Trane BACnet MS/TP - Total Number of Trane and Non-Trane Devices on a Bacnet Link

20 BAS-SVX51L-EN
Best Practices
Total Number of Trane and Non-Trane Devices on a BACnet
®
Link
This section describes the best practices when there are only Trane devices on a BACnet
®
link, and when
there are both Trane and non-Trane devices on a BACnet
®
link.
Important: In an all-Trane device network, the maximum number of devices on a BACnet
®
MS/TP link
is 60. Each Tracer
®
SC+ supports three BACnet
®
MS/TP links.
Limit the quantity of devices on a link when there are both Trane and non-Trane
devices on the link.
BACnet
®
MS/TP is not implemented consistently amongst controller manufacturers. Verify that the third-
party device has BTL certification.
• Baud Rate: BACnet
®
MS/TP link baud rate must be set to the same value for all controllers on the
link, which is defined by the device with the slowest baud rate. For example, a third-party device that
is capable of only 19.2k baud will require all BACnet
®
MS/TP devices on the same link be set to 19.2k
baud. Baud rates that are lower than the Trane default of 76.8k baud will adversely affect the
performance of the entire BACnet
®
MS/TP link.
Note: Third-party devices may support auto baud functionality. Trane recommends that the auto
baud feature of a device not be used and the baud rate be manually set to the appropriate
value. Consult the appropriate documentation to determine how to verify and modify the baud
rate as needed.
• Best practice is to place slower devices on a separate MS/TP network.
• Object Lists: The BACnet
®
2008 standard specifies that an object list is required for all BACnet
®
devices. Some non-Trane controllers do not contain a BACnet
®
Object List. Tracer
®
SC+ requires an
object list to exist in a BACnet
®
controller in order to discover the points that reside in the BACnet
®
MS/TP device.
• Impedance: Trane controllers allow for more controllers (60) to reside on a link, compared to the
BACnet
®
MS/TP standard (32). The addition of non-Trane controllers to a BACnet
®
MS/TP link may
reduce the number of devices below the Trane limitation (60). Some non-Trane controllers may be
limited to a quantity of 32 based on their device impedance.
• Integrating non-Trane devices may require a separate link, potentially limited to 30 devices.
Populate BACnet
®
links evenly when 30+ devices are present.
Split BACnet
®
MS/TP links of more than 30 devices equally among the three links on the Tracer
®
SC+.
Table 6, p. 20 contains some criteria to use. The fewer devices there are per link, the better the
communication performance will be. For example, if link 1 has 45 devices, link 2 has 15 devices, and link
3 has 30 devices, we recommend moving 15 devices from link 1 to link 2 so that each link has 30. The
quantity does not have to be exactly the same; an equal amount is optimum but not always practical.
Note: It is not necessary to change links that contain less than 30 devices. Doing so will not significantly
improve performance and may result in unnecessary wiring.
Note: When possible, place non-Trane devices on their own link.
Table 6. Criteria useful in determining when to divide a link
When to divide the link
Baud rate
Placing devices that do not support the 76.8K baud rate on a separate link is
preferred to reducing the baud rate of Trane unit controllers
Critical devices
• Isolate critical devices to their own link
• A critical device is performing intense data logging or TGP2, or displaying lots of
data on Tracer ES graphics
Non-Trane devices Isolate non-Trane devices to contain possible performance issues
Device count greater than 30
• Equally load non-critical links
• There is less value in dividing links when there are fewer than 30 devices
• Performance is proportional to number of devices