Dot Matrix Printers  

Dot matrix printers came onto the market around 1970, notably the Centronics machines which used a mechanism made by Brother.   Digital Equipment Corporations first LA series dot matrix printers appeared at about the same time. Quite who invented the idea seems uncertain but presumably it has roots in TV raster generation and both probably have their roots in embroidery. The idea of rasterising information dates back at least to Alexander Bain's idea for a facsimilie machine. info.

Dot matrix printer uses a set of seven or more "pins" to make the character image as a matrix.  The matrix used for each character is held in memory and as the printhead scans across the page the printer looks up the patterns and drives the pins back and forth. It's the same pixelation principle that makes the lit and unlit pixels on a screen and the displays on railway departure boards.

Dot-matrix print seems to have been an idea where the time was right. It wasn't really possible to make a dot matrix printer using discrete transistors and diodes, the electronic complexity would have been too great. Printing calculators tended to use number wheels. A dot matrix device needs a read-only memory for the character to be printed and counters for the position to be printed if not for the whole line.

the invention of the dot matrix principle was coincidental with the invention of the microprocessor but obviously the new kind of printer very quickly adopted microprocessor based circuitry. The main use for early microprocessors wasn't home computing, it was to provide the video and printing terminals for the new multitasking computer systems.

In the 1970's and 80's dot matrix printers became more and more sophisticated and lower in cost so that they were widely used in home computing.   Epson became the number one producer but there were a couple of dozen manufacturers:

Anadex, Apple (using a mechanism from C.Itoh, Brother, Bull, Citizen, DEC, Fujitsu, Facit, Genicom, IBM, Newbury Data, OKI, Olivetti, Panasonic, Philips, Printronix, Seiko, Star, Tally and Zebra. Other manufacturers badged printers, for instance ICL printers were often based on OKI. HP were (we think) badging Olivetti.

Several of the today's printer makers are in the list. Brother made the transition to laser printers and Epson to inkjets. Canon and HP weren't notable dot-matrix printer makers and that might be why they were free to develop inkjet and laserprinter mechanisms; they weren't locked into the benefits of the existing technology. Apple don't provide printers at present. Anadex, C.Itoh and Facit don't exist any more. Several other companies who once made printers now do other things.

Dot matrix machines are still used where low transaction costs or a carbon copy are priorities. For instance in a hire shop they might print a form, the customer signs it and gets the top copy, they file it and it's a legally binding document.

Graphics are never much good, typically looking like bad newsprint. With 24 pins in a column about 4mm high printers are capable of around 150 dots per inch at one pass but with two passes and interpolated dots they might achieve 250 dots per inch. Colour ribbons didn't last long, the printers took an age to actually render a picture and the result usually looked dingy when finished.

Dot matrix print reached one of its technical limits with a 48 pin head. Each pin then had a diameter just under 0.1mm and the print resolution was about 300 dpi. The problem was that the few printers made to this kind of specification proved difficult to maintain, the fine pins in the printhead head were too easily damaged.

Another limit is that mechanical pins can't move much faster than 3,000 times per second so a serial dot matrix can't print much more than one or two lines per second. The power required to move the pins faster rises (probably the square of frequency) so that fast printheads become very hot. Designers overcame this by surrounding the printhead with a heatsink, or even incorporating a fan in the carriage. In continual use the printhead still gets too hot, ideally a thermistor in the printhead gives temperature readings to the printer's controller which then slows the print speed for a while until things cool down.

With crude graphics, no very convincing colour ability and limitations on print speed it might seem surprising that many manufacturers still have some dot matrix printers in their catalogues. There are two or three things dot matrix printers do very well:

Multipart

Multi-part stationery. Dot matrix is an impact technology, the blow from the print pins goes through the first sheet and on to a second, third and so forth. The paper can have carbons between although these days carbonless copy paper is more usual. So the printer can give several copies. Of course it is perfectly possible to spit two copies of a document out of a laser printer but it would be difficult to prove with any certainty that they are identical without both parties to an agreement inspecting them both. If they have come from one printer in one pass the printing must be identical.

Economy

Economy. Dot matrix printers can often give unrivalled economy. Printronix claim their printers can have operating costs a sixth those of a laser printer (all cost claims are contentious). All a dot matrix printer does is knock sticky ink out of a ribbon onto the page. There are precision engineered parts in the printhead but the ribbon is cheap to manufacture and uses processes developed a century ago for typewriters. Laser and inkjet printers are both considerably more complicated. Of course ribbons are not the only cost. Some dot matrix printers have long lasting printheads that can be refurbished, but many do not. And because the economies of scale have gone dot matrix heads can seem very expensive when they have to be replaced. Printheads designed for heavy use usually do last several years in continual service.

One aspect of the dot-matrix printers economical behaviour can conflict with another. The ribbon will go on providing faded print when there is very little ink left in it and it has built up a coat of dust and grit. Ink may act as a printhead lubricant, so running the ribbon dry might be a false economy and reduce the printhead life.

Speed

Speed   Individual pins on a dot matrix printer can't work very quickly, which is one reason the typical printer has at least 9 pins to make a line of characters in one go. (So far as we know no-one has ever built a single pin printer, it would be too slow). Putting 18 pins in a single vertical row doesn't make a printer faster.

Using two 9 pin printheads roughly doubles the speed of the printer; one prints the left side and the other the right side of the page. (Actually its a bit less than double because the right hand head tends to have a bit less to do.) There is an issue getting the two heads properly aligned but its a matter of fine-tuning the timing. The printer produces a test pattern and the user presses keys until a match is achieved.

There have been printers with three and four heads as well, so that roughly doubles speeds again. There is a problem, printheads have quite a lot of mass and pushing them back and forth rapidly over a few inches takes quite a bit of energy and puts stress on the drive belt.

Parallel dot matrix printers or "line matrix" as Printronix call them have a pin per print position across the page. Instead of a printhead scanning back and forth across the page there is a continual band of print pins across the page width. This is potentially very much faster - but such machines are expensive to make.

To have a dot at 0.1mm intervals and get 250dpi printing across the 13.6 inch print width typically provided by dot matrix printers would need 3400 pins and drive coils. That would be expensive to arrange. It isn't impossible, because the pins don't all have to be in a row. There can be several offset rows and the printer's logic can take into account their positions in determining the firing time.

Shuttle printers put some number of pins like 85 or 170 across the page width. those pins would just produce vertical columns but the pins and their drivers sit on a shuttle mechanism which vibrates rapidly back and forth over a centimeter or so as the printer works. Each pin is responsible for two or four character positions.

There is actually a continuum between having 36 pins in four serial matrix printheads and the 56, 85 or 170 that might be in shuttle printers. The printer's logic has some work to do sorting out which pin is responsible for which bit of the line at what time but that is the sort of thing that can be worked out by clever software or a specially designed ASIC.

The main manufacturer of parallel dot matrix printers is Printronix.


Mass Market and Production

From 1970 to the early 1990s the market for dot matrix printers grew rapidly. They were the best option across a broad market.

Prior to dot matrix printers being available computers had either

Drum printers contain a metal drum with the character set formed round its circumference in each of 132 character positions. The drum rotates at fairly high speed on one side of the paper. On the other side of the page a hammer bank with a hammer in each of 132 1/10th inch wide character positions waits for the required character and its solenoid activates just as the character comes into position. If the drum rotates at 3000 rpm it will take a 50th of a second to print a line and since a typical page has about 50 lines it will take about a second to print. So fast printers have been around for a long time.

A typical drum would have 64 characters around it giving it a diameter of 10.6 inches. A drum 10.6 inches in circumference with 132 rings of characters - 8448 characters - is rather difficult to make. When drum printers were common the 132 hammer drive circuits were quite expensive as well. formed Getting the full ASCII character set would need a 16 inch diameter Unless the hammer timing is perfect the print isn't all that great.

  • band printers: a metal belt like that for a bandsaw but with characters embossed or pressed around its surface. Again, there is a hammer bank

Drum or band printers. Inkjet and laser printers were both introduced at reasonable prices in 1984 and transformed the market.

Serial dot matrix reached its economic limits with prices getting as low as $75 or so in the early 90s as manufacturers committed to the design tried to appeal to a market that was increasingly attracted to inkjet printers.

Prices for new machines are much higher now because dot matrix printing is a niche market.