6100 BSC Programming Manual

6100 BSC Concepts and Context
S5066-003
PAD SYN...SYN STX TEXT ETX BCC PAD
Application-Supplied
BSC-Supplied
At Least
2 SYN
Characters
BCC Accumulation
Includes These
Characters
1 or 2
Characters
Figure 1-4. Basic Message Format
If you were using a line monitor to observe the activity on the
line, you would also see these other parts of the message,
invisible to the application:
• The leading pad. This character is present at the beginning
of each transmission; it helps to synchronize the modems and
has a value of 16 hexadecimal in ASCII, 32 hexadecimal in
EBCDIC. (The IBM manual allows another value for the pad,
but 6100 BSC supports only these values.)
• Initial SYN pattern. This pattern allows the stations to
synchronize. It consists of at least two SYN characters; you
define the number as a SYSGEN parameter. The value of SYN is
the same as that of the leading pad: 16 hexadecimal in ASCII,
32 hexadecimal in EBCDIC.
• The block check sequence (BCC). The block check is a
one-character or two-character sequence, the result of a
running computation. It is used by the communicating stations
for error control. The computation begins with the character
after the STX and proceeds through the final ETX. The station
sending the message computes and transmits the block check
sequence; the receiving station does the same computation and
compares its result to the received sequence. If the sender
and receiver have the same results, the message is assumed to
be error-free.
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