It appears that line printers are what used to be called dot matrix. They are impact printers they strike thru a ribbon onto the paper. They are still used as alarm printers in control rooms and any place that very fast continous feed printers are needed. They are also needed where a multilayer form is used as in Invoices. The name threw me as I worked on Dot matrix printers several years ago and never heard them called line printers. BTW they are very durable you can just about print on grocery bags. Another use is specialty labels such as hi temp Capton and Kevlar labels used on ckt boards that can stand a solder bath....ETC...ETC..
Line printers need be neither dot matrix nor huge - although they often are! Lines printers are called this because they are capable of printing one full line of characters simultaneously.
Most modern printers fall into two categories: pages printers (lasers and the like) print a complete page on one go; character printers print one character then move on to the next. Dot matrix printers typify this latter category.
A line printer must be mechanically constructed so that all the characters in one complete line can be printed more or less together. There are three main methods - one truly fullfils the requirement and the others goes a long way towards it. Line printers do have one advantage. Printers can be divided into two categories - imapact (where something hits the paper) and non-impact (where there is no striking mechanism), such as a laser printer. Line printers are of the impact type, which means they can print on multipart stationary - something even the most up to date laser cannot do! Nowadays, it is this, rather than speed, which determines whether or not a line printer is used. On the subject of noise, as someone has already mentioned, the larger machines can be incredibly noisy. However, small line printers, as described first below, can be quiet enough for use in shops and elsewhere without undue noise pollution.
The first method consists of a row of discs, or print wheels, placed side by side to make a cylinder, or drum. (Imagine a stack of coins lying horizontally, one coin for each print position across the page). Each disc has character shapes embossed on its edge. The electro-mechanical print mechanism spins each print wheel separately until the correct character for the each print position is in place. The paper is then pressed against the
drum, and the image transferred to the paper via an inked ribbon that lies between the two. This, the only true line printer, has the disadvantage of being unable to print any characters not available on the print wheels or in anything other than the defined print positions. It is also pretty slow but is cheap to manufacture and maintain. Whilst there were a few large machines built using this type of mechanism most are actually smaller than your average PC printer, being found in printing calculators, shop tills and instrumentation printers, where the number of print positions (characters per line) is generally forty or fewer.
The second type of line printer mechanism, known as a drum printer, consists of a solid drum - just like the type described above but without the separate discs. (Again, imagine a cylinder made up from several coins but this time the coins cannot spin in relation to each other.) Here, because each character must actually be impressed on the paper separately, each print position has its own individual hammer. The print drum spins continously very fast. For each print position in turn, when the correct character is in place opposite the paper the hammer for that position fires and ipresses the character on the paper. These printers are generally very large machines and extremely expensive and can only print characters in set positions. (These are the printers people tend to recall as line printers and where the impression of size has come from.) Unlike the type described above, which can have its print wheels changed individually if necessary, the solid print drum precludes even this minor flexibility. However, print speeds can be measures in pages per minute like a laser - and do remember that this technology preceded lasers by many, many years.
Finally, the matrix line printer combines the flexibility of a dot matrix print head and the speed of a line printer. The characters formed are not limited to those embossed on a print wheel and printing is not limited to predefined character positions. Graphics can also be printed. The method is to contruct the print mechanism out of two, three or four separate matrix print heads. Each head covers a half, a third or a quarter of the width of the paper. Whilst not as fast as the drum printer it is roughly two, three or four times faster than a fast dot matrix. These are now the most common type of line printer and are somewhat larger than the typical domestic/office dot matrix printer. Strictly speaking they do not print a whole line at a time (neither does the drum printer, of course) but they are often referred to as such because they are fast, which is what the phrase line printer is synonymous with.