Allen-Bradley 1779-KFMR User Manual page 82

transmit a response frame without being polled. The characteristics of a
response are determined by the type of respond opportunity, the state of
the node, and the mode within the state of the node.
Asynchronous Respond Opportunity
With two-way simultaneous transmission, each node always has an
asynchronous respond opportunity (ARO). ARO is a respond opportunity
in which the node can initiate transmission of response frames without
being polled by the receipt of a command frame with the P bit set to 1.
Asynchronous transmission of response frames may be initiated at the
first opportunity. The 1779-KFM interface will always transmit a
response frame at the first opportunity after receiving a command frame.
an asynchronous transmission can initiate data transfer and/or report
status changes [e.g., N(R) change, transition from ready to busy or vice
versa, or establishment of an exception condition)].
A node shall transmit a response frame with the F bit set to 1 only in
response to a received command frame with the P bit set to 1. The F bit
set to 1 is not to be interpreted as the end of transmission. Additional
response frames with the F bit set to 0 may be transmitted following the
response frame which had the F bit set to 1.
When a node receives a command frame with a P bit set to 1 while it is in
the middle of transmitting a frame, it must set the F bit to 1 in the earliest
possible response frame it transmits.
Each node shall use a response timeout function which will cause
initiation of appropriate recovery procedures if previously transmitted
unsolicited response frames have not been acknowledged within a defined
timeout period.
Logically Disconnected State
The logically disconnected state (LDS) prevents a node from appearing on
the link in a fully operational sense during unusual situations or
exceptional conditions. Otherwise, such operations could cause:
Unintended Contention
Sequence Number Mismatch
Ambiguity of Status
Chapter 6
HDLC Data Link Layer
6 13