User manual

IEC61131 User and Reference Manual
April 22, 2008
451
Application Layer
The application layer in DNP is responsible for the processing of complete messages for requesting,
or responding to requests, for data.
The following shows the sequence of Application Layer messages between one master and one
outstation.
Master Outstation
Send Request --------------------> Accept request and process
<-------------------- Optional Application
Confirmation
Accept response <-------------------- Send Response
Optional Application
Confirmation -------------------->
Important change detected
Accept response <--------------------- Send Unsolicited Response
Optional Application
Confirmation -------------------->
The complete messages are received from and passed to the pseudo-transport layer. Application
layer messages are broken into fragments with each fragment size usually a maximum of 2048 bytes.
An application layer message may be one or more fragments in size and it is the responsibility of the
application layer to ensure the fragments are properly sequenced.
Application layer fragments are sent with or without a confirmation request. When a confirmation is
requested the receiving device replies with a confirmation indicating the message was received and
parsed without any errors.