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17.2. Termination jumper
Position of the termination jumper is shown in the figure below. The termination jumper should
be connected on the first and last device on data bus segment. When the jumper is connected
termination is enabled and a 120Ω resistor is connected between RS-485 nodes. When the
jumper is removed the termination of the cable is disabled.
17. BACnet MS/TP network recommendations
17.1. Wiring and topology
BACnet wiring requirements
•
Use Shielded Twisted Pair cable (0.2 mm
2
).
•
Maximum cable length for a data bus segment must not exceed 1200 m.
•
Only a daisy-chained data bus topology as shown in drawing is acceptable.
•
Connect cable shields together. Connect cable shield to the Protective Earth (PE) at one
point only.
•
Connect termination jumper on the first and last device on the data bus (see below).
•
Maintain same polarity between devices on a single power supply.
17.3. Maximum number of devices on data bus
The C35-VAV-…-B/RS is categorized as BACnet MS/TP Master device. The maximum
number of Master devices supported by BACnet MS/TP on one data bus segment is 128.
However, it is recommended that no more than 50 devices are connected to one data
bus segment. A BACnet router device should be used for projects where more data bus
segments need to be connected, shown below.
17.4. Isolated RS485
Devices with RS-485 with galvanic isolation are marked with suffix /RS-G. Galvanic
isolation is required if there is no guarantee that the potentials at the earth grounds
of different nodes in the system are within the common-mode range of the receiver.
The benefits of galvanic isolation extend beyond safety and protection from dangerous
voltages. They also provide error-free communication in the presence of high edge rate
transients, noise and high common mode voltage that would otherwise render a non-
isolated network inoperative.
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