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This is a one-page outline of dot-matrix printer mechanisms.

The heart of a dot matrix printer is the printhead. The printhead is a compact assembly of electromagnetic coils and moving pins. When the electromagnets operate the pins shoot forward by around a millimetre.

The printer is built so that paper passes through it using a platen oller, a forms tractor or both. The idea of the platen roller was inherited from typewriters and the forms tractor from bandprinters. A line-feed motor  drives the platen and tractors. Normally the line-feed motor pushes the paper through the machine one text line at a time.

The paper feeds through the "print station", a part of the printer where the prithead contacts the ribbon and the swath of paper to be printed. The printhead sits on a carriage within the print station so that its nose with all the pins faces into the paper - almost touching.

Between the pins and the paper is an inked fabric ribbon, which can be supplied on spools or in a cartridge. When the pins strike the ribbon ink is forced onto the paper.

The carriage moves back and forth horizontally under control of a motor. The printhead pins operate as the carriage moves so making a dot-pattern on the page that people interpret as text or pictures. When the carriage reaches one side printing stops, the line feed motor advances the paper and the carriage everses and prints the next line - bidirectional printing.

The ink ribbon is usually in an endless loop, a mobius strip. Alternatively the spools keep turning backwards and forwards so that there is always fresh ribbon under the head. Some printers have a motor to drive the ribbon, others use all sorts of clever little cogwheel and ratchet contraptions.

The pins fire rapidly as the carriage moves, so rapidly that the dot they make doesn't smear on the page. The faster the pins can fire the faster the printer can work. A pagewidth of text is typically made up from around one or two thousand dots and each pin in the head normally produces these in under a second. Each pin needs to be able to move several thousand times a second. Pin speed is a key limiting factor on dot matrix design.

There are usually at least 9 pins in a printhead arranged vertically. This produces a swath that is one line of recognisable Latin text characters of the type used in the US and Europe.

The printers can be adapted to print Arabic, Hebrew, Chinese, Cyrillic, Korean and Japanese scripts. More pins usually help do a better job, particularly with complicated scripts like Chinese. 24 pin printers also produce better looking latin text. Some 48 pin printers were made but they weren't reliable.

Printers are typically designed to take in bytes of ASCII digital code, so the letter "A" is 100 0001 and "Z" is 101 1010. The printer typically takes in a complete line of characters before it starts to print. For each set of pin positions across the page the printers internal logic looks up the character it is printing, and the column of dots required for its current position within that character and feeds the correct pattern to the circuits driving the pins. In older printers the mechanism doing this might be a collection of dedicated logic circuits but in most recent printers all the control tasks are done by a microprocessor.

A microprocessor driven printer can also store the pixel pattern for a line or even a whole page in memory and use that instead of interpreting ASCII into dot patterns. This allows the printer to handle graphics as well as text. The problems are that:

• the printer needs a language to describe the patterns - and this wasn't standardised for a long time,
• and it needs some memory to store patterns - and early printers didn't have much memory.

Dot matrix printers aren't usually very good at graphics because the mechanical pins need to be around 0.1mm in diameter or they are too easily worn and bent. Inkjet printheads don't actually touch the paper so they can have nozzles a tenth the size at  0.01mm in diameter or less without problems. Inkjets are broadly similar to dot matrix printers, they just make much smaller pixel dots more quickly so they are better at graphics.

Dot matrix printing is used to record commercial transactions in Point of Sale machines and in workshops and warehouses. The mechanism is more obust than an inkjet in some ways (it can cope with some dust). Dot matrix printers can also make a carbon-copy of what has been printed, which can be important in commercial work.

For very simple commercial work thermal direct printing may be a better choice.

For office work laser printing works more quickly - although each page costs more.

The advantage of dot matrix printers is that they can get on with their job and print a couple of thousand pages without user intervention, the paper unfolds from a box, passes through the printer and re-folds behind it. A good ribbon can print a crate of paper. If you want straight forward commercial records on paper the dot-matrix printer is a good choice.

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© Graham Huskinson 2010