Specifications

Table 20: Common Problems
Possible CausesSymptom
• Device is failing to respond to the NAE's request.
• Power or other failure occurred at the device.
• Communication is disabled at the device.
• 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.
Devices Go Offline
• The device may be connected to the wrong bus.
• The baud rate of a new device is incompatible with the baud rate of the running network.
• No device on the network is configured to use a specific baud rate (normally the NAE),
but all devices are set to use auto baud. At least one device, typically the bus supervisor
(NAE), must have an assigned baud rate. Set the baud rate in the bus supervisor and
set all other devices to Auto baud.
• Device failed to download Main Code.
• The BACnet Router needs a BBMD defined or the BBMD is defined incorrectly.
• The same Network Number is being used on the MS/TP side of more than one BACnet
Router at the site.
• A BACnet Router has failed or locked up. This problem may occur if you use a router
that is not tested nor recommended.
• Duplicate addresses exist
Device Does Not Come Online
Correcting Physical Bus Problems
The MS/TP bus is subject to a number of physical factors that can affect performance. Consider the following common
physical attributes:
• Check wires.
- Verify proper wire gauge, connections, polarity, and lengths.
- Look for opens and shorts.
• Check terminations.
- Verify that EOL terminations are only at the ends of daisy chains.
• Check addresses.
- Check for duplicate addresses.
- Verify that the address range is sequential.
• Check for and eliminate T-Taps (wire configurations that create a T shape) and star configurations.
• Check for consistent baud rates.
• Check for sources of interference.
Correcting Bus Overload Problems
Excessive data communication on the MS/TP bus can degrade system performance. To reduce the load on the bus,
consider the following:
• Unmap points you do not need.
•
Minimize the number of COVs on the bus; see Counting the COVs.
- Check and increase the COV increment for noncritical points.
- Use a timed COV interval for fast changing points. Set the Use COV Min Send Time attribute to True.
- Lengthen the COV timed interval COV Min Send Time 10-255 in the device object. The default is 15 seconds.
54MS/TP Communications Bus Technical Bulletin: Appendix: Maximizing and Troubleshooting the MS/TP Bus