TECO F510 series - Page 433

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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 fugure.
4.7.2 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?