Engineering Documentation

Table Of Contents

Chapter 2 – Network Electrical Systems

RS-485 MS/TP Communications

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125-3002

Before the RS-485 reference terminator is installed, the third wire (yellow) must be

tested with a DMM to insure it is not already connected to earth ground. If the wire is

connected to earth ground the fault condition must be remedied before terminating the

wire using the RS-485 reference terminator.

Fig. 20: Network RS-485 Reference Terminator.

BACnet Nodes on Siemens Controllers or Third-Party

Equipment (Using 1.5 pr cable)

Not all Siemens Building Technologies provide a 3-wire MS/TP network interface. In

order to connect to a 3-wire network use the following diagram as a guide. This guide

may also be used when connecting to third-party controllers that support a 2-wire

interface. If the Master node supports a 3-wire network, then wire the network in the

same manner as the BACnet slaves. The RS-485 common must be referenced to

earth ground through the RS-485 reference terminator (550-974P10) at one end of the

network, (master end preferred).

Depending on the manufacturer, the third wire on 3-wire network interfaces has

several names (for example: Ref, Ground, Com. SC (Signal Common), R (for

Reference), GND, SG (Signal Ground)).

SBT chose the General Ground symbol ( ) as the international symbol for

Equipotential Point versus protective/earth ground or noiseless ground. Some early

BACnet controllers may be marked with the earth symbol ( ) or the S designation.

This pin is not the termination point for the shield of the communications cable.

NOTE:

The symbol ( ), is the symbol being used to represent RS-485 communications

common reference.

Early versions of some controllers may show the earth ground symbol ( ) or the

"S" designation.