1-On-1 Communication Voltage Levels; Network Communication Voltage Levels - Johnson Controls MS-FIT100-0 User Manual

1-ON-1 Communication Voltage Levels

In the 1-ON-1 mode, the FIT is only connected to a single controller that is not connected to a
network. The communication voltage readings in this mode should be in the range of 4–10 VDC
differential voltage and 2 to 5 VDC for the +, – line voltages. The important thing to note is that
the + and – line readings typically are equal or within approximately 0.2VDC of each other. The +
line voltage will usually read equal to or slightly higher than the – line voltage. See Figure 20 for an
example.
If the readings taken are below 4V differential or the + and – readings are not within 0.4VDC of each
other, then the controller may be damaged. Damaged controllers typically have a larger voltage
difference (greater than 0.4VDC) between + and - readings. See Figure 21 for an example.

Network Communication Voltage Levels

When the FIT is connected to a network of controllers it will measure the communication voltage
levels of each individual controller. When looking at communication voltage levels on a network,
it is important to note that the voltage levels are affected by many factors, such as wire length,
wire gauge, number of controllers, network topology, EOL terminations, and the FIT connection
on the bus. The FIT's connection location affects the voltage readings taken. The controllers closer
to the FIT typically have higher voltage readings than the controllers farther away from the FIT. It
is therefore important to take note of the network location where the FIT is connected since the
voltage readings are affected.
Differential voltage levels in the range of (2.5V to 8V) are expected on most properly functioning
buses. The + line voltage and – voltage should be approximately equal or within 0.2VDC of each
other on a properly wired and terminated bus.
Since the FIT is reading and calculating these voltages, they are only accurate for the bus segment
to which the FIT is connected. If using communication voltages to troubleshoot, each bus segment
should be read independently. Voltage readings where there are large voltage differences between
the + line and – line can indicate a wiring problem such as a shorted, pinched, or open wire
connection.
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Figure 20: 1-ON-1 Communication Example 1
Figure 21: 1-ON-1 Communication Example 2
MS-FIT100-0 Field Inspection Tool User Guide