Bacnet Ms/Tp Communication Data Bus Fundamentals; Bacnet Ms/Tp Data Transmission Essentials; Bacnet Ms/Tp Data Bus Is Polarity Sensitive - Acuity Controls nLight ECLYPSE User Manual

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Table of Contents

CHAPTER 11

BACnet MS/TP Communication Data Bus

Fundamentals

This chapter describes the BACnet MS/TP Communications Data Bus operating principles.

BACnet MS/TP Data Transmission Essentials

Certain ECLYPSE controllers support BACnet MS/TP to BACnet/IP routing. See the Controller's datasheet for more infor-

mation. To enable BACnet MS/TP to BACnet/IP routing, see Routing.

The BACnet MS/TP or Modbus RTU network option is selected in the controller's web interface. BACnet MS/TP and Mod-

bus RTU communications are made by connecting directly to separate RS-485 ports. When the ECLYPSE Controller is

configured for BACnet MS/TP, values from the connected BACnet MS/TP controllers can be used in ENVYSION graphics

hosted on the ECLYPSE Controller. Furthermore, the ECLYPSE Controller acts as a BACnet/IP to BACnet MS/TP bridge

that allows BACnet objects to be shared among BACnet intra-networks through BBMD. See

ment Device Service

(BBMD).

The BACnet MS/TP data bus protocol is part of the BACnet

(RS-485) physical layer standard for data transmission (herein called the data bus). Multiple data buses can be logically

tied together as each BACnet MS/TP data bus is assigned a unique Network Instance that distinguishes it from other data

buses in the BACnet MS/TP Local Area Network (LAN). An example of an interconnected BACnet MS/TP data bus is

shown in

Adopting a Numbering System for MAC Addresses, Device Instance Numbers, and Network

EIA-485 is a standard that defines the electrical characteristics of the receivers and drivers to be used to transmit data in a

differential (balanced) multipoint data bus that provides high noise immunity with relatively long cable lengths which makes

it ideal for use in industrial environments. The transmission medium is inexpensive and readily-available twisted pair

shielded cable.

While there are many possible LAN topologies for an EIA-485 data bus, only devices that are daisy-chained together are

allowed with BACnet MS/TP (see

is connected to the data bus through a repeater (see

End-of-line (EOL) terminations are critical to error-free EIA-485 data bus operation. The impedance of the cable used for

the data bus should be equal to the value of the EOL termination resistors (typically 120 ohms). Cable impedance is usu-

ally specified by the cable manufacturer.

BACnet MS/TP Data Bus is Polarity Sensitive

The polarity of all devices that are connected to the two-wire BACnet MS/TP data bus must be respected. The markings to

identify the polarity can vary by manufacturer. The following table summarizes the most common identification labels for

BACnet MS/TP data bus polarity.

Device Manufacturer

Common identification labels for BACnet MS/TP data bus polarity by other

Manufacturers

Table 4: Common Identification Labels for BACnet MS/TP Data Bus Polarity for other Manufacturers

When interfacing with BACnet MS/TP devices from other manufacturers, refer to the documentation provided with the device to correctly wire

the device.

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Only a Daisy-Chained Data Bus Topology is

Using Repeaters to Extend the Data