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From: jones@pyrite.cs.uiowa.edu (Douglas W. Jones) Newsgroups: alt.folklore.computers Subject: Re: ASCII control codes Date: 31 Aug 1995 19:37:58 GMT

From article 4239uk$ko1@metro.ucc.su.OZ.AU, by adrianw@cassius (Adrian Whichello):

Some are obviously still used for their original purpose (CR, LF, the
tabbing codes etc.) but what did ENQ, EM, SUB, FS, etc. do? Thanks,

- - - here's a reply I posted a year ago in June - - -

Subject: Re: original uses of ASCII control codes Newsgroups: alt.folklore.computers References: 1994Jun02.064649.131411@ua1ix.ua.edu

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:

NAME	HEX/CTL	USE

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	19 ^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.