Allen-Bradley 1779-KFMR User Manual page 86

frame nor does it increment its receive variable (R) upon accepting a
supervisory frame.
The encoding of the control field in the supervisory format is as follows:
Control Field Bit No.
Designates
Supervisory
Format
Commands/Responses
RR Receive Ready
RNR Receive Not Ready
REJ Reject
A supervisory frame contains an N(R), which indicates the N(S) expected
in the next data frame to be received by the node transmitting the N(R).
The N(R) is a response that acknowledges that data frames through
N(R)—1 have been received by the node transmitting the supervisory
frame.
Each command/response code specifies both a command and a response:
Receive Ready (RR) is used by a node to indicate that it is ready to
receive data frames and to acknowledge data frames through N(R)—1.
One node may use the RR command with the P bit set to 1 to solicit a
response from the other node.
Receive Not Ready (RNR) is used by a node to indicate a busy
condition (the temporary inability to accept additional incoming data
frames) and to acknowledge data frames through N(R)—1.
Reject (REJ) is used by a node to request retransmission of data
frames starting with fame N(R). Data frames through N(R)—1 are
acknowledged. A REJ frame is one way to report the end of a busy
condition. After sending a REJ frame, a node must not send another
REJ frame until the resulting REJ exception condition is cleared upon
acceptance of a data frame with an N(S) equal to the N(R) of the REJ
frame or until after a timeout has occurred.
Chapter 6
HDLC Data Link Layer
Time
0 1 2 3 4
1 0 C/R P/F
Code
0 0
1 0
0 1
5 6 7
N (R)
Receive Sequence
Number Modulo 8
Command: 1 = Poll
Response: 1 = Final
6 17