198
•
Fieldbus Communication
BACnet/IP
WAGO-I/O-SYSTEM
750
BACnet/IP Controller
Virtual terminal services (Virtual Terminal Service)
•
VT-Open Service
Establishes a connection to a network
•
VT-Close Service
Breaks the connection off
•
VT-Data Service
Transmits data between network members
4.2.1.2.3.1 Client-Server
Communication
In Client-Server communication, Clients and Servers are considered to have
different functions. A Clients makes a request as a service user of a Server
(service provider). The Server then sends its response back to the Client.
BACnet devices can work, depending on function, both as a Client and as a
Server. For example, a device with the ReadProperty service can read a value
in another object; in the same manner, however, a value in one of its own ob-
jects can also be changed from the outside.
4.2.1.2.3.2 Prioritization
Prioritization for Output Objects
BACnet applications can access Objects through Services and change their
properties. Access prioritization is required to regulate and organize access of
various applications to the properties. The BACnet standard distinguishes be-
tween 16 different priority levels.
A priority can only refer to current values (Present_Value) of output objects.
For this reason they are also called "command properties".
The processing sequences for individual applications can be changed using
prioritization. Thus, the simultaneous access for several uses of the same ob-
jects is regulated (see Fig. 4-1). The use on the highest priority level with the
lowest number has priority.
