Emerson E2 operation manual BACnet Overview, BACnet Communication, Master Slave Token Passing

6.4BACnet

BACnet is a communications protocol for building

automation and control networks available for E2 versions

4.02and 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.

6.4.1BACnet Overview

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.

Figure 6-11- BACnet Objects

6.4.2BACnet Communication

BACnet supports MS/TP or IP based communication.

6.4.2.1Master Slave Token Passing

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).

BACnet The RS485 Network and Hardware Setup • 6-13