Remote Field Bus restrictions - 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

Compared to the local MS/TP field bus, the Remote Field Bus has several operational restrictions.

Number of equipment 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 1). 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.
Note: While the router may support at least 10 BACnet MS/TP devices, broadcast messages are put on the MS/TP bus through the router site so the controllers may not be able to handle the broadcast receive rate.

Maximum Change of Values (COVs)

The total number of COVs per network 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 equipment controller on the bus. The FDD tool helps you find chatty devices and helps decrease the COV rate by updating the COV increments.

Equipment controller Main Code release

For Remote Field Bus compatibility, the Main Code for the equipment controllers you connect to remotely must be at Metasys system Release 5.3 or later. Remote field bus communication is not as robust with controllers that are running older releases of the Metasys system.

CCT Passthrough loading

Generally, you can upload and download code and applications with CCT Passthrough for devices directly connected to a Remote Field Bus (subject to router message load levels). However, you cannot use CCT Passthrough to download code to an IOM connected to the SA bus of an equipment controller. In these situations, use a Wireless Commissioning Controller at the remote location.

Device and point mapping

When you define a Remote Field Bus, be sure to map all devices and points at the remote controllers through the same Remote Field Bus. For most networks, a detrimental increase in router traffic occurs if multiple integrations use the same router and network number. This is true regardless of where the multiple integrations are defined or if a BACnet Integration is used.