AM3270/TR3271 Reference Manual

Control Characters
Bisync Protocols—Concepts and Terminology
1–6 086705 Tandem Computers Incorporated
Table 1-1 lists the bisync transmission control characters, and describes their use and
the appropriate response, if any.
Table 1-1. Bisync Transmission Control Characters (Page 1 of 2)
Bisync Control
Character
(Mnemonic) Function Action and Response(s)
SYN Synchronous idle Allows the receiver to synchronize characters.
SOH Start of Header Indicates start of header block.
STX Start of Text Indicates start of text block.
ETX End of Text Indicates end of text block started with SOH or STX.
Has the same function as ETB except that it also
indicates no more blocks to send. The BCC follows.
ETB End of Transmission
Block
Indicates end of a text block that started with SOH or
STX. The BCC follows.
ETB requires a response from the receiver that
indicates its status: ACK0, ACK1, NAK, WACK, RVI.
EOT End of Transmission Indicates end of a complete transmission sequence
containing one or more blocks, which in effect clears
the link.
Also used to indicate “nothing to transmit” to a poll
request. Also can be used as an abort signal.
DLE Data Link Escape DLE STX turns on data transparency, DLE ETX or DLE
ETB turn it off .
Used to create two-character sequences. For example,
in EBCDIC an RVI is sent as DLE@
DLE ENQ, DLE DLE, DLE EOT are used for control
checks during transparent transmission.
ENQ Enquiry Requests a response from another device, used in link
setup and polling/selecting sequences to contact the
remote device.
Also used to request retransmission of ACK/NAK when
original response was missing or contained errors.
ACKx Acknowledge ACK is a positive acknowledgement:
ACK0 is even, ACK1 is odd.
Indicates that the last block was received without error,
and the receiver is ready to receive the next block.
Also used to acknowledge multipoint selection.
NAK
(or NACK)
Negative
acknowledgement
Negative acknowledgement of a data block or a poll
request. Also indicates the data block was received but
there was an error.
WACK
(or WAK)
Wait before
Transmitting
WACK is a positive acknowledgement.
Indicates that the last block was received without error,
but the receiver is not ready to receive the next block.
The sender responds with an ENQ. The receiver sends
WACKs until it is ready to receive.