Telecommunications - Bi Sync Protocol - Data Link Control

Mixed with the character codes are a number of codes for data link control, some of these control sequences are one character long and some are two characters long. The use of these characters is described later.

control sequences Hex coding
SYN synchronous idle 32 16 3A
STX start of text 02 02 0A
ITB end of intermediate transmission block IUS=1F US=1F US=1D
ETB end of transmission block 26 17 0F
ETX end of text 03 03 2E
ENQ enquiry 2D 05 2D
ACK0/ACK1 Alternating affirmative acknowledgments DLE'70' DLE 0 DLE -
WACK Wait before transmit positive acknowledgment DLE'6B' DLE ; DLE W
NAK Negative Acknowledgment 3D 15 3D
DLE Data Link Escape 10 10 1F
RVI Reverse Interrupt DLE7C' DLE < DLE 2
TTD Temporary Text Delay STX ENQ STX ENQ STX ENQ

Link + higher layer control

SOH start of heading 01 01 00
EOT end of transmission 37 04 1E
ESC escape 27
DLE EOT Disconnect Sequence for a switched line


For Asynchronous links both bit and character synchronization are done by the start and stop bits on every character, for a synchronous protocol such as Bisync synchronization is handled as follows:


Bit - synchronization is handled by the modem/Kilostream terminating equipment, a clock signal is derived by the modem, for odder types of modem up to 16 bit transitions may be necessary at the start of transmission for bit synchronization so a PAD - 01010101 sequence (=hex 55) may be sent before the message.


At least 2 contiguous SYN characters are inserted at the start of the message to establish character synchronization and then at 1 second intervals.

Because Bisync is designed to allow multidrop SYN characters are not normally sent continuously between messages so character-synchronization must be achieved at the start of each message.

Once character-synchronization is achieved and the receiving station will not re-synchronize until the end-of-message or until a 3 second receive timeout has expired having not detected a SYN character.

Example (EBCDIC)


11110001001100100000001000001101000000 ...

The receiving end searches through the datasteam one bit at a time until it finds a match, in the case of EBCDIC the SYN character is hex 32 sent lsb first, once it has found this it knows that the following bytes continue on immediately after.

slide ibm - bsc2 ascii

Format of message blocks

The message blocks do the work carry the text/data), the general format is as follows:

<--- optional --------->


PAD SYN SYN | SOH | heading ... | STX | text ... | ETX | BCC |

'55' +-----+-----------------+-----+---------------+-----+-----|

or ETB

or ITB

Message Block

PAD SYN SYN is followed

by SOH (if heading included)

or STX (text)

The message block ends with,

ETX BCC end of text (line turn round)
ETB BCC end of block but not end of text (line turn round)
ITB BCC end of block but not end of text (no line turn round)

Where BCC is a checksum sequence as described later,

Maximum length of message block

BSC does not define a maximum block length, however devices such as the 3274 Perform inbound text blocking. Each Block of data can contain a maximum of 256 text characters. Of that total, each block contains the STX and ETB (or ETX) data-link control characters. Two address bytes (CU poll address and device address) precede the read heading in the first block only and are included in the 256-character total.

Common message blocks and control sequences

<--- optional ------->


Message Block P S S | S | heading ... | S | text ... | E E I | BCC |

A Y Y | O | | T | | T or T or T | |

D N N | H | | X | | X B B | |


5 <--- all characters after first SOH ---> <--->

5 or STX included in block check 8,12 or

accumulation (except SYN characters) 16 bit



Polling/ P S S | CU | CU | dev | dev | E |

Selection A Y Y | add | add | add | add | N |

D N N | | | | | Q |



End P S S | E | P

Of A Y Y | O | A

Text D N N | T | D



ACK 0 P S S | A |

A Y Y | C |

D N N | K 0 |



ACK 1 P S S | A |

A Y Y | C |

D N N | K 1 |



NAK P S S | N | P

negative A Y Y | A | A

acknowledge- D N N | K | D

ment ---


WACK P S S | W |

wait-before A Y Y | A |

transmit- D N N | C |

positive- | K |




RVI P S S | D |

Reverse A Y Y | L @ |

Interrupt D N N | E |



TTD P S S | S E |

temporary A Y Y | T N |

text delay D N N | X Q |


goto next document - Polling and Selection


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