In computer terminology, a fill character is a
character transmitted solely for the purpose of consuming time. It does this by filling a timeslot on a data transmission line which would otherwise be forced to be idle (empty). In this way, fill characters provide a simple way of timing required idle times.
Fill characters are usually used in response to some real-world limitation. For example, mechanical computer printers such as the earliest
dot matrix printer
A dot matrix printer is an impact printer that prints using a fixed number of pins or wires. Typically the pins or wires are arranged in one or several vertical columns. The pins strike an ink-coated ribbon and force contact between the ribbon ...
s may have been able to print 30 characters per second, but when a "carriage return" character was received and the printhead began returning to the left margin, there was a noticeable delay before the printing of the next line could begin. Unlike modern printers, these early printers contained essentially no
buffer
Buffer may refer to:
Science
* Buffer gas, an inert or nonflammable gas
* Buffer solution, a solution used to prevent changes in pH
* Buffering agent, the weak acid or base in a buffer solution
* Lysis buffer, in cell biology
* Metal ion buffer ...
ing, nor did they do any
handshaking
A handshake is a globally widespread, brief greeting or parting tradition in which two people grasp one of each other's like hands, in most cases accompanied by a brief up-and-down movement of the grasped hands. Customs surrounding handshakes ...
, so there would be no place to store the characters which would be received while the printhead was in the process of returning to the left margin, nor any way to tell the sender to temporarily stop transmitting characters. Instead, one or more fill characters would be transmitted to cover this time.
In its strictest definition, fill characters cause no action to be performed at all; they simply consume time. The
ASCII
ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
"null" character is commonly used for this purpose. In actual practice with printers, though, one of the time slots that would otherwise contain a fill character was usually used to contain the "line-feed" character that caused the paper to advance by one line. For some printers (such as
teleprinter
A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device that can be used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations. Initi ...
s), this was all the "filling" that was needed.
As described above, printers commonly required fill characters when the carriage was returned to the left margin. With other equipment, fill characters were occasionally required in other circumstances. For example the
VT05
:''"VT-05" can also refer to .''
The VT05 is the first free-standing CRT computer terminal from Digital Equipment Corporation introduced in 1970. Famous for its futuristic styling, the VT05 presents the user with an upper-case-only ASCII chara ...
video terminal sometimes required fill characters when a "scroll up" operation was performed; the process of shuffling data in the
shift register
A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one loca ...
memory of the VT05 was slow.
In a similar fashion, communications protocols often require fill characters at points when processing must be performed.
Digital typography
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