Allen-Bradley 1779-KFMR User Manual page 83

Chapter 6
HDLC Data Link Layer
6 14
While in the LDS, the response capability of the node is logically
disconnected from the data link; no data frames or supervisory response
frames are transmitted or accepted. The response capability is limited to
transmitting a disconnected mode (DM) response frame at each respond
opportunity and accepting a set asynchronous balanced mode (SABM)
command frame. The node must respond with DM to any valid command
frame received with the P bit set to 1.
A node in the LDS is always in the asynchronous disconnected mode
(ADM). ADM is a disconnected mode in which the response capability
of the node is logically disconnected from the data link and follows
asynchronous respond opportunity protocol ("Asynchronous Respond
Opportunity").
A node will enter the LDS upon receiving a disconnect (DISC) command.
Also, a node will automatically enter the LDS when any of the following
conditions occur:
The node power is turned on.
The node has a temporary loss of power.
The node's link-level logic is manually reset.
The node is manually switched from a local condition to a
connected-to-the-link condition.
While in the LDS, the node may not establish a frame-reject-exception
condition.
Initialization State
The initialization state (IS) is a transitional state leading to the data
transfer state. When the host computer transmits a SABM command
frame, it enters the initialization state. When the host computer receives
an unnumbered acknowledgment (UA) frame in response, it leaves the
initialization state and enters the data transfer state.
While in the LDS, the host computer can transmit a SABM command
frame to enter the IS. While in the data transfer state, the host computer
can also transmit a SABM command frame to enter the IS; this may be of
use when there is uncertainty that the link is functioning properly in the
data transfer state.