Voltage Levels; Communication Voltage Levels - Johnson Controls MS-FIT100-0 User Manual

Voltage Levels

There are two types of voltage levels read by the FIT.
• Communication Voltage Levels—The voltage when communication is active (also called BitVolts).
• Idle Voltage Levels—The voltage when communication is inactive. The FIT can still read this
voltage on an active communication bus by taking a reading when communication pauses.

Communication Voltage Levels

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 16.
Note: The waveform is uniform above and below the 0 V line which represents the COM line.
The bus EOL termination provides approximately 0.4 VDC of bias voltage to keep the bus in the
high state when communications are idle. The EOL also provides proper bus termination.
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
MS-FIT100-0 Field Inspection Tool User Guide
Figure 16: Communication Voltage Example
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