The ASCII Control Characters
Adrian Wichello

To understand the original intent of the ASCII control codes, you have to think of teletypes, using paper tape, configured in a multidrop system with relay logic used to turn on or off individual teletypes in the bunch, and you have to remember that the designers were pretty smart and they anticipated future developments, but they also managed to include provisions for things that never happened. Here are the ASCII control characters, and a few others, with comments on how they were supposed to be used and how this relates to current popular uses:

NULL 00 ^@ always ignored - leader and trailer on paper tape systems was typically made of sequences of NULLs.
SOH 01 ^A start of heading - imagine a heading containing, for example, the address of the recipient. You could have relay logic that scans for SOH, then enables the print mechanism if the following character matches this station's address. In early documentation, this was called start of message.
STX 02 ^B start of text - if the heading matched, start printing with the following character. In early documentation, this was called end of address.
ETX 03 ^C end of text - now is a good time to stop printing. Your message might continue after this with a checksum or other administrative stuff. In early documentation, this was called end of message. The common use of control C as a kill character stems from this - it indicates the end of your text addressed to some application.
EOT 04 ^D end of transmission - relay logic could decode this and, if there is a tape in the tape reader, it could begin transmitting its own message.
ENQ 05 ^E enquire - on receiving this, local relay logic would generate a response. In early documentation, this was called WRU or who are you. Teletypes had programmable response sequences that were encoded on a music box mechanism, and it was up to the customer to break plastic fingers off the drum to code how it responded to an ENQ.
ACK 06 ^F acknowledge - one possible response to ENQ. In early documentation, this was called RU or are you.
BEL 07 ^G bell - ring the bell in the terminal. Teletypes had real bells where most modern terminals have beepers of some kind. A sequence of BEL characters sent to a teletype sounded very much like a telephone ringing.
BS 08 ^H backspace.
HT 09 ^I horizontal tab.
LF 0A ^J linefeed.
VT 0B ^K vertical tab.
FF 0C ^L formfeed - page eject.
CR 0D ^M carrage return - on many mechanical devices, CR was slow. The sequence CR LF was always sent in that order so that the linefeed could be handled while the carriage was returning; a well adjusted Teletype could just finish the CR in this time (0.2 seconds), and a common sign that it was time to call the service man was that the first letter printed after a CR LF was printed "on the fly" on the way back to the margin.
SO 0E ^N shift out - if you've got a two-color ribbon, shift to the alternate color, usually red. There are obvious extensions of this to alternate character sets.
SI 0F ^O shift in - undo whatever SO does. For mysterious reasons that have no apparent connection to old or modern ASCII standards, DEC liked to use control O as a break character to suppress teletype output.
DLE 10 ^P data link escape - an escape character is generally a prefix for something else. DLE was expected to be used as a prefix on characters in the user data stream that might otherwise be interpreted as data link control characters, for example, flow control characters. In some early documentation, this was called DC0 or device control zero.
DC1 11 ^Q device control 1 - turn on the paper tape reader. In early documentation, this was called XON.
DC2 12 ^R device control 2 - turn on the paper tape punch.
DC3 13 ^S device control 3 - turn off the paper tape reader. In early documentation, this was called XOFF, The use of XON/XOFF (DC1/DC3) for flow control stems from their use to control the flow of data from the paper tape reader attached to a Teletype.
DC4 14 ^T device control 4 - turn off the paper tape punch.
NAK 15 ^U negative acknowledge - another possible response to ENQ. One flow control mechanism is to use ENQ to ask if the receiver has buffer space, and require the receiver to respond with either ACK (yes) or NAK (no). ENQ could also be used to enquire about whether a retransmission is required after sending a checksum. The popular use of control U to delete the current input line is only vaguely grounded in this definition.
SYN 16 ^V synchronous idle - if you're using a synchronous transmission protocol, and you have no data to send, you send SYN characters to keep the clocks synchronized. The receiver should ignore these, and the transmitter may have to insert them into the data stream once in a while.
ETB 17 ^W end of transmission block - used when a transmission must be broken into many blocks for some reason, for example, to place a checksum after each block. Early documentation called this logical end of media.
CAN 18 ^X cancel - take that back, what I just sent you is a mistake, ignore it.
EM 9 Y end of medium - there's nothing left on this reel of (paper) tape.
SUB 1A ^Z substitute - the next character is from an alternate character set. SUB X might be equivalent to SO X SI, or it might be an alternate mechanism for extending the character set. The common use of control Z as an end of file character has no obvious relation to the standard.
ESC 1B ^[ escape - the next character is to be interpreted as something other than text, for example, it might be an extended control character of some kind.
FS 1C ^\ file separator - useful if you have multiple logical files in one transmission.
GS 1D ^] group separator - useful if files are made of groups of records.
RS 1E ^^ record separator - COBOL anyone?
US 1F ^_ unit separator - are records made of units?
ALT 7D } Some early teletypes had an ALT MODE key that generated this code instead of ESC. This was interpreted as an escape code, which was no problem when nobody had lower case printers, but with the advent of full 96 character ASCII, there were obvious compatability problems.
PRE 7E ~ A few terminals had a PREFIX key that generated this code instead of ALT MODE, with all the same problems.
DEL 7F delete - remember, paper tape uses a hole to record each one and no hole to record each zero. DEL is all holes, so it can be punched over any other character to rub it out (on old teletypes, it was the RUB or RUB OUT key). If you mispunch a character, just back up the tape and overpunch it with a DEL. Software is expected to ignore DEL the same way it ignores NULL.

Posted by Adrian Whichello on 1994 Jun 02 on alt.folklore.computers

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