A do-nothing char, usually referred to as media fill time or padding. It has no information value but is regarded as 'time filler' (for speed of transmission); thus it gives hardware device time to perform some operation.
SOH or
Start of Header
Marks the beginning of the header information which may be of any length.
STX or
Start of Text
Marks the beginning of the data (TEXT) block and thus end of the header.
ETX or
End of Text
Marks the end of data (TEXT) block.
On Mac Enter Key.
EOT or
End of Transmission
Marks the end of transmission; usually sent to receiver instead of SOH char.
ENQ or Enquiry
Sent by one of the computers to establish communication to enquire the presence of the second computer.
In many cases used for identification or status enquiry.
ACK or
Acknowledge
Sent after an error free reception of a message, usually a block.
BEL or Bell
Signal sent to cause an attention.
May be audible, visual or of other type.
BS or Backspace
Moves the cursor back on the SAME line. Thus it cannot move the cursor from the beginning to the end of the previous line.
Also used for text formatting (non-MAC) eg bold, italics etc.
HT or
Horizontal Tab
Plain TAB character; however there is no ASCII representation for TAB spacing. On MAC this spacing info is in resource fork.
LF or Line Feed
Generates printing of a new line, but does not affect the column position of the cursor only it's active position is one line down. Not used by MAC; thus must be inserted or deleted during file transmission.
VT or Vertical Tab
Regarded as a 'super' line feed that allows us to jump more than one line at a time.
When agreed, it may act as a Carriage Return. (See CR).
Now not used at all; in future can be changed for a new char eg EOL.
FF or Form Feed
Advances the active position to a predeterm. line on the next page. Number of lines is calculated by the device itself (printer). Printers not supporting FF must receive LFs and CRs.
Some terminals clear screens upon receiving FF.
CR or Carriage Return
Advances the cursor active position to the first column of the SAME line. Used with LF to generate a new line; on Mac it acts as both LF and CR.
On some printers (not screens), CR is used for text formatting (non-MAC).
SO or Shift Out
Used in conjunction with SI and ESC to activate extended char set.
SO invokes alternate char sets which may be of control, char, or graphics type. To come back use SI.
See also SI and ESC.
SI or Shift In
Used in conjunction with S0 and ESC to activate extended char set.
Used after SO char to come back from the alternate char set. The use of SI and SO can be nested like FOR loops to allow the use of more char sets.
DLE or
Data Link Escape
Signals that next characters that follow have special meaning. Thus it is used to supplement existing control characters (creates control code sequence).
DC1 or
Device Control 1
Has no ASCII meaning. Used internally by the OS most commonly as XON (or release) char. Sent by a receiver when it wishes the sender to resume. See also DC3.
On MAC it also draws command char.
DC2 or
Device Control 2
Has no ASCII meaning. Used internally by the OS eg on MAC it draws check mark char.
DC3 or
Device Control 3
Has no ASCII meaning. Used internally by the OS most commonly as XOFF (or holdoff) char. Sent by a receiver when it wishes the sender to pause in sending data. See also DC1.
On MAC it also draws diamond char.
DC4 or
Device Control 4
Has no ASCII meaning. Used internally by the OS eg on MAC it draws apple char.
NAK or
Negative Acknowledge
Transmitted by receiver upon encountering an error in a message.
SYN or
Synchronous Idle
Used in synchronization systems to maintain synchronization on data free lines.
ETB or
End of Trans Block
Sent in place of ETX especially if the physical EOF is encountered before the logical EOF.
CAN or Cancel
Cancels previously agreed-upon number of preceding characters.
Some OSs use it to delete partially typed commands.
EM or
End of Medium
Indicates that the preceding character on the medium was the last usable one; not necessarily indicating exhausted medium.
SUB or Substitute
Placed in the data stream in place for any erroneous char, eg using non-ASCII char ($80 - $FF) on non-MAC should produce this char, typed as mirror image of '?'.
ESC or Escape
Used with SI and SO.
Used at the beginning of an Escape Sequence to signal the start of control information.
On Mac Clear Key.
See also SI and SO.
FS or
File Separator
Used with GS, RS, US chars to introduce hierarchical order to data. They may have arbitrary meanings but hierarchical order must be maintained.
FS is the most inclusive.
On Mac Left Arrow.
GS or
Group Separator
Used with FS, RS, US chars to introduce hierarchical order to data.
GS is the second most inclusive.
See also FS, RS, US.
On Mac Right Arrow.
RS or
Record Separator
Used with FS, GS, US chars to introduce hierarchical order to data.
RS is the second least inclusive.
See also FS, GS, US.
On Mac Up Arrow.
US or
Unit Separator
Used with FS, GS, RS chars to introduce hierarchical order to data.
US is the least inclusive.
See also FS, GS, RS.
On Mac Down Arrow.
DEL or Delete
Used to delete a character. The cursor may or may not move depending on the OS.
On some OS it may be used as NUL char for padding purposes.
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