Devices Provide Services

Having just objects with properties is not sufficient. We need services so that one BACnet device can access data from another or to command another device to take action. In order to learn what the outside temperature is, we would use the service called ReadProperty which allows the reading of properties within an object. In fact this service is the one service that all BACnet devices must process. For example, the BAS Remote’s point 2 knows the outside temperature. Another device functioning as a client makes a ReadProperty request to the BAS Remote which functions as the server. The request is made to the Present_Value property in the Analog Input, Instance 2 object. The BAS Remote receives the request and executes the command by providing the information.

A confirmed service is one where a service request is initiated and a response with data is expected. An unconfirmed service expects no reply. Depending upon the capabilities of a device, it may be able to initiate a confirmed service request or respond to a confirmed service request. Some devices can do both. The same applies to unconfirmed services.

There are 38 possible service requests and they are grouped into five categories (see Table 4.)

• Alarm and Event Services (11)
  
• File Access Services (2)
  
• Object Access Services (10)
  
• Remote Device Management Services (12)
  
• Virtual Terminal Services (3)
  

Table 4 — 38 services are grouped into five categories. Services are confirmed (C) or unconfirmed (U).