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.

Char Name	control sequences	EBCDIC	ASCII	6BIT
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 -
		DLE'61'	DLE 1	DLE T
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

Char Name	control sequences	EBCDIC	ASCII	6BIT
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

Synchronization

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

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.

Character-Synchronization

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)

<-syn--><-byte1><-byte2><-byte3>

11110001 00110010 00000010 00001101 000000 ...

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

cksum

---------------------------

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 |

---

acknowledgement

------

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 |

------

Polling and Selection