What are BACnet control points and devices in building automation systems?

Controller insights

In this is partial transcript from the March 2023 webcast “HVAC and BAS: Can BACnet control an HVAC system?”, Jason Gerke, PE, LEED AP BD+C, CxA, Principal, GRAEF; and Yanlin Zeng, PE, CEM, CMVP, LEED BD+C, Controls Engineering Supervisor, Southland Industries; discuss building, automated, and control networks (BACnet) control systems. The full webcast is available here.

BACnet IP versus MS/TP

Yenlin Zeng: BACnet master-slave/token passing (MSTP) uses shielded twisted pair cable that can only be connected in daisy chain, but the cable length can be up to 4,000 feet. For internet protocol (IP), an Ethernet cable like CAT-5E or CAT-6 is used, which can be connected in star, ring or series connections. IP cable can only run up to 100 meters. This is from controller to controller, controller to switch or switch to switch. If the distance is beyond a hundred meters, then it must switch to fiber.

BACnet IP is much faster than MS/TP. This is likely the main reason there are more and more IP applications. While IP has a little higher installation cost, it is much easier for troubleshooting. Security-wise, MS/TP network is not easy to hack into, but once in there is no protection.

For IP, there are two ways of protection. The first one is risk management framework by the that manages the switches to add security on the network. The other one is BACnet secure-connect. This is an addendum to BACnet protocol by the ASHRAE BACnet committee. It uses authenticating and encrypting to add a security on the communication. Only the recipient on the network has the key to decrypt the message and other devices on the network will not be able to determine the content of the secured message.

If a controller on the IP network is lost, it will not impact the IP network communication because the Ethernet switch built in the controller is fail patched proof. If the controller is lost, all the data will be transferring patched through, so the communication is just working fine.

Types of BACnet controllers

The other key component for building automation system network is controllers. There are many different controllers from different manufacturers. The BACnet testing laboratories is the organization oversees the global BACnet product certification program. They test the controller and see if this controller meets the BACnet standard.

• Building controller: A very powerful controller that can be used at router with BACnet broadcast management devices to o scheduling, alarming and trending. It is normally used for very complicated systems and most controller manufacturers make these types of controllers expandable input/output (I/O) modules. This makes the future expansion very convenient.

• Advanced application controller: A unitary controller, meaning it has fixed IO points. For future expansion, if more IO points are needed, an upgrade to a building level controller is required.

• Application-specific controller: Used mostly for terminal unit controls, it is factory programmable and configurable. The programmable one, has full programmable capability to create customized sequences. For the configurable type, the programs are already preloaded to the controller. Since it does not start from scratch, engineers only configure certain parameters. This is for standard application, but it will save a lot of programming time.

BACnet control points

The control points connect the brink of the controller to all the field devices. There are two type of control points: hardware points and software points. In hardware points, all the signals, called inputs, are sent from the field device to the controller. The controller receives these signals and will do some calculations, logic thinking and will send out commands to the field device. These commands are called outputs. Based on the different signal type, they are separated into analog and binary. For the analog, there are measures for temperature, humidity, pressure, flow and points like that. The signal normally is 4 to 20 milliamp, 0 to 5 BDC or 0 to 10 BDC.

The binary input output is just two states, 0 and 1, to indicating it’s running, it’s on or off or the damper is open or closed. Another unique state seen in most controller manufacturers is a universal input and output. These points can be configured as analog or binary as needed, so it’s very convenient.

Software points are basically about variables. It is also separated as analog variables and binary variables, which are used for internal programming or data transferring between controllers.

How do controllers find each other correctly?

There are three numbers to identify controllers. The first one is the BACnet Instance numbers, that’s a device ID. It must be unique on the entire network. The second one is the network number. The network number is used to identify the individual BACnet network segment. Again, no duplicate network numbers are allowed. The third one is the media access control (MAC) address. This must have a unique MAC address on a single MS/TP segment, but it can have the same MAC address on different MS/TP segments.

The building automation system (BAS) controller is very flexible, so it’s recommended to use BAS controller for complicated or customized applications. However, it’s not recommended to combine the BAS controller and the factory controller. This adds one more single point failure to the system and there are two control sources for one unit. The data transfer between the BAS and the factory controller is very critical.

The variable frequency drive integration, is very mature. All the manufacturers offer different communication cards like BACnet, Modbus, LON or some proprietary communication cards. Some clients will ask if they should hardwire more points or rely on the integration only?

It is recommended to hardware critical points such as Start Stop command, speed control and alarm points and then use integration with the drive to pull more data in the system. In case the connection is lost, the hardwired points are still there, so it can be started and stopped and the speed can still be controlled.

Alarms and analytics

Jason Gerke: There’s a distinct difference between these keywords. An alarm is the result of an evaluation of a single item against a single point in time. Analytics provide ongoing evaluation and comparison of the data, identifying trends, the period of time and which equipment is operating. The analytics system will provide the detailed review of what events are related and what else was happening at the time, so going back to the example you just heard about. An analytics system will identify opportunities for energy savings and focus maintenance.

ASHRAE Standard 135: BACnet — A Data Communication Protocol for Building Automation and Control Networks contains many control sequences. It’s regularly updated with new and approved sequences, so it can be used as a guide. Company standards and past resources can also be useful. The systems and sequences need to be evaluated for interrelated operation and ensure the protocol of one system works with the other. BACnet protocol provides this opportunity for the systems to operate together.

The use of analytics in a control system is the latest iteration of how a BAS can be leveraged to provide the best operational outcome for a facility. There are many opportunities for analytics to be utilized in a control system and there’s lots of different opportunities for them to be provided by different manufacturers.

An analytics system is like a full-time person that does nothing except look for patterns and changes to those patterns in a system. These systems work 24/7 to look for anomalies or trends that do not meet the original programming expectations or outside average or normal parameters. A system such as this will work all the time, does not get sick and does not get distracted. It can be a great resource in a constrained labor market to not only find problems before they occur, but also to quickly identify the source when issues do arise.

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