Specifications

If you anticipate critical peer-to-peer communication and therefore do not want field controllers to wait for the bus
supervisor to establish the baud rate, you can specify the baud rate for each device immediately at startup. Choose
the baud rate carefully if you use this method, since changing it requires changing each device separately.
TEC26xx Series thermostats automatically set their baud rate to the bus supervisor’s baud rate.
Typically, the baud rate setting on bus repeaters and third-party MS/TP devices is configured manually at the device,
and the baud rate setting must match the bus supervisor’s baud rate. A third-party device that does not support
auto-baud establishes the baud rate for the MS/TP network if the NxE is not connected. Refer to the manufacturer’s
product documentation for information on setting the device’s baud rate.
The Metasys software contains the following two device attributes that relate to the baud rate:
• Baud Rate Selection allows you to set the baud rate for the device using the System Configuration Tool (SCT)
for NAEs, or the Controller Configuration Tool (CCT) for FACs, FECs, VMA16s, and IOMs.
• Active Baud Rate allows you to view the baud rate at which the device is communicating on the active bus when
Auto baud is selected for the device.
For more information on Metasys system attributes, refer to the Metasys system Help (LIT-1201793).
The high baud rates capable on MS/TP buses limit the range of wire gauges used on the bus. The baud rate, wire
gauge, wire length, and the number of devices are related. Higher baud rates support more devices but require small
gauge wire (which provides lower capacitance). A lower baud rate may be required to use existing, larger gauge
wire (which has higher capacitance) but may support fewer devices. We recommend 38,400 baud using 22 AWG
stranded wire. This combination provides the best balance between performance and installation sensitivity.
For information on determining wire gauges, wire lengths, and the number of devices supported, see MS/TP Bus
Cable Recommendations.
Device Addresses on the MS/TP Bus
Each device connection on an MS/TP bus requires a device address to coordinate communication. Each bus has
a set of device addresses that is separate and independent from the device addresses on all other buses. Devices
connected to both an MS/TP bus and SA bus have two device addresses, one for each bus connection (Figure 7).
In the MS/TP bus hierarchy, device connections on separate buses can have the same device address. For example,
every bus supervisor connection on an MS/TP bus has a device address of 0 (zero), and the device address for the
first network sensor on any SA bus is 199. This is possible because separate buses are identified with different
network numbers. Figure 7 shows a simple example of an MS/TP bus and the device addresses for connections on
the FC bus and SA bus.
An NAE, NCE, or BACnet Router is the bus supervisor on an FC bus (or Remote Field Bus). The FAC, FEC, VMA16,
or the field controller on an NCE is the bus supervisor on an SA bus. Bus supervisors have a fixed device address
of 0 (zero) that cannot be changed (Figure 7). Depending on the model, a network sensor has a fixed address of
199 or an assigned (switch-selectable) address between 200 and 203. Table 4 provides a list of the valid MS/TP
device address values and address value ranges for MS/TP devices.
Each MS/TP master controller passes the token to the controller with the next known address. After 50 token passes,
each controller searches for other controllers that might have joined the network by attempting to send the token to
the controller with the next consecutive address, starting with one higher than its own, until one is found. While you
do not need to address devices on the trunk consecutively, you can improve performance by minimizing address
skipping. To help with address value selection, see Table 4.
Note: The devices on the bus do not need to be physically wired in sequential order.
Setting a Device Address
For most devices on an MS/TP bus, the (non-supervisory) device address is set by positioning the DIP switches on
the device’s ADDRESS DIP switch block. The DIP switch blocks are binary switch blocks, with each switch
representing a binary numerical value when the switch is in the ON position.
18MS/TP Communications Bus Technical Bulletin