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Introduction

BACnet MS/TP network recommendations

  1. BACnet wiring requirements
    • BACnet wiring requirements

    • Use Shielded Twisted Pair cable (0.2 mm2 ).

    • 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.

  2. Position of the termination jumper is shown in the figure to the left. 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.
    • Position of the termination jumper is shown in the figure to the left. 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.

  3. The C35-CBMS-…-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.
    • The C35-CBMS-…-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 left.

    • 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 nonisolated network inoperative.

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