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INTRODUCTION

Modern control networks such as EtherNet/IP™, Modbus/TCP and BACnet/IP® use Ethernet for communications due to its high speed, lowering cost and in some instances, the necessity to operate over structured wiring. Although the study of Ethernet does not require an understanding of application protocols, knowledge of application protocols has become increasingly important as modern networks are deployed. The latest protocols are all based upon Object Modeling which can be quite confusing to someone who has not been exposed to this abstract concept. This paper introduces object modeling, object properties, and services as they pertain to a physical BACnet/IP device. Although the majority of BACnet devices support the master-slave/token-passing (MS/TP) network, newer devices now function over Ethernet.


BAS Remote as a Physical Device

Although any other product could have been used, we will use as our physical device example the BAS Remote by Contemporary Controls for this discussion. This product is intended as a remote input/output device complying with the BACnet/IP standard. When connected to an Ethernet infrastructure, the BAS Remote provides eight input/output points that can be accessed by any device on a BACnet network. Six of those points are universal input/outputs—meaning they can be field configurable to be either an analog or digital, input or output. Two points are fixed as relay outputs. The BAS Remote supports the BACnet/IP standard as defined in Annex J of the BACnet standard and therefore, requires no router in order to connect to Ethernet. But how do other devices access the BAS Remote? To explain how this works, one needs to study BACnetís object model and services.


The BACnet Standard

As of January 2008, the current revision of the BACnet standard is ANSI/ASHRAE 135-2004 from the American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. If you purchase the standard from the ASHRAE bookstore, you will receive a 600-page bound copy along with a stack of errata. In the errata are revisions that have yet to make it to the printed bound copy, and interpretations of the standard made by committee members in response to questions, proving that the BACnet standard continues to evolve. Like any other standard, the BACnet standard is not written as a tutorial on BACnet although some tutorials exist on the web. However, since the BAS Remote is a relatively simple BACnet device, studying this product offers an opportunity to learn how any device becomes BACnet-compliant.


Object Modeling the BAS Remote

Observing the BAS Remote sitting on a table, it appears in its physical form as a plastic enclosure with screw connectors for powering the unit and for connecting to field devices such as sensors and actuators (see Figure 1). According to the BAS Remote data sheet, in its simplest form it has six universal input/output points and two relay outputs. The six universal I/Os can be individually configured to be an analog input, analog output or digital input. In addition, the circuitry is flexible in that it can accommodate several voltage and current levels such as 0-10 VDC or 4-20 mA. Contact closures or thermistors can be sensed directly. There is one RJ-45 connector for the Ethernet network. To someone in the industry, the BAS Remote would be considered an eight-point remote I/O device with an Ethernet connection. Obviously, the Ethernet connection allows network access to the various I/O points, but how is this accomplished in the BACnet world? Once configured for the needs of the application, how can each physical point on the BAS Remote be queried from any point on the network without any prior knowledge of the capabilities of the BAS Remote? Object modeling is the answer.



Figure 1 — The BAS Remote has six universal input/output points and two relay outputs.

Although the BAS Remote is an eight-point I/O device, we know little more. To understand this device or any other BACnet device, it is best to view these devices as a collection of objects. An object is an abstract concept that allows us to identify both physical items such as I/O points, and non-physical concepts such as software and calculations. In Figure 2 we notice that the BAS Remote can be represented as a collection of objects of varying types such as Analog Input, Analog Output, Binary Input, Binary Output and Device. Since the BAS Remote has eight points of I/O, there will be eight objects created plus one for the device itself. Since six of the I/O points are universal, the number of each object type will vary depending upon the final configuration. This is not true for the Device object since only one is allowed per physical device. There will be two Binary Output objects to correspond to the two relay outputs on the BAS Remote. In total, the BAS Remote can be modeled as a collection of nine objects with only five object types. As shown in Table 1 (next page), the current BACnet standard identifies 25 object types that would typically be found in building automation systems. BACnet-compliant devices are not required to implement all 25.



Figure 2 — The BAS Remote can be described as a collection of objects
with one identifying the device itself.

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