Timeout Functions 6 - Allen-Bradley 1779-KFMR User Manual

Chapter 6
HDLC Data Link Layer
6 20
the node continues to be busy, it must respond with an RNR frame to each
data or RR frame it receives.
When a node receives an RNR frame, it will cease transmitting data
frames at the earliest possible time by completing or aborting the frame it
has in process. If the RNR frame received is a command frame with the P
bit set to 1, the node must transmit a response frame with the F bit set to
1. The node receiving the RNR frame must perform a timeout before
resuming transmission of data frames unless it receives a frame which
indicates that the busy condition is cleared.
When the internal constraint ceases, the busy condition is cleared at the
node which has transmitted the RNR frame. When a node's busy
condition has been cleared, it must report that fact to the other node by
transmitting an RR, REJ, SABM, or UA frame (with or without the P/F
bit set to 1).
N(S) Sequence Error
An N(S) sequence error occurs when a node receives a data frame that has
no FCS error but contains an N(S) that is not equal to the receive variable
(R) at that node. The node receiving the out-of-sequence data frame does
not increment its receive variable (R); it does not acknowledge the
out-of-sequence frame or any frames that follow until it receives a data
frame with the correct N(S); it transmits a REJ response to request
retransmission of data frames starting with frame N(R).
When a node receives an out-of-sequence data frame which is otherwise
error-free, it accepts the N(R) value as an acknowledgment of previously
transmitted data frames; it also accepts the information in the P/F bit.
When a data frame is retransmitted, the N(R) value and P/F bit status may
have been updated and therefore could be different from that contained in
the originally transmitted data frame.
Only one sent-REJ exception condition at a time can be established from
one node to the other. A sent-REJ exception is cleared when the
requested data frame is received or when a timeout function expires.
When a node perceives by timeout that the requested data frame was not
received (because either the requested data frame or the REJ was in error
or lost), the REJ may be repeated.