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Impact printers are the oldest print technologies still in active production. Some of the largest printer vendors continue to manufacture, market, and support impact printers, parts, and supplies. Impact printers are most functional in specialized environments where low-cost printing is essential. The two most common forms of impact printers are dot-matrix and daisy-wheel.
The technology behind dot-matrix printing is quite simple. The paper is pressed against a drum (a rubber-coated cylinder) and is intermittently pulled forward as printing progresses. The electromagnetically-driven print head moves across the paper and strikes the printer ribbon situated between the paper and printhead pin. The impact of the printhead against the printer ribbon imprints ink dots on the paper which form human-readable characters.
Dot-matrix printers vary in print resolution and overall quality with either 9 or 24-pin printheads. The more pins per inch, the higher the print resolution. Most dot-matrix printers had a maximum resolution around 240 dpi (dots per inch). While this resolution is not as high as those possible in laser or Inkjet printers, there is one distinct advantage to dot-matrix (or any form of impact) printing. Because the the printhead strikes the surface of the paper with enough force to embed characters on the page, it is ideal for environments that frequently print on carbon copy, special multi-sheeted documents with carbon on the underside that will create a mark on the sheet underneath it when enough pressure is applied. Retailers and small businesses often use carbon copy as receipts or bills of sale.
If you have ever seen or worked with a manual typewriter before, then you understand the technological concept behind daisy-wheel printers. These types of printers have printheads composed of multi-plated metallic or plastic wheels cut into petals. Each petal has a letter (in capital and lower-case), number, or punctuation mark on it that is raised. When the petal is struck against the printer ribbon, the resulting letter is embedded in ink onto the paper. Daisy-wheel printers are loud and slow. They cannot print graphics, cannot change fonts unless the wheel is physically replaced with a wheel of a different font, and are generally not used in modern computing environments. However, Red Hat Linux does include the Common UNIX Printing System (CUPS), which has a comprehensive printer compatibility list in case your environment requires use of daisy-wheel printers.
Another type of impact printer somewhat related to daisy-wheel is the line printer, which has multiple columns of characters lined up instead of a petaled wheel. As the roller moves the paper forward one line, the appropriate characters strike a ribbon onto the paper, causing an entire line to be printed at one time, rather than one character or area of text. Line printers are much faster than dot-matrix or daisy-wheel printing; however, they are quite loud and produce lower print quality.
Ink ribbons and paper are the primary recurring costs of impact printers. Of all the printer types, however, impact printers have relatively low consumable costs. Impact printers require a continuous, uncut ream of paper that has perforations between each page. Pre-punched holes on either side of the page help the paper move against the print drum smoothly, preventing paper jams or print misalignment.