6100 MPS-B Programming Manual

INTRODUCTION TO 6100 MPS-B
Message Formats
The parts of the message that must be supplied by, and which are
visible to, your application process are:
• The station address, which is the Message Control Word (MCW),
or if MCW is zero, the sequence header. The sequence header
begins with EOT and is terminated with ENQ (see "Request
Message Format" in this section).
• The SOH (Start of Heading) or STX (Start of Text) character.
This character signals the beginning of the text part of the
message.
• The text, which is application-dependent. It usually contains
application data. It also may contain control information for
a target station or for processing by higher-level
communication software.
• The ETX (End of Text) character. This character signals the
end of the text part of the message.
Your application includes these characters in its output buffer
when sending a message and finds them in its input buffer when
receiving a message.
If you use a line monitor to observe the activity on the line,
you will see these other parts of the message (invisible to your
application process):
• The initial SYN pattern. This pattern allows the stations to
synchronize. It should consist of at least two SYN
characters; you define the number as a SYSGEN parameter.
• The BCC (Block Check Character). The block check is the
result of a running computation. It is used by the
communicating stations for error control. The computation
begins with the character immediately following the SOH or
STX and proceeds through the terminating ETX. The station
sending the message computes and transmits the BCC character;
the receiving station performs 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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