AEA PK-232 Technical Reference Manual page 57

PK-232 TECHNICAL MANUAL
A.3.4.2.1 Receive Ready (RR) Command and Response
Receive Ready is used to do the following:
1. to indicate that the sender of the RR is now able to receive more I frames.
2. to acknowledge properly received I frames up to, and including N(R)-1, and
3. to clear a previously set busy condition created by an RNR command having been sent.
The status of the DXE at the other end of the link can be requested by sending a RR command
frame with the P-bit set to one.
A.3.4.2.2 Receive Not Ready (RNR) Command and Response
Receive Not Ready is used to indicate to the sender of I frames that the receiving DXE is tempora-
rily busy and cannot accept any more I frames. Frames up to N(R)-1 are acknowledged. Any I
frames numbered N(R) and higher that might have been caught between states and not acknowl-
edged when the RNR command was sent are not acknowledged.
The RNR condition can be cleared by the sending of a UA, RR, REJ, or SABM frame.
The status of the DXE at the other end of the link can be requested by sending a RNR command
frame with the P bit set to one.
A.3.4.2.3 Reject (REJ) Command and Response
The reject frame is used to request retransmission of I frames starting with N(R). Any frames that
were sent with a sequence number of N(R)-1 or less are acknowledged. Additional I frames may
be appended to the retransmission of the N(R) frame if there are any.
Only one reject frame condition is allowed in each direction at a time. The reject condition is
cleared by the proper reception of I frames up to the I frame that caused the reject condition to
be initiated.
The status of the DXE at the other end of the link can be requested by sending a REJ command
frame with the P bit set to one.
A.3.4.3 Unnumbered Frame Control Fields
Unnumbered frame control fields are either commands or responses. Fig. 8 shows the layout of U
frames implemented within this protocol.
Set Asynchronous Balanced Mode SABM
Disconnect
Disconnected Mode
Unnumbered Acknowledge
Frame Reject
Unnumbered Information
PK232TM Rev. A 5/87
Fig. 8 – U frame control fields
Control Field Type
7
Res 0
DISC Cm 0
DM Res 0
UA Res 0
FRMR Res 1
UI Either 0
A-12
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
Control Field Bits
6 5 4 3 2
0 1 P 1 1
1 0 P 0 0
0 0 F 1 1
1 1 F 0 0
0 0 F 0 1
0 0 P/F 0 0
1 0
1 1
1 1
1 1
1 1
1 1
1 1
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