Baud rates on the MS/TP Bus - 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

An MS/TP bus can be configured to communicate at one of four different baud rates. All of the devices on an MS/TP bus must communicate at the same baud rate.

The baud rate setting determines the rate at which devices communicate data over the bus. The baud rate settings available on Metasys MS/TP devices are 9600 bps, 19.2 kbps, 38.4 kbps, 76.8 kbps, and Auto. The baud rate setting for Metasys devices is set in the Metasys software.

Note: The BACnet Standard only requires support for 9600 and 38400 baud. Some third-party vendor devices might not support all the baud rates supported by the Metasys system. Reducing the baud rate to accommodate these devices affects overall performance.

We recommend setting all MS/TP bus supervisors to 38,400.

We recommend setting all equipment controllers on the FC bus or Remote Field Bus (CGMs, CVMs, FACs, FECs, VMAs, XPMs, and IOMs ) to Auto. In the Auto setting, the Metasys devices listen for the bus supervisor to communicate first; the devices then automatically set their baud rate to the bus supervisor’s baud rate. CGMs, CVMs, FACs, FECs, VMAs, XPMs, and IOMs ship with a default baud rate setting of Auto.

If you anticipate critical peer-to-peer communication and therefore do not want equipment 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.

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 or SNE 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 SNE or NAEs , or the CCT for CGMs, CVMs, FACs, FECs, VMAs, XPMs, 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 that provides lower capacitance. A lower baud rate may be required to use existing, larger gauge wire that 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.