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 | |||
| 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
| 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>
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
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 |
------
goto next document - Polling and Selection
