6100 BSC Programming Manual

6100 BSC Concepts and Context
The meaning of DLE to the receiver is "The next character is not
transparent; it is really a control character." In processing
incoming transparent text, BSC ignores any control character not
preceded by DLE. The receiver interprets DLE DLE to mean "Ignore
the first DLE, and treat the second as data." The receiver
strips DLE from the block except where it precedes STX or is
preceded by DLE; in these two cases, DLE is delivered to the
application.
Except in the one case described below, transparent mode
stays in effect only until the end of the block.
Transparent mode has special implications for messages blocked
with ITB. First of all, the application does not put the ITB
characters in its buffer. (An ITB character in the buffer would
be treated as data.) Rather, you specify a block length as a
SYSGEN parameter. Then if the application sends a transparent
message longer than the block length--e.g., if the message is 200
characters long and the block length is 100 characters--BSC
inserts the following sequence at intervals equal to the block
length:
DLE ITB BCC BCC SYN SYN DLE STX
The receiver strips this sequence from incoming data and delivers
the message as a single block to the application. Notice that
the sending application supplies only one DLE; BSC initiates
transparent mode for every subsequent block of the message. This
practice has an important implication: while in other cases you
can mix transparent and normal modes in the same message--by
issuing a separate request for each block and starting
transparent blocks with DLE STX--in this case one text mode
applies to the whole message. Also notice that the SYN pattern
between the blocks is SYN SYN, not DLE SYN; transparent mode is
never in effect between the blocks of a message.
Some devices that use BSC may not support transparent text mode.
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