Allen-Bradley 1779-KFMR User Manual page 85

Chapter 6
HDLC Data Link Layer
6 16
Control Field Bit No.
The data frame control field contains two sequence numbers:
N(S) is the send sequence number which indicates the sequence number
of the data frame containing the N(S).
N(R) is the receive sequence number which indicates the N(S) expected
in the next data frame to be received by the node transmitting the N(R).
The control field of a data frame contains only data commands and
responses. Therefore, it does not have a sub-field for command/response
codes to distinguish between different commands and responses.
Each data frame control field contains both a command and a response.
The command is to receive the accompanying data field. The response is
an acknowledgment that data frames through N(R)—1 have been received
by the node transmitting the data frame.
Supervisory Format
Supervisory command/responses perform basic supervisory link control
functions such as:
Data Frame Acknowledgment
Polling
Temporary Interruption of Data Transfer
Error Recovery
Supervisory frames do not contain a data field. Therefore, a node does
not increment its send variable (S) upon the transmission of a supervisory
Time
0 1 2 3
0 N (S) P/F
Send Sequence
Number Modulo 8
Designates
Data Transfer Format
4 5 6 7
N (R)
Receive Sequence
Number Modulo 8
Command: 1 = Poll
Response: 1 = Final