6100 BSC Programming Manual

6100 BSC Concepts and Context
The ETB character (End Transmission Block) has a value of 17
hexadecimal in ASCII, 26 hexadecimal in EBCDIC. It completes a
block and turns the line around, permitting the remote station to
reply. As shown in the figure, ETB is permitted at the end of
the heading or of a block of text. In each case, the block check
sequence immediately follows the ETB; the subsequent block begins
with STX, which starts a new block check accumulation. The last
block in the message can end with either ETX or ETB. (Many
devices require ETX, but 6100 BSC does not enforce the
restriction.)
Each block that ends with ETB or ETX requires a separate
application request, whether you are reading or writing the
block. If the remote station responds to any block with a
negative acknowledgement, BSC sends that block again; it need not
retry any earlier transmission. Notice that BSC starts each
transmission with a pad and initial SYN pattern, and ends each
transmission with a trailing pad, as described above.
The ITB character (End of Intermediate Transmission Block) has a
value of 1F hexadecimal in ASCII or EBCDIC. It completes a block
but doesn't turn the line around; thus it lets you concatenate
multiple blocks in a single transmission. ITB is permitted
anywhere in the data portion of the heading or in the message
text, as in Figure 1-6. BSC adds a block check sequence after
each ITB character.
NOTE
ITB is sometimes called IUS in ASCII or US in EBCDIC.
If a message is blocked with ITB, only the first block of the
heading is required to start with SOH; only the first block of
the text is required to start with STX. The last block of the
transmission must end with ETB or ETX. Some devices require
ETX, but 6100 BSC does not enforce the restriction.
An application can send or receive multiple blocks that end with
ITB, without making a separate request to send or receive each
block. The buffer size of each station must be large enough to
hold the whole transmission.
After an intermediate block, a new block check accumulation
begins with the first character of the next block, even if that
character is STX. If the remote station finds an error in any of
the block check sequences in a message, it responds to the sender
with a negative acknowledgement; BSC repeats transmission of all
intermediate blocks.
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