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Article: Commandable versus Writable

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Frequently Asked Questions (FAQ)


Does a multi-state variable have a limit on the maximum number of states?

  • Yes. The minimum is 1. The limit is vendor dependent with a protcol maximum of a 32 bit number. In theory it could be different for each object in a device although we have not encountered a device like that. To learn the limit read the object property called “Number_Of_States”. You can read this property using the CAS Bacnet Explorer.

What is a BACnet priority array? Read More

  • The Priority Array is a read-only property that consists of commands that are categorized by priority levels of decreasing order. NULLs may also exist in this property type as well. The highest priority will reside with the lowest array index and a non-NULL value will represent the active command.

What are BACnet objects and object properties? Read More

  • 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 . The other objects represent the device’s data.

How is the BACnet architecture designed? Read More

  • BACnet consists of a collapsed architecture that corresponds to four layers of the Open Systems Interconnection (OSI) model. The four layers from the OSI model within the BACnet architecture includes:
    • Application
    • Network
    • Data Link
    • Physical

What is BACnet BBMD? Read More

  • BBMD stands for BACnet/IP Broadcast Management Device. It is a technology used by BACnet to discover devices from other networks.

What is BACnet MSTP? Read More

  • The MS stands for Master – Slave although in practice there are not many slaves out there. The TP stands for Token Passing. This flavor of BAcnet is most commonly used to connect field devices to controllers / routers / control applications.

What is the best BACnet MSTP topology configuration? Read More

  • The best MSTP topology is a single trunk that in-outs on the terminal blocks of each device it connects. What do we mean by best? We mean the choice which is least likely to cause problems. Read more for diagrams and other examples.

How can I build a safe and functional BACnet MSTP network using RS485 cables? Read More

  • BACnet MSTP uses RS485 (also known as EIA485) as it physical layer. Here is some simple advice for RS485 networks (for details read more):
    • RS485 is a 3 conductor network. You take a huge risk by not installing the 3rd conductor.
    • Always connect the ground reference conductor first if you are connecting a powered on device or laptop.
    • You can get away without the shield.
    • Take care where you run your cables. It seems obvious not to wind your cable around other cables or sources of electricity/magnetism.
    • All cables offer impedance (resistance). Some cables are designed so that the impedance is relatively independent of distance. You want one of these cables.

Why you need 3 wires for 2 (two) wire RS485? Read More

  • RS485 needs 3 conductors and a shield. Many people say its a two wire network but it is not.

Where can I find more BACnet developer resources? Read More

  • The Internet offers a wide variety of sites related to BACnet development from personal to commercial sites. Two of the more impressive, detailed websites are the following (read more for a breakdown of what these sites contain):


BACnet Objects
What is a BACnet object? Read More

  • BACnet object(s) are concepts that allow the communication and organization of data related to inputs, outputs, software, and calculations to be performed.

What is a BACnet Notification object? Read More

  • The Notification object is a BACnet object that is used to send event notifications within a BACnet system. When a device has a problem or a fault, such as the present value of a property rising above a predetermined level, a notification object would be created to inform the rest of the BACnet system that there is something wrong.

What is a BACnet Multi-state object? Read More

  • Multi-state objects are very important in operational state description of a BACnet device. There are three types of Multi-state objects described in the BACnet standard:
    • Multi-state Input
    • Multi-state Output
    • Multi-state Value

What is a BACnet Life Safety object? Read More

  • BACnet Life Safety objects are the objects related to fire, safety, and security applications. The two types of these objects include (as described in the BACnet standard):
    • Life Safety Point Object Type
    • Life Safety Zone Object Type

What is a BACnet File object? Read More

  • BACnet File objects are data files that are described by the objects’ properties, and are accessed through File Services. File Services are used for the access and manipulation of files within BACnet devices.

What is a BACnet Command object? Read More

  • The Command object is a very important part of the operation of a BACnet device. It writes to a group of object properties through action code(s) inside the Present_Value property. When this writing occurs, the Command object will start a sequence of actions in the BACnet device(s).

What is a BACnet Group object? Read More

  • A Group object is a collection of other objects, with one or more of their properties. It is essentially used to simplify the information exchange between BACnet devices through a shorthand method to specify all group members at once. The developer should be aware that a group can be formed through any combination of object types.

What is a BACnet Schedule object? Read More

  • A Schedule object is a periodic schedule of events that may repeat within a range of dates. The schedule object is categorized into two types of days:
    • Normal Days in a Week
    • Exception Days

What is a BACnet Loop object? Read More

  • The Loop object is defined by the standard in terms of the object properties representing the external, physical attributes of any category of control loop. The Loop object is a detailed and complex object in the standard.

What is a BACnet Averaging object? Read More

  • The Averaging object is a recording medium of visible value characteristics over a specific time interval. These specific sampled values may take the following forms in the object:
    • Boolean
    • Integer
    • Unsigned
    • Enumerated
    • Real

What is a BACnet Event Enrollment object? Read More

  • The Event Enrollment object is information related to the management of events for BACnet systems. Events are essentially changes in the value of any property concerning any object that meets specific criteria.

What is a BACnet Trend Log object? Read More

  • The Trend Log object is a property monitor for a specific object. When certain conditions are reached, a log is produced with the property value and a date/time stamp which is placed into a buffer for future retrieval. This buffer can be optionally fixed in size by the developer. The Trend Log object offers the developer two ways in which the value is logged:
    • Periodically
    • During a change in value

What is a BACnet Pulse Converter object? Read More

  • The Pulse Converter object is a process monitor represented by counts or pulses. The type of processes monitored can include those tasks in maintaining a building, such as water usage, power usage, natural gas usage, etc…


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