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ECLYPSE APEX
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 controller models 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
.
The BACnet MS/TP or Modbus RTU network option is selected in the controller’s web interface. BAC-
net MS/TP and Modbus RTU communications are made by connecting directly to separate RS-485
ports. The Connected System Controller integrates up to three RS-485 ports when equipped with one
ECY-RS485 extension module allowing the controller to support more than one trunk or communica-
tion protocol at a time. 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 Se-
ries 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. See
Broadcast Management Device Service (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
Numbering System for MAC Addresses, Device Instance Numbers, and Network Numbers
.
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
Only a Daisy-Chained Data Bus Topology is Ac-
). A spur is only permitted when it is connected to the data bus through a repeater (see
Repeaters to Extend the Data Bus
).
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 re-
spected. The markings to identify the polarity can vary by manufacturer. The following table summa-
rizes the most common identification labels for BACnet MS/TP data bus polarity.
BACnet MS/TP Communication Data Bus Fundamentals
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