Fill character
Encyclopedia
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
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
ing, 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. 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
"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
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
video terminal sometimes required fill characters when a "scroll up" operation was performed; the process of shuffling data in the shift register
memory of the VT05 was slow.
In a similar fashion, communications protocols often require fill characters at points when processing must be performed.
Character (computing)
In computer and machine-based telecommunications terminology, a character is a unit of information that roughly corresponds to a grapheme, grapheme-like unit, or symbol, such as in an alphabet or syllabary in the written form of a natural language....
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
Dot matrix printer
A dot matrix printer or impact matrix printer is a type of computer printer with a print head that runs back and forth, or in an up and down motion, on the page and prints by impact, striking an ink-soaked cloth ribbon against the paper, much like the print mechanism on a typewriter...
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 (computer science)
In computer science, a buffer is a region of a physical memory storage used to temporarily hold data while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device or just before it is sent to an output device...
ing, 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. 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
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
"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
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
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
VT05
The VT05 was the first free-standing CRT computer terminal from Digital Equipment Corporation. Famous for its extremely futuristic styling, the VT05 presented the user with an upper-case only 5x7 dot-matrix display of 20 rows by 72 columns. The terminal only supported forward scrolling and direct...
video terminal sometimes required fill characters when a "scroll up" operation was performed; the process of shuffling data in the shift register
Shift register
In digital circuits, a shift register is a cascade of flip flops, sharing the same clock, which has the output of any one but the last flip-flop connected to the "data" input of the next one in the chain, resulting in a circuit that shifts by one position the one-dimensional "bit array" stored in...
memory of the VT05 was slow.
In a similar fashion, communications protocols often require fill characters at points when processing must be performed.