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 |
------
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