Remote Field Bus rules and specifications - Metasys - LIT-12011034 - General System Information - Metasys System - Release 11.0

MS/TP Communications Bus Technical Bulletin

Product name
Metasys System
Document type
Technical Bulletin
Document number
Release 11.0
Revision date

Table 1 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 or SNE, make sure the trunk number has not been used on the NAE or SNE. 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 or SNE. (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). For an example of bus segments, see Figure 1.
  • 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 1. Remote Field Bus rules




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)

OAS: 18 RFB only

ODS: 16 RFB only

SNC2515x: 4 (3 RFB + 1 FC)

SNC1612x: 4 (3 RFB + 1 FC)

Note: The SNC requires Release 11.0 or higher to support the Remote Field Bus integration.

SNE22xxx: 8 (6 RFB + 2 FC or 7 RFB + 1 FC)

SNE11xxx: 4 (3 RFB + 1 FC)

SNE105xx: 4 (3 RFB + 1 FC)

Number of devices and Bus segments

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 equipment controller (CGM, CVM, FAC, FEC, VMA, IOM, or XPM) to the Remote Field Bus as a Metasys Family device.

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).

Cable length for FC Bus and Bus segments

When all of the devices connected on the Remote Field Bus are Metasys CGMs, CVMs, FACs, FECs, VMAs, IOMs or XPMs, the cable length limits (using 22 AWG 3-wire twisted, shielded cable) are as follows:
  • Each bus segment can be up to 1,520 m (5,000 ft) in length.
  • Each RFB can be up to 4,570 m (15,000 ft) in length.
When one or more third-party family device is connected on the Remote Field Bus, the device and bus segment limits are as follows:
  • Each bus segment can be up to 1,220 m (4,000 ft) in length.
  • Each RFB can be up to 3,660 m (12,000 ft) in length.
Note: If third-party devices are connected to the bus, the cable lengths should be reduced (if necessary) to match the third-party vendor recommendations.
Note: Do not place subordinate devices on the Remote Field Bus. Subordinate devices on the Remote Field Bus are not supported.

When using fiber-optic connections: 2,010 m (6,600 ft) between two fiber modems.

Network number and instance number for router

BACnet Network Number assigned to the IP side of the Router must match the BACnet IP network number used in the supervisory engines. The default IP network number is 1001. Do not use 2000 or 65000 as the network number; these numbers are used for existing embedded networks.

BACnet Network Number assigned to the MS/TP side of the BACnet Router must be unique on the site across the entire BACnet Site. This may include multiple buildings, cities, states, and even countries. It is very important to know the network to which you are adding one of these routers.

Device Instance Number assigned to the BACnet Router must be unique on the site.

BACnet router settings

Follow the manufacturer's instructions to configure the BACnet Router’s IP address, BACnet Device ID, MS/TP Network Number, and BBMD. Note that some routers do not have a device object in which a device name and id is required.

Set Max Info Frames = 80 to 100 (all devices are Johnson Controls field devices) or 20 (some or all devices are third-party field devices)

Recommended cable

22 AWG Stranded, 3-Wire Twisted, Shielded Cable

Note: The recommended cable type provides the best bus performance. See MS/TP Bus cable recommendations for information on alternative cable types and lengths that may be used in MS/TP applications.

EOL termination

The EOL switch must be set to On (or an EOL terminator installed) on the two devices located at either end of each bus segment on a Remote Field Bus. Use an EOL terminator for the BACnet Router as it has no EOL switch. The EOL switches must be set to Off (or EOL termination disabled) for all other devices on the bus segment on a Remote Field Bus. See EOL termination on the Remote Field Bus for more information.