Telecommunications - Bi Sync Protocol - Data Transfer

Interrupts and Aborts

Forward Abort

A Block of text can be terminated prematurely by using an ENQ character which signals the receiver to "disregard this block" NAK is always the reply in this situation.

RVI Reverse Interrupt

The RVI control sequence is a positive response used in place of the ACK0 or ACK1 positive acknowlegement. RVI is transmed by a receiving station to request termination of the current transmission because of a high priority message which it must transmit to the sending station.

Flow Control

WACK (Wait-before-transmit-positive-acknowledgement) allows a receiving station to indicate a "temporary not ready to receive"


S S | S |

Y Y | O | ---> message

N N | H |



S S | W |

<--- Y Y | A |

N N | C |




S S | E |

Y Y | N | ---->

N N | Q |


. continues to send WACK in response

. to ENQ until ready to continue



S S | E |

Y Y | N | ---->

N N | Q |



S S | A |

<---- Y Y | C |

N N | K |




S S | S |

Y Y | O | next message -->

N N | H |


TTD - Temporary Text Delay

The TTD control sequence is sent by a sending station in message transfer

state when it wishs to retain the line but is not ready to transmit.

The normal response to TTD is a NAK





Transparent Data (optional - can be used in EBCDIC or ascii)

If information which is not EBCDIC or ascii text is to be sent in a message then a bit pattern equivalant to a control code may occur, to avoid this transparent mode must be used. In transparent mode all data-link control sequences must start with DLE as follows,




DLE SYN = SYN inside message only

message starts with SYN SYN as usual


DLE DLE used to permit transmission of DLE as data when a bit pattern

equivalent to DLE appears within the transparent data.

One DLE is disregarded the other is treated as data.


EOT and NAK must be followed by a trailing 'FF' PAD character

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