MS/TP Bus overview - 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

The BACnet protocol MS/TP communications bus is a local network that connects supervisory controllers and equipment controllers to field point interfaces. The bus is based on BACnet standard protocol ANSI/ASHRAE Standard 135.

The BACnet MS/TP protocol is a peer-to-peer, multiple manager protocol based on token passing. Only manager devices can receive the token, and only the device holding the token is allowed to originate a message on the bus. The token is passed from manager device to manager device using a small message. The token is passed in consecutive order starting with the lowest address. Subordinate devices on the bus only communicate on the bus when responding to a data request from a manager device.

Important: Do not connect MS/TP devices and N2 devices to the same bus. MS/TP communications buses follow different protocol and wiring rules from N2 communications buses, and MS/TP devices and N2 devices are not compatible on the same bus.

An MS/TP bus is used for two types of buses: a Field Controller bus (FC) and a Sensor Actuator (SA) bus (Figure 1). The MS/TP bus can also be extended remotely over the IP network with the addition of a BACnet IP to MS/TP router.

The FC bus, SA bus, and Remote Field Bus are networks of daisy-chained devices. Each bus has only one bus supervisor, depending on which controllers are connected. On a local FC bus, the bus supervisor is the network engine . On the local SA bus, the bus supervisor is a field controller. On the Remote Field Bus, the bus supervisor is the BACnet IP to BACnet MS/TP Router (hereafter called the BACnet Router), which works with the network engine when a Remote Field Bus is installed.

Note: When you install a BACnet Router for adding a Remote Field Bus to the job site, you need a site-level, unique MS/TP network number and device object ID (if it contains a device object) for the BACnet Router. In addition, it is critical that you determine if a BACnet Broadcast Management Device (BBMD) is required for the BACnet Router. If you configure the BACnet Router as a BBMD, assign a static IP address to both the router and the NxE or ODS where you are configuring the MS/TP bus. For an existing installation, contact the BAS Manager, Building Manager, or IT department for information on available network numbers, device object IDs, and existing BBMDs for crossing IP subnets. Information on configuring a BBMD in a BACnet Router (if it supports them internally) is available from the router vendor.

The bus supervisor communicates with devices on the supervised bus and with devices on the next (higher level) bus on the network. The bus supervisor typically starts the communication on the FC bus, remote FC bus, or SA bus. If an SA bus, FC bus, or Remote Field Bus does not have a bus supervisor, the manager device with the lowest device address value on the bus and a specific baud rate selected starts the communication.

The WRG1830/ZFR183x Series Wireless Field Bus System enables wireless communication on an MS/TP bus, allowing you to create wireless connections between Metasys General Purpose Application Controllers (CGMs), VAV Box Controllers (CVMs), Advanced Application Field Equipment Controllers (FACs), Field Equipment Controllers (FECs), or VAV Modular Assemblies (VMAs), Input/Output Modules (IOMs) Expansion Modules (XPMs), and Network Automation Engines (NAEs), Network Control Engines (NCEs), SNCs, and SNEs.

See Enabling equipment controllers for wireless operation and Related documentation for detailed information about the WRG1830/ZFR183x Series Wireless Field Bus System.

Note: The ZFR1800 Series Wireless Field Bus System is not supported on the Remote Field Bus.