BACnet devices share data using BACnet objects over a properly configured BACnet network that carries this data using Ethernet, IP, or MS/TP protocols. At the device level, MS/TP network trunks connect individual BACnet controllers. The BACnet architecture in an I/A Series MicroNet BACnetwork uses these networking protocols, especially MS/TP, to allow communication between controllers and tools either directly over MS/TP or by connecting multiple MS/TP networks using BACnet over IP or Ethernet to form an internetwork with the Plant Controllers and UNCs acting as routers between the networks.

1. F-27360 MicroNet BACnet Wiring and Networking Practices Guide 2-1
Chapter 2
Networking Practices
Introduction to BACnet
In BACnet systems, BACnet devices use BACnet objects to share data. To
allow this sharing of data, a BACnet network must be properly configured.
On a properly configured network, the BACnet protocol carries these data
and uses Ethernet®, Internet Protocol (IP), and Master Slave Token Passing
(MS/TP) for network communication. At the device level, MS/TP network
trunks connect individual BACnet compliant controllers.
Architecture Overview
Introduction As implemented in an I/A Series MicroNet BACnetwork, the BACnet
architecture uses one or more networking protocols to allow communication
among controllers and engineering tools. At the device level, Master Slave
Token Passing (MS/TP) networks can be used to connect up to 127 unitary
or VAV controllers and MS/TP tools to a Plant Controller. With 127 devices
connected to a Plant Controller, all 128 MS/TP addresses on the Plant
Controller are used. Similarly, up to 31 devices (unitary, VAV, Plant
Controller, or MS/TP tools) can be connected to each of the two network
trunks of a UNC 510-2. Multiple BACnet MS/TP networks can be connected
by networking the Plant Controllers and UNCs using BACnet over IP or
BACnet over Ethernet. This is referred to as a BACnet internetwork. In such
a configuration, the Plant Controllers and/or UNCs manage communication
throughout the internetwork and serve as routers. Engineering tools can be
used to manage controllers throughout an internetwork by connecting them
to an MS/TP network trunk or by connecting to the IP network.
Figure-2.1 shows a BACnet internetwork comprising four or five individual
networks. There are thee individual MS/TP network trunks, each managed
by a UNC or Plant controller and running individual BACnet devices.
Ethernet or IP can be used as the networking technology for the backbone,
adding a fourth network. If appropriate for the installation, both Ethernet and
IP can be used on the network backbone. This would add a fifth network to
the internetwork as shown in Figure–2.1.