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Best Practices
The following sections contain the best practices for various elements of the BACnet
Configuring and Wiring the BACnet
This section describes the best practices for configuring and wiring a BACnet
recommendations and techniques to get the best results.
Wire and Network Characteristics
Does the wire that has been strung through the building have the correct physical characteristics? All wire
is not created equal and is designed to fulfill a specific application. In this case, the wire is being used to
convey information via small changes in voltage that occur at specific time intervals. Wire that has the
wrong characteristics may result in failure of the network. The wire used should have the following physical
characteristics. This information is typically marked on the label of the wire spool or contact the supplier
for this information.
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Understanding Non-isolated and Isolated MS/TP Device Types
All BACnet
(plus shield) communication cable. A device that requires two wires is a non-isolated device. A device that
requires three wires is an isolated device. The terms isolated and three-wire, or non-isolated and two-wire
are used interchangeably in the field. Trane BACnet
The term isolation refers to the electrical design of the EIA-485 transceiver on the device. With an isolated
device design, the input signal circuitry is completely separate from the rest of the circuitry on the device.
Power and ground are not shared and the transceiver input "floats" at some unknown voltage. In order
to guarantee that two or more isolated devices can communicate, a common reference voltage is needed
between the input circuitry of all devices. The third wire of a 3-wire cable is used to provide this voltage
reference. The reference wire is not grounded anywhere in the system. See
A non-isolated device design shares the power and ground between the input signal circuitry and the rest
of the circuitry on the device. In this case, the reference voltage is provided by grounding the device to
the enclosure. The ground wire of each enclosure will run back to a common, earth ground elsewhere in
the building.
Table 1.
• Tracer
• Tracer
• Tracer
• Tracer
• BCI-I, BCI-R, BCI-C
• Symbio™ 700
• Symbio™ 800
1
The electrical impedance value of Trane purple wire is less than the specified range. Trane has tested this wire and authorizes its use for BACnet
8
Style—Braid or foil shielded, twisted pair wire.
Impedance—100 to 130 ohms
Capacitance—between any two conductors, less than 100 pF per meter.
Capacitance—between any conductor and shield wire, less than 200 pF per meter.
Maximum Length—4000 feet with 18 AWG conductor wires.
Maximum Length—2000 feet with 22 AWG conductor wires.
®
MS/TP devices are designed to communicate via either a two-wire (plus shield) or three-wire
Trane non-isolated and isolated devices
Non-isolated Trane Devices
®
UC400
®
UC600
®
UC800
®
SC+
MS/TP networks.
®
MS/TP Link
1
.
®
MS/TP devices are largely non-isolated.
Isolated Trane Devices
• Trane Communicating Thermostats.
• TR200.
®
MS/TP link.
®
MS/TP link. Follow these
Figure 2, p.
9.
®
BAS-SVX51L-EN
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