In this initial power on mode, the FIT is just monitoring FC/SA bus communications and is not actively participating in communications.
The Bus Health consists of eight bars indicating the health of the FC/SA communication bus. The higher number of bars the higher the health. When the FIT is first connected to the bus, the Bus Health starts out at eight bars and then the communications are monitored for errors. The number of bars is reduced if communication errors are occurring. Communication errors can occur for various reasons such as noise from electrical sources, long wire lengths, heavily loaded buses, loose terminations, duplicate addresses, or improperly wired buses. Some communication errors are tolerated as long as they do not cause issues such as controllers going online and offline, erratic communications, or poor system control.
A Bus Health reading of eight bars is obviously desired, but it is considered acceptable to have a Bus Health reading of four bars or more. Bus Health is basically calculated by counting communication errors over time. A Bus Health of four bars equates to two communication errors per minute, which is tolerable for FC/SA communications.
When the FIT powers on, it may display No MSTP. This can mean the FIT is seeing bus traffic, but a token is not passing between addresses. This occurs when one or multiple FEC/VMA controllers on the bus have an address switch position eight (normally labeled 128) in the ON position. This switch is not to be used on a hard-wired Johnson Controls MS/TP Bus. It is reserved for Wireless bus installations. If you see this condition, validate that all FEC/VMA MS/TP hard-wired controllers have the 128 switch in the OFF position.
No MSTP can also be displayed if an MS/TP bus which is active has installation errors significant enough for the FIT to fail determining a baud rate. If this condition is experienced, use the FIT’s Voltage > IdleBus to validate proper voltages (see Voltage Levels for more information). In addition, the FIT’s baud rate could be adjusted from the default autobaud in the System Menu to the specific baud rate of the bus that is being tested (see System Menu for more information).