COMPUTER PRINTER AND DISPLAY TERMINALS BASIC INFORMATION

A serial or character printer, whose name resulted from the fact that they print one character at a time, was the display mechanism first used with terminals. The first serial printers were ‘fully formed’ impact printers in which different types of mechanisms, including a daisy wheel, type-ball or rotating cylinder which formed characters from a single piece of type, were used to strike a ribbon to produce a printed image.

The editing capability provided by the terminal was minimal, typically permitting the operator to delete a previously entered character or the current line, since the terminal transmitted and received data on a line by line basis. A second type of impact printer which grew in popularity during the 1970s and 1980s to where it has virtually replaced fully formed printers is the dot matrix printer.

The dot matrix printer employs a matrix of pins in its print head. The first dot matrix printers used a rectangular matrix of dots, typically 7 dots high by 5 dots wide or 9 dots high by 7 dots wide. The pins in the matrix are selectively ‘fired’ to form each character. Printing of characters results from the movement of the print head containing a column of 7 or 9 pins across the paper, with the printer selectively firing the pins at 5 or 7 successive intervals to form each character.

Until the mid-1980s, the matrix of pins used to form characters resulted in a considerable amount of white space between dots. This space made the dot matrix pattern easily discernible to the eye and limited the use of printed output produced by this type of printer to draft correspondence.

By the mid 1980s, advances in print head technology resulted in the inclusion of more pins on some print heads. The additional pins were used to considerably reduce the space between pin impacts in forming a character.

Other dot matrix developments included two-pass printing in which the first pass of printing a line was followed by the printer feeding the paper upward by a slight amount, perhaps 1/256th of an inch, prior to the line being printed a second time.

One result of placing more pins on the print head and using a two-pass printing technique was. a higher quality print. Since this print resembled the letter quality print of a full impact printer, it became known as near letter quality (NLQ).

The firing of additional pins to form a better print image required additional time, resulting in NLQ printing being slower than conventional dot matrix printing. Thus, most modern dot matrix printers have two or more user selectable print modes – draft and NLQ – with the draft print mode providing

a considerably faster print rate than the NLQ print mode.

A second major category of printers employs non-impact technology to form characters. Non-impact printers include thermal matrix ink jet and laser devices. The thermal matrix printer forms characters by applying a voltage to pins in a matrix, causing the pins to be heated. The heated pins interact with heat-sensitive paper used in these printers, resulting in the formation of characters.

The ink jet printer has a nozzle consisting of a matrix of holes out of which ink is squirted to form characters. Thus, both thermal matrix and ink jet printers are based upon dot matrix technology. In comparison, the laser printer uses a rotating drum and a small amount of current to generate a magnetic field, which results in toner from a cartridge adhering to distinct locations on paper passing around the drum. Laser printers have resolutions between 300 and 1200 dots per inch (dpi) and through software can form characters in almost any shape.

The key limitation associated with the use of printers for both input and output is the elementary editing capability provided by this type of terminal device. Data entered from the keyboard are either printed and transmitted as each character is pressed or stored in a buffer area.

The buffer storage area contained in most ASR and KSR terminals is only capable of holding one line of data or 72 to 80 characters depending upon the type of terminal. By using the backspace key to eliminate a previously entered character, an operator can perform elementary editing.

Once the carriage return key has been pressed, however, the entire line is transmitted, resulting in an operator having to re-enter the line with any changes he or she desires to correct a previously entered line. As an alternative to the use of the backspace key, an operator can simultaneously press the control (Ctrl) key and an alphabetic key, canceling the present line and removing its contents from buffer storage. This action causes a carriage return and line feed to be automatically generated, permitting the operator to begin his or her data entry anew.