6100 BSC Programming Manual

6100 BSC Concepts and Context
S5066-004
PAD SYN...SYN STX TEXT ETX BCC PAD
At Least
2 SYN
Characters
BCC Accumulation
Includes These
Characters
1 or 2
Characters
SOH
HEADING
DATA
Application-Supplied
BSC-Supplied
Figure 1-5. Message with Heading
As the figure shows, the heading begins with the character SOH,
which has a value of 01 hexadecimal in ASCII or EBCDIC. By
convention, SOH is followed by a special character, which
applications can interpret to mean "That last character should
have been an SOH." The special character acts as insurance in
case of an error in transmitting SOH. The requirements of a
device might dictate your choice of special character; otherwise
the restrictions are only that it can't be a percent sign (%),
and it shouldn't be a character that will appear after STX.
(This second restriction prevents heading data from being
mistaken for message text.) The heading ends with a data field,
which contains information needed by the device or application.
The sending application includes the heading in its output
buffer--normally along with the rest of the message, as shown in
Figure 1-5. The receiving station finds them in its input
buffer.
If a message begins with a heading, the accumulation of the block
check sequence begins right after the SOH. It covers the rest of
the transmission, including the STX character.
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