Specifications

Figure 5: Wiring BACnet Router
Remote Field Bus Restrictions
Compared to the local MS/TP field bus, the Remote Field Bus has several operational restrictions.
Number of Field Controllers on a Remote Field Bus
The number of controllers that can successfully operate on a Remote Field Bus is generally less than that of a local
field bus, and is heavily dependent on the capabilities of the router used. Generally, a good quality router can support
at least 10 BACnet MS/TP devices, and some can support more. (See Table 10.) If you notice that devices go offline
randomly, or you notice a large number of BACnet message retries, you may have exceeded the maximum number
of supported devices for the router. You may be able to use a larger number of devices if you mitigate the message
traffic that originates from field devices located on the Remote Field Bus.
Maximum Change of Values (COVs)
The total number of COVs per supervisory engine must remain at less than 500 per minute. To determine and tune
the number of COVs per minute on a Remote Field Bus, use the FDD tool, but only if you have at least one
Johnson Controls field controller on the bus. The FDD tool helps you find chatty devices and helps decrease the
COV rate by updating the COV increments.
14MS/TP Communications Bus Technical Bulletin