Specifications

MS/TP Bus Overview
The MS/TP bus is based on BACnet® standard protocol SSPC-135, Clause 9.
The BACnet MS/TP protocol is a peer-to-peer, multiple master protocol based on token passing. Only master 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 master device to master device using a small message. The token is passed in consecutive order
starting with the lowest address. Slave devices on the bus only communicate on the bus when responding to a data
request from a master 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 (Figure 1) is used for two types of buses: a Field Controller bus (FC) and a Sensor Actuator (SA)
bus. The MS/TP bus can also be extended remotely over the IP network with the addition of a BACnet IP to MS/TP
router (Figure 2).
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 supervisory
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 supervisory
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
NxM 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 master device with the lowest
device address value on the bus and a specific baud rate selected starts the communication.
The ZFR1800 Series Wireless Field Bus System enables wireless communication on an MS/TP bus, allowing you
to create wireless connections between Advanced Application Field Equipment Controllers (FACs), Field Equipment
Controllers (FECs), or VAV Modular Assemblies (VMAs), Input Output Modules (IOMs), and Network Automation
Engines (NAEs) or Network Control Engines (NCEs).
See Enabling Field Controllers for Wireless Operation and Related Documentation for detailed information about
the ZFR1800 Series Wireless Field Bus System.
Note: The ZFR1800 Series Wireless Field Bus System is not supported on the Remote Field Bus.
5MS/TP Communications Bus Technical Bulletin