Specifications

Check for duplicate addresses in the following ways, depending on the severity of the situation:
• If bus performance is degraded, check the address switch settings at the devices with unreliable communications.
• If the bus communications problems are severe, and there is no communication or you cannot determine where
communication is unreliable, partition (disconnect and isolate a portion of the bus for testing purposes) and test
the bus portion connected to the NAE (or Wireless Commissioning Converter [MS-BTCVT-1]).
Common Problems
Several factors can influence the behavior of the MS/TP bus. In addition, certain problems can affect the bus in
multiple ways and have more than one symptom, making the exact diagnosis difficult. For example, duplicate
addresses on the bus can degrade performance, make a device go offline, or stop communication completely. Table
20 lists common problems that you should consider when troubleshooting an MS/TP bus. The first row, Multiple
Symptoms, lists possible problem causes that can have multiple symptoms.
Table 20: Common Problems
Possible CausesSymptom
• Excessive bus errors are occurring.
• A device was added or changed with a duplicate address (may not be the same address
as some devices having problems, and may have happened some time before the
problem was noticed).
• Wiring errors or wire problems exist.
• The baud rate was changed on some devices on the network, but not all devices.
• Max Master was changed incorrectly (this may have happened some time before the
problem was noticed).
• A download is in progress.
• There is a fault at a device.
• A repeater is needed or configured incorrectly.
• There is a duplicated Device Object name or instance.
• The EOL termination is improperly set.
• The BACnet Router used with a Remote Field Bus is defective or has poor performance.
• The same Network Number is being used on the MS/TP side of more than one BACnet
Router, NAE, or third-party gateway at the site.
Multiple Symptoms
• Excessive bus traffic exists (bus overload).
• Baud rate may be set too low.
• Too many devices may exist on the network.
• Unaccounted devices are on the network (that is, not mapped to the NAE).
• Unusually slow devices are on the network or devices that are slow to pass the token.
• Multiple integrations (Remote Field Bus integration or BACnet integration) are using
the same BACnet Router.
• More than one supervisory device is using the same BACnet Router.
Poor Performance
53MS/TP Communications Bus Technical Bulletin: Appendix: Maximizing and Troubleshooting the MS/TP Bus