Bacnet Ms/Tp Data Transmission Essentials; Bacnet Ms/Tp Data Bus Is Polarity Sensitive - Distech Controls ECL/ECB-PTU Series User Manual

BACnet MS/TP Data Transmission Essentials

The BACnet MS/TP data bus protocol is part of the BACnet® ANSI/ASHRAE™ Standard 135-
2008 that uses the EIA-485 (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
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 envi-
ronments. 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
ted when it is connected to the data bus through a repeater (see
the Data Bus on page 28).
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 termina-
tion resistors (typically 120 ohms). Cable impedance is usually specified by the cable manu-
facturer.

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.
Table 1: Common Identification Labels for BACnet MS/TP Data Bus Polarity for Distech Controls' Products
Distech Controls Product
ECB Series Controllers
ECB/ECL-VAVS
ECB/ECL-VAV
ECB-PTU Series Line-Powered
Controllers
EC-BOS
Thermostat
Repeater
12
Figure 22.
Typical Data Bus Connection Terminals
Inverting
NET -
NET -
NET -
-
-
Data-
Data1-
Figure 5). A spur is only permit-
Using Repeaters to Extend
Non-inverting Reference
NET + 24V COM
NET +
(Ground symbol)
NET + COM
+ S
+ Ref
Data+ N/A
Data1+
Network Guide