User manual

IEC61131 User and Reference Manual
April 22, 2008
452
Pseudo-Transport Layer
The pseudo-transport layer formats the larger application layer messages into smaller packets that
can be handled by the data link layer. These packets are referred to as frames in DNP. The pseudo-
transport layer inserts a single byte of information in the message header of each frame. This byte
contains information such as whether the frame is the first or last frame of a message as well as a
sequence number for the frame.