Allen-Bradley 1779-KFMR User Manual page 87

Chapter 6
HDLC Data Link Layer
6 18
Unnumbered Format
Unnumbered commands are used to change mode. Unnumbered
responses are used to acknowledge the change in mode. Unnumbered
frames do not contain a data field. Therefore, a node does not increment
its send variable (S) upon the transmission of an unnumbered frame nor
does it increment its receive variable (R) upon accepting an unnumbered
frame. The encoding of the control field in the unnumbered format is as
follows:
Control Field Bit No.
Designates
Unnumbered
Format
Commands
SABM Set Asynchronous Balanced Mode 1
DISC Disconnect
Responses
UA Unnumbered Acknowledgment
DM Disconnected Mode
FRMR Frame Reject
Mode-setting commands are transmitted to change the mode at both
nodes. Once established, a mode remains in effect until one node
transmits another mode-setting command and the other node accepts it by
transmitting the appropriate response. If the command frame has its P bit
set to 1, the response frame must have its F bit set to 1.
Each node must restrict the transmission of mode-setting commands so
that only one such command is outstanding (not acknowledged) at any
given time. This eliminates the requirement for a node to queue responses
and prevents any ambiguity regarding the meaning of the response.
Set asynchronous balanced mode (SABM) is a command to place the
nodes into the asynchronous balanced mode (ABM) which is in the data
transfer state (DTS). Upon receiving and accepting the SABM command,
a node transmits an unnumbered acknowledgment (UA) response. If a
node is in the asynchronous disconnected mode (ABM) and is unable to
Time
0 1 2 3
1 1 C/R P/F
Code
1
0 0
0 0
1 1
1 0
Command: 1 = Poll
Response: 1 = Final
4 5 6 7
C/R Code
Extension
1 0 0
0 1 0
1 1 0
0 0 0
0 0 1