BACnet MS/TP Communication Data Bus Fundamentals
110 ECLYPSE User Guide
BACnet MS/TP Data Transmission Essentials
Certain models of the ECY Series Controller support BACnet MS/TP to BACnet/IP routing
according to the controller model purchased. See the Controller’s datasheet for more
information. To enable BACnet MS/TP to BACnet/IP routing, see Routing on page 78.
The ECY Series Controller can support either BACnet MS/TP or Modbus RTU network on its
RS-485 port. This option is selected in the controller’s web interface. When the ECY Series
Controller is configured for BACnet MS/TP, values from the connected BACnet MS/TP
controllers can be used in ENVYSION graphics hosted on the ECY Series Controller.
Furthermore, the ECY Series Controller acts as a BACnet/IP to BACnet MS/TP bridge that
allows BACnet objects to be shared among BACnet intra-networks through BBMD.
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 11-16 on page 128).
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 Figure 11-4). A spur is only
permitted when it is connected to the data bus through a repeater (see Using Repeaters to
Extend the Data Bus on page 122).
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 usually 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.
Table 11-1: Common Identification Labels for BACnet MS/TP Data Bus Polarity for
Distech Controls’ Products
Typical Data Bus Connection Terminals
ECB-PTU Series Line-Powered Controllers
BACnet/IP to MS/TP Adapter
BACnet/IP to MS/TP Router
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