The purpose of the first set of ASCII character codes or the set of control code characters is to provide a way for a computer to control a peripheral connected to the computer with a cable such as RS-232 serial interface or RS-485 serial interface or IEEE 1284 parallel interface.
These control codes had several purposes:
- control the behavior of the device (carriage return to return print head to beginning of line or bell to ring the bell on a teletype to catch the attention of an operator)
- provide standard message format characters used in messages to a device (STX to indicate start of text or ETX to indicate end of text)
- to trigger out of band communication (use ESC character within text to indicate the beginning of a command to the device or the DC1 character for an XON indicator telling the remote device to resume sending text or the DC3 character for an XOFF indicator telling the remote device to stop sending text)
RFC 20 ASCII format for Network Interchange provides a description of the proposed control codes and their function. For instance here is a partial list from section 5.2 control codes of RFC20:
NUL (Null): The all-zeros character which may serve to accomplish
time fill and media fill.
SOH (Start of Heading): A communication control character used at
the beginning of a sequence of characters which constitute a
machine-sensible address or routing information. Such a sequence is
referred to as the "heading." An STX character has the effect of
terminating a heading.
STX (Start of Text): A communication control character which
precedes a sequence of characters that is to be treated as an entity
and entirely transmitted through to the ultimate destination. Such a
sequence is referred to as "text." STX may be used to terminate a
sequence of characters started by SOH.
ETX (End of Text): A communication control character used to
terminate a sequence of characters started with STX and transmitted
as an entity.
EOT (End of Transmission): A communication control character used
to indicate the conclusion of a transmission, which may have
contained one or more texts and any associated headings.
ENQ (Enquiry): A communication control character used in data
communication systems as a request for a response from a remote
station. It may be used as a "Who Are You" (WRU) to obtain
identification, or may be used to obtain station status, or both.
ACK (Acknowledge): A communication control character transmitted
by a receiver as an affirmative response to a sender.
BEL (Bell): A character for use when there is a need to call for
human attention. It may control alarm or attention devices.
The original teletypes or teleprinters used a print head with paper which was replaced by cathode ray tube terminals which emulated the behavior of the original teletypes on the surface of the CRT screen.
The most famous example of the CRT was the DEC VT-100 terminal whose use of ANSII control codes became a defacto standard for CRTs. The VT-100 control codes are also commonly emulated in terminal windows on various Unix, Linux, MS-DOS, and other operating systems (see also the curses user interface programming library).
The VT100 was introduced in August 1978, replacing the VT50/VT52
family. Like the earlier models, it communicated with its host system
over serial lines at a minimum speed of 50 bit/s, but increased the
maximum speed to 19,200 bit/s, double that of the VT52.2
The major change within the system was the control system. Unlike the
VT50/52's proprietary cursor control language, the VT100 was based on
the emerging ANSI X3.64 standard for command codes.[a] At the time,
computer vendors suggested that the standard was beyond the state of
the art and could not be implemented at a reasonable price. The
introduction of low-cost microprocessors and the ever-falling cost of
computer memory addressed these problems, and the VT100 used the new
Intel 8080 as its internal processor.4 In addition, the VT100
provided backwards compatibility for VT52 users, with support for the
VT52 control sequences.5
ASCII control code applications
The ASCII control codes were designed to provide a standard set of codes for common serial message protocol needs. While the structure or format of a message was proprietary as was the data sent in a message, the ASCII control codes provided standard codes that everyone could agree to use in the control part of their messages.
Modern protocols use text such as JSON or XML because these messages are transported over high speed networks for the most part. However early computer equipment typically used RS-232 or RS-485 serial communication equipment and the messages between the devices were as compact and concise as possible. So rather than sending a text message such as "ACK", three characters, a device would send a single character, the ACK control code when acknowledging a message.
An example of ASCII control codes is when a PC is sending text with formatting directives to a thermal receipt printer. The receipt printer is connected to the PC with a serial cable, often a DB-9 cable or a USB cable. The PC sends a series of text characters to the receipt printer however some of the characters are printed normally which others are printed double high and double wide.
This change in the size of the printed text is communicated to the receipt printer by embedding print format commands in the stream of text characters by using the ESC or escape control code. When the printer sees the ESC control code then it knows that there will be a printer command following so it looks for the printer command character sequence, makes the change to its printing behavior, then continues process the text that is sent until it sees another ESC control code.
Also while processing the text string, when the receipt printer sees an ASCII control code that indicates a change in the position of the print head such as a carriage return or tab character or backspace character or line feed character, the receipt printer changes the position of the print head and then resumes printing text.
The Wikipedia topic IPTC 7901 describes the use of these control characters with news service messages beginning with the formal approval of the IPTC protocol in 1979 which sounds to be similar to an RSS feed protocol. The actual specification is available from the IPTC web site as The IPTC Recommended Message Format, 1995.
Data flow control
With slow speed serial communication between two machines in the case of a large amount of data such as a file transfer, the sending device could easily send more data, faster than the receiving device could process it. This is why the RS-232 serial communication protocol specification has several hardware flow control pins in the connector for data set state communication. However there were many cases where cables with the additional wires were not available so the use of software flow control using ASCII control codes, DC1 for XON and DC3 for XOFF, was introduced.
Serial communication modems also used ASCII control codes for software flow control as well as for application such as faxes (see Wikipedia topic Command and Data modes (modem)).
