Emerson E2 User Manual

BACnet is a communications protocol for building 

automation and control networks available for E2 versions 
4.02 and above. Any of the three serial ports (COM2, 
COM4, and COM6) can be used to communicate with 
devices over BACnet. E2 supports BACnet devices that 
communicate using MS/TP and IP.

BACnet is defined by three basic concepts:

• Object - The general reference to sensors, actua-

tors, and other functional elements that make up a 
BACnet device. The objects fall into categories 
specified by BACnet protocol. Analog Input object 
and Analog Output object are two of the most com-
monly used objects. BACnet devices are defined on 
the network as a collection of “objects”. 

• Object Property – Each object has several proper-

ties required by BACnet protocol. The most com-
monly used property is Present Value. Other 
common properties include reliability and status 
flags. Optional properties for objects such as analog 
input include minimum and maximum range, high 
and low limits, etc. 

• Services – Along with the definition of objects, 

BACnet protocol defines “services”. These include 
object access services, alarm and event services, 
file access services, and a few more. Object access 
services are the most commonly used since these 
provide the fundamental “read/write” access to 
object properties.

Data inside a BACnet device is organized as a series of 

objects. Each object has a type and a set of properties. 
There is always at least one object in a device – it is used 
to represent the device itself. This is called the Device 
Object. The other objects represent the device’s data.

BACnet supports MS/TP or IP based communication.

Master Slave Token Passing or MS/TP is the protocol 

used over RS-485 for BACnet. Each device on the link is 
considered the “master” when it has the token. If it does 
not have immediate need to use the token, it is required to 
pass the token along to the next device. All devices on the 
link that do not currently have the token are regarded as 
slaves, and are expected to listen to any messages the cur-
rent master may have for it. Because some devices take 
turns being master, the link is effectively peer to peer (not 
all MS/TP devices can be master).

It is possible for a device to be a constant slave, though 

it is uncommon, because in this case a device cannot send 
such request as I-Am, notifications, etc.

MS/TP devices differentiate themselves with a MAC 

address. MAC addresses are unique identifiers set per 
device and must be unique on the MS/TP line. 

It should be noted that MS/TP BACnet devices do not 

use their unique identifier for accessing them. Instead, a 
second identifier called a Device ID is used. This is 
because BACnet traffic can be routed across IP or MS/TP 
(i.e., the physical layer is different). The Device ID has a 
much larger addressable range.

MS/TP requires only the baud rate to be specified and 

consistent. All other line disciplines are specified by the 
standard and are therefore not configurable. MS/TP also 
requires that the E2 to have a MAC address. This is con-
figurable per MS/TP route (serial port).