Exception Condition Reporting And Recovery - Allen-Bradley 1779-KFMR User Manual

Exception Condition Reporting
and Recovery
enter the ABM when it receives a SABM command, it transmits a
disconnect mode (DM) response.
From the time that one node sends a SABM command until the other node
sends a response, the nodes are in the initialization state (IS). When a
node transmits a SABM command or accepts a SABM command, it sets
to zero its send and receive variables. Previously transmitted data frames
that are unacknowledged at this point remain unacknowledged.
The host computer should transmit a SABM command after each power
up. The host computer could also transmit a SABM command at other
times when there is uncertainty that the link is functioning properly in the
data transfer state. However, the host computer should be cautious about
sending a SABM command when it has not yet sent a response to each
data frame it has received; these unacknowledged data frames would be
lost because the 1779-KFM interface would not be able to retransmit them
after initialization.
Disconnect (DISC) is a command to perform a logical disconnect; this
places the nodes into the asynchronous disconnect mode (ADM) which is
in the logically disconnected state (LDS). In this disconnected mode, the
nodes can only transmit and accept mode-setting commands and
responses.
If a node is in the asynchronous balanced mode (ABM) when it accepts a
DISC command, it transmits a UA response. If a node is already in the
asynchronous disconnected mode (ADM) when it accepts a DISC
command, it transmits a DM response.
This section specifies the procedures for recovery following the detection
or occurrence of an exception condition at the link level. These exception
conditions are those situations that may occur as the result of transmission
errors, station malfunction, or operational situations.
Busy Condition
A busy condition occurs when a node temporarily cannot receive or
continue to receive data frames due to internal constraints (buffering
limitations). A node reports its busy condition by transmitting an RNR
frame with the N(R) indicating the next data frame it expects. Although a
node transmits an RNR frame to indicate that it is too busy to accept any
more data frames, it may still continue to transmit data frames. As long as
Chapter 6
HDLC Data Link Layer
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