Envoy ACP/XF Application Programming Manual

Features Common to Bit-Synchronous Protocols
EnvoyACP/XF Application Programming Manual–132179
B-3
Control Field
•
Identifies the sequence number of sending frames (when sending data)
•
Acknowledges frames received
The primary or combined station uses the control field to instruct the addressed
secondary or combined station to perform a specific operation. The secondary or
combined station also uses the control field to respond to the remote primary or
combined station.
The control field can consist of either 1 octet (8 bits) or 2 octets (16 bits), as shown in
Figure B-3. The standard 1-octet control field allows modulo-8 frame sequencing; the
extended control field (having 2 consecutive octets) allows modulo-128 frame
sequencing.
Note. The use of extended control fields in IBM environments is subject to certain restrictions.
For information on these restrictions, see “IBM Extended Control Fields” in Section 3, Normal
Response Mode (NRM) Protocols.
Figure B-3. Control Field Format
I-Frame
S-Frame
Basic 1-Octet Format Extended 2-Octet Format
U-Frame
0 1 2 3 4 5 6 7
1 1 U P/F 1 1 U
S S1 Nr Nr
Nr NrNs0
0 1 2 3 4 5 6 7
Ns0P/F
8 9 10 11 12 13 14 15
First Octet Second Octet
LSB MSB LSB MSB LSB MSB LSB MSB
LSB MSB LSB MSB
Legend
Ns
Nr
LSB
MSB
= Number Sent Variable
= Number Received Variable
= Least Significant Bit
= Most Significant Bit
S
U
P/F
= Supervisory Message Encoding
= Unnumbered Message Encoding
= Poll/Final Frame
009
P/F
S S0
P/F 1 0 0 0 P/F0 0
0 00 00 0 0U U U U
P/FU U U UP/F