Envoy ACP/XF Application Programming Manual

EnvoyACP/XF Application Programming Manual–132179
B-1
B
F
eat
s Protocols
Bit-synchronous protocols provide the following features:
•
Synchronous data transmission
•
Message-framing format, in which specific bit patterns delineate message blocks
•
Data-code transparency
•
Multiple-frame transmission before acknowledgment, which reduces the requisite
number of line turnarounds
•
Error detection for addressing, control, and data integrity through cyclic redundancy
checking
Message Formats
In a bit-synchronous communications environment, messages are divided into
transmission units called frames. All bit-synchronous frames adhere to the same general
format (Figure B-1). The bit-synchronous frame fields are described after Figure B-1.
Flag
All frames begin and end with a flag sequence: a leading 0 bit followed by six 1 bits and
a trailing 0 bit (01111110). All bit-synchronous stations constantly monitor the line for a
flag sequence, which indicates the start of a message frame and aids in synchronization.
Your application does not have to deal with the flags. The flags are automatically added
and stripped by the communications line interface processor (CLIP).
Figure B-1. Bit-Synchronous Frame Format
008
* Using SCF or a SETMODE procedure call, you can extend the size of these fields when
using ADCCP. In extended modes, the address field can be up to four octets long, and the
control field can be two octets long.
Flag
1 Octet
Address
Field
1 Octet*
Control
Field
1 Octet*
Frame Check
Sequence Field
2 Octets
Flag
1 Octet
Information Field
(Variable)
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