TECO L510S Series - Page 191

App 3-19
All BACnet devices have the application programs to manage the requirements of device
motion and executing services. Take work station for example, the application program needs
to keep the display value of every input so it requires sending the service request to the object
of other device to update the display value of input. The application program of the device
needs to respond to the service requiremtents. Refer to the following figure.
2. BACnet Protocol Structure
BACnet is the communication protocol by way of protocol stack so the pocket is composed
of stacked layer types. Refer to the following figure.
When application program sends the BACnet service request for the pocket, it requires
requesting for executing BACnet request program in the application layer via application
program interface. The requirements of the program are sent to the application layer and
application protocol data unit (APDU) consists of Application Protocol Control Information
(APCI) and Servie Data of application program. Then APDU passes its messages downward
to BACnet request program in the network layer. APDU becomes Network Layer Protocol Data
Unit (NPDU) composed of Network Service Data Unit (NSDU) and Network Protocol Control
Information (NPCI). And so forth for the data link layer and physical layer to complete the full
service for the packet.
BACnet Service
Request
Application Program
Application Layer
Network Layer
Data Link Layer
Physical Layer
A-BACnet.Request
N-UNITDATA.Request
DL-
UNITDATA.Request
MA-
UNITDATA.Request
P-UNITDATA.Request
APCI
Service
Data
APDU
NSDUNPCI NPDU
LSDU LPDULPCI
MSDUMPCI
PSDUPPCI
MPDU
PPDU
Application
Program
Object
Object
Object
Object
BACnet Device
Request for
Service
Respond to
Service
Network
Service request
Read property
Service Reply?