| 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 |
cariage 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. |
