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BACnet MSTP troubleshooting

BACnet MS/TP troubleshooting and exploring with U+4 interface and BACnet Explorer

July 16, 2019

Do you know which devices have duplicate addresses on a network? Do you know where to look if there is no activity on the BACnet MS/TP network?

To answer these and other similar questions, you need to have the appropriate tools: One to discover all the devices on a network (e.g. BACnet Explorer) and one (e.g. BAS-o-matic or Wireshark) to actually capture the packets.

But first, what is BACnet MS/TP? BACnet defines the MS/TP (master-slave/token-passing) as a network designed to run at speeds of 115,200 bits per second or less over twisted pair wiring. MS/TP is widespread, easy to set up, and doesn't require expensive infrastructure. As with any other network, problems may arise. In the article "Troubleshoot MS/TP Network" we outline six reasons for troubled communication on a BACnet MS/TP network. If you work with this network, you need to monitor its health and troubleshoot if there are any problems. 

We have a special device for this task called U+4 USB to BACnet MS/TP Interface. The U+4 device helps connect a computer or laptop to a BACnet/MSTP network. The USB port on the U+4 connects to the PC, and the screw terminals help connect the BACnet/MSTP network to the U+4 hardware. This allows applications like BACnet clients (e.g., Cimetrics BACnet Explorer) and diagnostic programs (e.g., Cimetrics BAS-o-Matic or Wireshark) access to the BACnet/MSTP network.

Cimetrics U+4 MSTP

Using a simple adapter may seem like an obvious choice to access the network, but we have identified several problems that may arise when using such devices. First, simple adapters convey a stream of bytes from the wire to the Windows Operating System and even transmit it reliably. But, inevitably, they cannot keep track of the timing of the bytes which may lead to incorrect packet assembly or even phantom packets. This can occur more frequently if the network is noisy and bytes get distorted during transmission. Lastly, although you can monitor data through a simple adapter you cannot transmit the data.

There are also solutions that use USB to UART converters with the MS/TP implementation running inside of a PC. Because Windows is not a real-time system, such implementations cannot work reliably and may create havoc on the MS/TP network when they respond outside of the real-time requirements. The U+4 has a microprocessor that handles the time-critical functionality on the MSTP network.

Once installed, the U+4 mimics an Ethernet card. The user can choose this newly installed “Ethernet” card for communications with a BACnet MS/TP network.

To learn how you can configure U+4 and use it with BACnet Explorer, BAS-O-Matic or Wireshark, please check the documentation tab on U+4 page, the Explorer manual, and the following video:

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