States And Modes; Frame Structure 6 - Allen-Bradley 1779-KFMR User Manual

Chapter 6
HDLC Data Link Layer
Frame Structure
6 4
Logical States and Modes
The host computer and the 1779-KFM interface communicate with each
other in three logical states:
Data Transfer State (DTS) — While in the data transfer state, a node
may transmit and receive data. In this state, the nodes are always in the
asynchronous balanced mode (ABM). In this mode, a node may
initiate transmission without receiving permission from the other node.
An asynchronous transmission of this type may contain either single or
multiple frames. These frames could transfer data. These frames could
alternately indicate status changes at the transmitting node (e.g., the
number of the next expected frame, transition from a ready to a busy
condition or vice versa, occurrence of an exception condition) or status
changes of the link.
Initialization State (IS) — The initialization state is a transitional
state leading to the data transfer state. The host computer should
command initialization after each power up. The host computer could
also command initialization at other times when there is uncertainty
that the link is functioning properly in the data transfer state.
Logically Disconnected State (LDS) — While in the logically
disconnected state, the node is logically disconnected from the link. In
this state, the nodes are always in the asynchronous disconnected mode
(ADM). In this mode, a node is not permitted to transmit or receive
any data frames; however, a node is permitted to transmit or receive a
single frame indicating the node status. A node may transmit this
frame without getting explicit permission from the other node.
A frame is the unit exchanged at the data-link layer. A frame is a
sequence of contiguous bits bracketed by and including beginning and
ending flag sequences. All transmissions on the RS-422/449 link are in
frames that conform to the following structure:
Time
FLG ADR CTL DATA
(From Preceding Layer)
FCS FLG
2 Bytes