The communications on the BACnet MS/TP (RS-485) bus occurs at a user-defined baud rate. You must configure the baud rate for the automation server RS-485 bus(es) as part of the commissioning process for the RP and MP controllers and RP-IO modules that support MS/TP communications.
The RP and MP controllers and RP-IO modules support MS/TP baud rates of 9600, 19,200, 38,400, and 76,800 baud. The higher baud rate translates into higher performance observed in terms of refresh rate for information presentations, number of variables transferred between controllers or host, time taken to download the database or upgrade the firmware, and so on. However, as you increase the baud rate, you will approach the point where the network wiring characteristics (resistance, capacitance, reflected signals from insufficient termination, number of nodes, and so on) begin to cause an excessive number of detected errors at the baud rate. For short RS-485 buses, or longer buses with good bus wiring characteristics, the highest supported baud rate of 76,800 baud may be a practical choice. A speed of 38,400 baud is suggested as the preferred baud rate. Lower baud rates may be applicable when you have indications that the detected communication errors are not caused by other settings, cables, or controller deficiencies that cannot be corrected. Lower baud rates are more “forgiving” of the bus wiring characteristics.
On the RP and MP controllers and RP-IO modules, the RS-485 port dedicated (or reconfigured) for MS/TP communications should be connected to an operating BACnet MS/TP (RS-485) bus from an automation server that has the baud rate configured to the baud rate planned for the MS/TP network. When power is applied to the controller or I/O module, the MS/TP baud rate is detected, and the RP and MP controllers and RP-IO modules adjust to the MS/TP bus baud rate and the devices are discovered on the MS/TP network in the automation server.
After the controller or I/O module detects and adjusts the MS/TP baud rate in the device, an automatic adaption of the baud rate will not be performed again, without operator activation. This operation helps secure the selected baud rate and avoids inadvertent changes in the baud rate later in system installation or operation when such changes may be difficult to recognize and recover. If the MS/TP capable RP or MP controller, or RP-IO module, was previously connected to an MS/TP bus operating at a different baud rate, or if the status of the controller or I/O module is unknown, you can reactivate the one-time automatic baud rate adjustment by performing a network reset on the device. The network reset is achieved by pressing the reset button (on a powered device) for a period of 10 to 19 seconds. The network reset should be performed when you find that the controller or I/O module added to the network is not discovered by the MS/TP network in the automation server.
For more information, see RP Controller Reset Modes .
For more information, see MP Controller Reset Modes .
For more information, see IP-IO and RP-IO Module Reset Modes .