HP 262SA Reference Manual page 144

Data Communications
TEXT TERMINATION. When the terminal is receiving text (Text-In Mode) it will accept only
the block protocol control character ETB (octal 27), ETX (octal 3), or ENQ (octal 5) as a text
block terminator. Table 7-6 lists the characters used for control of block data transfers.
ETB Indicates the end of a block with one or more blocks to follow.
ETX Indicates the end of the current block and the end of the text transfer.
ENQ Indicates that the current block has been aborted. The terminal will respond
to the ENQ with a NAK to request the retransmission of the aborted text
block.
When the terminal is sending data (Text-Out Mode), it will terminate text blocks with either an
ETB or an ETX character.
All characters sent or received between the STX character and the terminating character must
not be more than 40 milliseconds apart for asynchronous operation. Synchronous operation
requires SYN or DLE SYN characters to be sent as fill characters if no text characters are
ready for transmission. SYN insertion must also be performed at one second intervals within
text blocks.
The terminal may send an STX ENQ as a Temporary Text Delay (TTD) notification instead of
the next block ofdata. This indicates that there is more text to come but that it is not ready to be
transmitted. A TTD should be answered with a NAK to request the transmission of the text
block, or an EOT to reset the terminal to control mode.
Table 7-6. Control Characters Used in Multipoint Data Transmission
ASCII
CONTROL CODE
CHARACTER (HEX) DESCRIPTION
Data link control characters. These characters are used to frame messages and ack-
nowledgements for both transmitted and received text blocks. They are also used to control all
communications in an orderly fashion.
DLE
10
Data Link Escape. This is the first character in two
byte control characters. The DLE character is usually
treated as data when used alone.
ACKO (DLE 0)
10 30
Acknowledge O. These control characters are sent by
the terminal after being selected to tell the computer
that the terminal is ready to accept a text block. They
are also sent by the receiving station (computer or
terminal) after even text blocks (2, 4, etc.) to tell the
sending station (terminal or computer) that the block
was received properly (see ACKl). The alternating
ACKO/ACKI
sequence is initialized to ACKO following
select sequence or to ACKI after a poll sequence.
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