7.2. Impact Printers

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 printhead 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 have a maximum resolution of 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 printhead must strike the surface of the paper with enough force to transfer ink from a ribbon onto the page, it is ideal for environments that must produce carbon copies through the use of special multi-part documents. These documents have carbon (or other pressure-sensitive material) on the underside and create a mark on the sheet underneath when pressure is applied. Retailers and small businesses often use carbon copies as receipts or bills of sale.