common point on one of the VCCX2 BACnet connections to provide the grounding for the overall shield. The shield can also be used as the common for all isolated connections since the VCCX2 controllers are grounded common.
   5. Termination Resistors: A 120 Ohm termination resistor must be installed at both ends only of the trunk. Termination resistors should not be placed on stubs.
4.  Device Configuration 
   1. MAC Address: Every device must have a unique MAC address ranging from 0 to 127. A duplicated MAC address will cause multiple devices to attempt talking on the RS485 line simultaneously, often resulting in intermittent communication failures.
   2. Baud Rate: All controllers on the network must utilize the same baud rate (e.g., 38,400 bps). Since all controllers on an MS/TP network are masters, a single device set to the wrong baud rate will cause periodic disruptions in communications for the entire network.
   3. Unique Device IDs: Every device on the BACnet network must have a unique BACnet device object instance number.
5.  Isolate and Test 
   1. Comm Driver Chip Check: Measure the voltage with a meter between T and the shield. Then, measure the voltage between R and the shield. Both measurements should be approximately 2.4–3.2 Volts DC. If either voltage is too high or too low, the comm driver chip is damaged and requires replacement.
   2. Isolation Test: Isolate the controller and test communication using independent BACnet software, such as YABE (which is free to download).
6. 1. If the above information is insufficient to determine the problem, capturing network traffic on the MS/TP network is necessary for better analysis. AAON tech support can assist with this process. It requires:
      • A computer with the WireShark application installed.
      • The mstpcap.exe utility installed under WireShark.
      • An RS485 to USB adaptor connected to the computer.

        Capture Network Traffic