Allen-Bradley 1779-KFMR User Manual page 80

of data frames, and indicating temporary interruption of capability to
receive data frames. The functions of N(R) and P/F are independent.
Use the unnumbered format to provide additional link control functions.
This format contains no sequence numbers; therefore, it has a 5-bit
sub-field for a command/response code.
Frame Variables and Sequence Numbers
Each node must maintain a send variable (S) to keep track of the send
sequence number [N(S)] of each data frame it transmits. Each node must
also maintain a receive variable (R) to keep track of the send sequence
number of each data frame it correctly receives (Figure 6.3).
Figure 6.3
Each Node Keeping Track of Data Frame Sequence Numbers
Computer
S
R
S indicates the sequence number of the next data frame to be transmitted
by that node. S shall take on the value 0 through 7. Only data frames
contain N(S), the sequence number of the data frame being transmitted.
Prior to transmission of a data frame, N(S) is set equal to S. S shall
increment by one with each completed data frame transmission. (S will
not increment when a data frame transmission is aborted.)
R indicates the N(S) expected in the next data frame to be received by that
node (0 through 7). R shall increment by one upon receipt of an error-free
data frame whose N(S) = R. All data frames and supervisory frames
contain an N(R). Immediately before transmitting or retransmitting a data
or supervisory frame, N(R) is set equal to R. Thus, by a node transmitting
an N(R), it acknowledges that it has correctly received all frames
numbered up to and including N(R)—1.
Chapter 6
HDLC Data Link Layer
Frames 1779 KFM Interface
N (S) N (R)
N (S) N (R)
S
R
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