12 Network Guide
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 Figure 22.
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 Figure 5). A spur is only permit-
ted when it is connected to the data bus through a repeater (see Using Repeaters to Extend
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
Typical Data Bus Connection Terminals
Inverting Non-inverting Reference
ECB Series Controllers
ECB/ECL-VAVS
NET - NET + 24V COM
ECB/ECL-VAV NET - NET +
(Ground symbol)
ECB-PTU Series Line-Powered
Controllers
NET - NET + COM
EC-BOS - + S
Thermostat - + Ref
Repeater Data-
Data1-
Data+
Data1+
N/A
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