Specifications

Remote Field Bus Rules and Specifications
Table 10 provides rules for the Remote Field Bus (RFB). In addition to these rules:
• Configure the BACnet Router before you connect it to the building network. If the BACnet Router is commissioned
using IP, use an Ethernet patch cable.
• For each Remote Field Bus you add to an NAE, make sure the trunk number has not been used on the NAE.
Enter a Network Address between 1 and 65534 that is unique for the site. Use trunk numbers 3 to 20 to configure
Remote Field Buses to the NAE. (Trunk numbers 1 and 2 are reserved for the local field buses.)
• Connect the MS/TP port of the BACnet Router to only one bus segment on the Remote Field Bus. Only a
daisy-chain topology is allowed on the Remote Field Bus (no T or Star topology configurations). See Figure 9
for an example of bus segments.
• If you create a new remote field bus with SCT by copying an existing remote field bus, you need to change the
Remote Field Bus trunk number because the number in the copied bus always reverts to 1.
• BACnet Broadcast Management Devices (BBMDs) may be required to allow BACnet IP broadcast messages to
reach the BACnet Router. BACnet IP broadcast messages are used to determine the BACnet Router's IP address.
• Define only one Remote Field Bus for each BACnet Router. Performance may be severely reduced if more than
one Remote Field Bus or integration is assigned to the same BACnet Router.
• Do not split the mapping of remote MS/TP devices from a single BACnet Router into more than one Remote
Field Bus or integration. Use only one integration to map all devices from the MS/TP side of the BACnet Router.
Table 10: Remote Field Bus Rules
Rules/LimitsCategory
Total number of field buses (RFB and FC bus):
NAE55: 8 (6 RFB + 2 FC or 7 RFB + 1 FC)
NAE45: 4 (3 RFB + 1 FC)
NAE35: 4 (3 RFB + 1 FC)
NCE25: 1 (1 FC; does not support Remote Field Bus)
NAE85: 16 RFB only
NAE-Lite: 2 (1 RFB + 1 FC)
ODS: 16 RFB only
General
Total number of remote field devices per Router:
32: if all connected devices are Metasys products
16: if one or more connected devices are manufactured by a third-party vendor
Note: To take advantage of MS/TP diagnostics, you must add at least one Metasys field
device (FEC, FAC, VMA16, or IOM) to the Remote Field Bus as an FEC Family
device.
Number of Devices and Bus
Segments
Bus segments on a Remote Field Bus are connected with repeaters (only). Up to two cascaded
repeaters may be applied to a Remote Field Bus (to connect three bus segments).
31MS/TP Communications Bus Technical Bulletin