Category: Tutorials

UDP/IP - Virtually everyone has heard the term TCP/IP. This is the term often generically applied to anything Internet or anything using "standard" networking. The UDP side of the stack operates in parallel to TCP, and is automatically included in most implementations of an Ethernet based protocol stack. The difference is that TCP is considered a "connection" protocol and all communication takes place in a session that has overhead to ensure delivery of all packets. UDP is considered "connectionless", has minimal overhead, and allows the application to deal with whether the packets were delivered or not. UDP is used where network speed and efficiency is important, and BACnet IP uses the UDP side of the IP stack. It is appropriate to use UDP for BACnet since messages are typically short and independent of each other. TCP is used for FTP file transfer, for example, where communication involves a lengthy series of long messages.

Object - The general reference to sensors, actuators, 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 a couple of the most commonly used objects.

Object Property - Each object has several properties required by BACnet protocol. The most commonly 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.

COV - Change Of Value - A COV notification is sent any time the object's value changes by an amount specified as a default parameter, or in the subscription request. Devices wishing to receive the notice of change need to "subscribe" to the COV by sending a subscription request to the device from which a COV notification is desired.

BBMD - Stands for BACnet/IP Broadcast Management Device. The BBMD is used to implement BACnet IP across a large network. Intended typically for campus wide management, it is technically just as possible to implement a single BACnet IP network that spans the country. In order for BACnet devices to operate as a system, they must be able to broadcast messages. However, standard IP technology dictates that routers do not forward broadcast messages. The BBMD resolves this problem by providing a re-broadcast on the local domain for any message originally broadcast on another domain. It is not necessary for all BACnet IP devices to support BBMD. Only one device on an IP domain needs to function as the BBMD. It will be configured to interact with BBMD's on other domains to provide the broadcast support.

MS/TP Mac Address  or Station ID - This is an 8-bit number, which for master/slave devices is 0-127. This Mac address is used locally on the RS-485 link to physically address devices on the link, and is not passed through routers. It is comparable to the Modbus RTU slave address.

MS/TP Max Masters - Determines how many possible masters the various devices on an MS/TP link will search for in the "poll for master" sequence. If this setting does not match in all devices on the link, erratic behavior can occur.

BACnet IP Addressing - The standard Ethernet IP address is used to identify devices on the IP network. The IP address is used to physically route messages on the network, while BACnet device instance still identifies a device in the BACnet system.

Device Instance - This is the logical address that matters to BACnet. Whether on an MS/TP link or IP network, the device instance is unique across all subnets and routed links.

Client/Server versus Master/Slave - We tend to think of master and slave at the MS/TP level, and start thinking in terms of client/server when we talk about IP networks. Functionally, the client and master are synonomous while the server and slave are synonomous.

Bus – The physical communications platform used for inter-device communications and connection to the BMS computer. This does not define the protocol used. Types of Bus include:

Proprietary - A defined protocol that is unique to an individual or company. The raw code is not in the public domain.

Open Technology - A solution based on uniformity and interoperability. A philosophy used to ensure competitive pricing and client focused benefits.

Integration - The process of translating from one protocol to another.

Interoperability - The ability to integrate with another vendors equipment.

Interconnectivity - Devices that are the same between manufacturers, allowing swapping of brands. (A device with the same communications protocol may be interoperable, but may have different internal variables and not a straight swap).

Native - Using a common protocol between vendors on a common Bus.

Gateway – A hardware or software device used to translate between protocols (Hardware solutions are preferred due to failsafe operation).

DDC System - Direct Digital Control System.

BMS - Building Management System.

EMS - Energy Management System.

I/O - Inputs/Outputs.

Hardware - Physical devices used to make decisions for controlled outputs, based on changes in input conditions.

HMI – Human Machine Interface (A controller or network display).

Software - The computer interface for the hardware, or separate utilities to perform value-added tasks

HLI- High Level Interface. The ability to send and receive a greater amount of data over a single communications Bus

Protocol – A defined communications language (Such as BACnet, Modbus, Lonworks, Meter-bus, KNX, DALI, Innotech)