The FIT can measure the voltage levels on the RS-485 MS/TP communications on the FC/SA bus. The RS-485 communications produce a differential voltage between the + and – communication wires. The differential voltage is the addition of the + wire voltage with respect to the COM wire and the – wire voltage with respect to the COM wire. The FIT will measure the (+, COM) voltage and the (–, COM) voltage, and the sum of these voltages is the differential voltage. If you view the differential voltage of a RS-485 communication on an oscilloscope, it resembles Figure 17.
When reading an FC Bus with multiple controllers, this Communication Voltage is the reading displayed by the FIT during the Monitor or Scan modes. It can also be displayed though the Voltage menu in submenu item BitVolts, but the reading displayed through the Voltage menu is from the controller on the bus that is communicating at the time of the sample was read when no address is provided by the FIT. Ideally, the + to COM and – to com readings should have an absolute value almost equal and no more than approximately 0.2 VDC apart. When the two readings have an absolute value greater than approximately 0.2 VDC difference, it could mean there is a loose connection on the bus cable at the controller, some other cable problem near the address displayed, or a problem with the controller itself.