Tips For Correcting Bus Installation Problems - Johnson Controls MS-FIT100-0 User Manual

1/7/2021
Voltage
Bit Voltage
Differential
Voltage, Total
Voltage
1
These voltages should typically not be read with a Volt-Ohm-Meter (VOM). Use the
FIT instead. Idle bus voltages are read with a VOM when communication is not
occurring. When FIT is connected to a bus and then powered on, it will display
Offline if the bus is Idle.

Tips for Correcting Bus Installation Problems

The MS/TP Bus is subject to a number of installation factors that can affect
performance. Consider the following common physical attributes when trying to
resolve communication problems:
Validate controller/device power is available and undisrupted.
Check wiring for:
- Proper wire gauge, cable type, connections, polarity, and lengths.
- Opens and shorts.
- Confirmation that one shield ground point exists per bus segment
(made at the source of the bus segment —the NAE, Side B of a Repeater).
Check EOLs to verify that EOL terminations are only at the ends of daisy chains
for each bus segment.
Check addresses for:
- duplicate addresses
- address ranges are sequential
Check for and eliminate T-Taps (wire configurations that create a T shape) and
star configurations.
Check for consistent baud rates (often baud rates problems occur with vendor
devices and repeaters).
Check for sources of electrical interference.
https://docs.johnsoncontrols.com/bas/internal/api/webapp/print/b4118696-0b8a-4733-98fb-fdd04b065b15
Communication
State Meaning
Active + or – with respect
to COM when
communication is
occurring.
Active The + to COM
Reading plus the
absolute value of
the - to COM
Reading when
communication is
occurring.
PDF Export
Acceptable Range
+2.0 to +4.0 VDC (+ to
Com), -2.0 to -4.0 VDC (- to
Com). When comparing the
absolute value of these two
readings, there should be
no more than
approximately a 0.2 VDC
difference.
2.5VDC to 8VDC is
acceptable.
1
33/60