AEA PK-232 Technical Reference Manual page 56

PK-232 TECHNICAL MANUAL
A.3.2.4.4 Received Sequence Number N(R)
The received sequence number is in both I and S frames. Prior to sending an I or S frame, this
variable is updated to equal that of the received state variable, thus implicitly acknowledging the
proper reception of all frames up to and including N(R)-1.
A.3.3 Functions of Poll/Final (P/F) Bit
The P/F bit is used in all types of frames. It is used in a command (poll) mode to request an imme-
diate reply to a frame. The reply to this poll is indicated by setting the response (final) bit in the
appropriate frame. Only one outstanding poll condition per direction is allowed at a time. The pro-
cedure for P/F bit operation is described in A.4.2.
A.3.4 Control Field Coding for Commands and Responses
The following commands and responses, indicated by their control field encoding, are to be use by
the DXE:
A.3.4.1 Information Command Frame Control Field
The function of the information (I) command is to transfer across a data link sequentially num-
bered frames containing an information field.
The information-frame control field is encoded as shown in Fig. 6. These frames are sequentially
numbered by the N(S) subfield to maintain control of their passage over the link-layer connection.
A.3.4.2 Supervisory Frame Control Field
The supervisory frame control fields are encoded as shown in Fig. 7.
Control Field Type
Receive Ready
Receive Not Ready
Reject
C or SABM Layer 2 Connect Request
D or DISC Layer 2 Disconnect Request
I
RR
RNR or NR Receive Not Ready. TNC Buffer Full
RJ or REJ
FRMR
UI
DM
PK232TM Rev. A 5/87
Fig. 6 – I frame control field
Control Field Bits
7 6 5 4 3
N(R) P
Fig. 7 – S frame control fields
7
RR N(R)
RNR N(R)
REJ N(R)
The Frame identifiers:
Information Frame
Receive Ready. System Ready To Receive
Reject Frame. Out of Sequence or Duplicate
Frame Reject. Fatal Error
Unnumbered Information Frame. "Unproto"
Disconnect Mode. System Busy or Disconnected.
A-11
APPENDIX A – AX.25 LEVEL 2 PROTOCOL
2 0
N(S) 0
Control Field Bits
6 5 4 3
P/F 0
P/F 0
P/F 1
2 1 0
0 0 1
1 0 1
0 0 1
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