Troubleshoot MS/TP network. 6 reasons for troubled communication

by Svetlana Lyons July 24, 2014

Very often we see in the field or get a call from a customer complaining that not all MS/TP devices on their network are visible in their BACnet Explorer or some other network analysis program. Here are some suggestions from our MS/TP experts why you can’t see some devices on a MS/TP network and what to do about it.

1. The most likely reason is that the network is configured with a wrong termination value or the wrong number of network bias resistors.

2. All devices on the network may not be set to the same baud rate.

3. Some devices on the network are configured, or designed, to be MS/TP slaves, and therefore are not discoverable (unless you have an MS/TP slave proxy device).

4. Devices are not properly grounded

5. The wrong wire is used; for example, telephone cable is not appropriate cabling. Something like Belden 485 should be used.

6. There are too many MS/TP devices on one network. To be safe, the number of devices on an MS/TP network should be under 32.

HOW TO TROUBLESHOOT:

MS/TP network problems can be diagnosed using a  U+4 interface with with network monitor/analyzer software such as  BAS-O-matic network analyzer or a free software such as  Wireshark.Instructions on how to use the U+4 with Wireshark and/or BAS-o-matic can be downloaded from this here.

Begin troubleshooting by starting a BACnet Discovery Tool (e.g. BACnet Explorer). Capture for approximately one minute, then examine the captured packets to determine if there are "I-Am" packets from all MS/TP devices (BACnet Explorer broadcasts "Who-Is" packets, and all MS/TP devices should answer with "I-Am"). If the capture shows "I-Am" from some devices but the devices don't appear in Explorer, the problem is in Explorer; however , if the capture doesn't show "I-Am" packets from some devices, there is a problem with the network or devices (e.g. some devices have the same MAC address, or there is a problem with wiring). In this case try connecting only part (e.g. half) of the network and see if you see all the devices on this network. Then connect another part/half. If parts/halves of the network work separaately but together they don’t, most probably there are extra unnecessary network bias resistors or terminating resistors. Other causes can be a very long trunk; too many devices (more than 32) on one trunk; the connection is not properly grounded; or there are MAC address conflicts.




Svetlana Lyons
Svetlana Lyons

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