I saved this from usenet a long time ago...
Subject: Re: original uses of ASCII control codes
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
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
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
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
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.
My laptop's battery is running out so I don't have much time to continue,
but one cutie is ENQ. Teletypes (at least 33's and 35's) had what was
called the "here is" drum. This was a small plastic drum with plastic
fingers which could be broken off. You could thus code a short message
on the drum (anybody know how many characters?) I saw this used as an
automatic "log on" way back in the 60's. CMU (then CIT) had a Bendix
(Control Data) G-20 computer system with 16 dial in lines (110 baud, Bell
103A's, real ones!) and a number of teletypes placed around the campus (as
well as in professor's homes...) Each of these had a unique number assigned
to it. You could locally activate the Here Is drum by pressing the "HERE IS"
key and the teletype would respond with something like CIT REMOTE 20. When
you would dial in however, the computer wanted to know which teletype you
were calling from (so it could call it back when the program you submitted
was done, it was a batch RJE system and "knew" the phone number of each
remote) so it sent the ENQ character which "INQUIRED" or is that "ENQUIRED"
the teletype causing it to run its here is drum. Imminent power failure...