Printers

In computer terminology, a printer is a device that is used to produce "hard-copy" output, as opposed to "soft-copy" which is volatile and short-term (such as text on a computer screen).

Categories[edit]

Printers can be categorized in two groups: paper and 3D.

Paper[edit]

Most printers in existence are designed to place text and/or images onto paper. Some printers, such as modern laser printers, handle single pages, whereas older printers used continuous rolls or fan-fold paper. High-speed printers used tractor feeds that fed continuous paper through the printing mechanism, keeping position by the use of projections that fit through evenly spaces holes on the left and right margins of the paper. Most printers used either 80-column-wide (9.5") or 132-column-wide paper.

3D[edit]

3D printers are used to create 3-dimensional objects. The object is built one small layer at a time from bottom to top. Most consumer 3D printers can create plastic objects up to about 10" in any dimension.

History[edit]

Early printers came in two forms: with keyboards and without keyboards. In fact, some early printer lines used the exact same printing mechanism, with some models having additional features such as keyboards and/or paper tape punches/readers. The ASR33 teletype was one of these printer lines. With an associated keyboard, the printers could be used as terminals. This practice continued for many years for cheaper printers, including the Digital Equipment Corporation LA line. The DECWriter II (model LA36) was one of the most popular printing terminals to supplant the teletype.

Teletypes operated at 110 baud (about 11 characters per second), whereas the newer dot-matrix printers achieved 1200 baud (about 120 characters per second). Later printer speeds where measured in lines per minute or pages per minute.

Even as video terminals replaced printing terminals, printers were still needed to create hard-copy output. Technological advancement was then focused on increasing print speed, increasing print quality, adding color, and reducing cost.

The 2010's saw the introduction of consumer-priced 3D printers.

Printing Mechanisms[edit]

Printer mechanisms have varied over time, with some falling out of favor as technology advances and costs come down. Typically, two or three different printing technologies vied for prominence at any given time with the older technologies providing a cheaper alternative and the newer technologies providing advantages in speed and/or capability.

Solid print heads[edit]

The original printers used a metal cylinder with the character glyphs embossed on the surface. The cylinder was rotated and raised/lowered so that the appropriate glyph faced the paper. The head was then forced forward against an inked ribbon and then the paper, forming the image on the paper in ink. The head was moved left to right with the process repeated for each column. When the head reached the right side, it returned to the left side to begin the next line.

Line printers also used metal cylinders (called "drums"), except that the cylinders were oriented horizontally with up to 132 stripes that circumscribed the drum - one for each column. Each stripe had all printable glyphs, so that if you looked at the drum while rotating it, you would see a line of 132 "A"s across the width, then a line of 132 "B"s, and so forth. The printer spun the drum at high speed and when the appropriate letter for a given column has spun into position, a hammer behind the paper would quickly push the paper into contact with the embossed glyph on the drum, with an inked ribbon between them, thus placing the character onto the paper. Since more than one instance of a given character would often occur on the same line, multiple hammers would strike the paper at the same time (for each column that used that character on that line), thus an entire line of text could be transferred to the paper during a single rotation of the drum.

Band printers used a similar approach except that the glyphs were embossed on a belt that spun in a horizontal direction. The belt had only a single instance of each character, but as the belt traveled across the paper, hammers would strike when the appropriate character was in the appropriate position - often with many hammers striking simultaneously when the appropriate glyphs lined up with the appropriate column.

Because of their mechanical nature, these types of printers tended to be extremely noisy.

Dot-matrix[edit]

The next advance in printing terminals used a different type of print-head called a dot-matrix printer. The advantage is that these print heads could print faster than entirely mechanical print heads. Dot-matrix printer technology consists of narrow metal pins organized in rows and columns (typically 7x9). For a given glyph, a pattern of the pins were pushed out, through an ink ribbon, creating a representation of the glyph on the paper. The sound of the metal pins hitting the paper made the printers moderately noisy, but far less than solid print head printers.

Thermal[edit]

Thermal print heads used special heat-sensitive paper and a print head that heated up a matrix in an appropriate configuration to transfer the image to the paper, much resembling the dot-matrix printer output, but with almost no noise. However, the special paper cost more than normal paper and was subject to stray marks if something brushed against it hard enough to cause friction that the paper reacted to. As a consequence, this technology never really caught on.

Ink-jet[edit]

Ink-jet printers superseded dot-matrix printers. They worked by spraying droplets of ink through extremely fine nozzle​s, which would then create the character on the paper, usually in a dot-matrix pattern. However, because the angle of the ink drops could be controlled, the resolution of characters could exceed 100 dots per inch (dpi), which was far superior to the quality of dot-matrix technology. Because the only mechanical movement was moving the nozzle from side to side, and the feeding of paper, these printers were extremely quiet. The printers often came with several colored ink cartridges which allowed color printing of graphics. The printing mechanism was simple and so the cost of the printers was often less than $100. However, the ink tended to be extremely expensive, quickly used up, and on infrequently-used printers, it would often congeal in the nozzles, causing printing issues.

Electrostatic[edit]

Electrostatic printers use the same technology as copy machines. A laser is used to create patterns on a charged metal drum as the drum turns, thus creating an electrically-charged version of the text or image to print. The drum rotates through a supply of toner, which sticks to the charged parts of the drum, but not the non-charged parts. The drum is then pressed against paper which transfers the toner to the paper. Finally, pressure and heat is used to fix the toner onto the paper, which is then discharged from the printer with the completed page on it. Some laser printers support multiple colors of toner and because the toner particles are so fine, the quality of text is as much as 1,200 dpi even on cheaper consumer modules, which is close to the 1,440 dpi quality of typeset.