Impact printing is the oldest, simplest, and probably the most dependable form of printing ever developed. Images are literally slammed against a page as a matrix of individual dots; the dot patterns form letters and graphics. Impact printers are known collectively as dot-matrix impact (DMI) printers. 3-2 shows a comparison between character (e.g., typewriter) and dot-matrix ', print. Notice how the DMI figure appears rougher and less defined than its die counterpart.
The Dot Mechanics
Each dot is generated by an individual metal print wire driven through a solenoid as illustrated by 3-3. When an electrical pulse reaches the solenoid, it energizes the coil and produces a brief, intense magnetic field. This field "shoots" its print wire against the page. After the pulse passes, the solenoid's magnetic field collapses. A return spring pulls the wire back to a rest position. In actual practice, DMI solenoids and print wires are very small assemblies. A typical print wire might only travel about 0.5 mm. This distance is known as wire stroke. Not all DMI heads hold their print wire directly within the solenoid's coil. While this approach might work well for smaller, general-purpose heads, heavy-duty heads need larger coils than can be stacked vertically. Each solenoid is mounted offset from one another, then connected to their respective print wires using a mechanical linkage as shown in 3-4. Due to the additional mechanical components, heavy-duty print heads operate somewhat slower than "direct-drive" heads, and they usually do not last as long because of the additional mechanical wear.
Driving the Dot
Solenoids require a substantial amount of electrical energy very quickly to develop a magnetic field strong enough to move a print wire. Figure 3-5 shows a driver circuit similar to one used in everyday DMI printers. A printer's ECU produces a narrow logic pulse, which is sent along to a transistor driver. In 3-5, a gate array ASIC (marked U3) is responsible for generating the logic signal. However, logic circuits alone cannot handle enough power to operate a solenoid directly, so a transistor driver is used. A highpower driver transistor (such as the 2SD1929) acts as a switch that turns the solenoid on and off.
Dot Specifications
DMI print heads use an array of 9 or 24 print wires arranged invertical columns as shown in 3-8. There are three major mechanical specifications that you should be familiar with. Wire diameter specifies the diameter of each print wire (normally expressed in millimeters). This tells you how large each dot will be. The distance between the center of each dot is known as wire pitch (also expressed in millimeters). Finally, the height of each fully formed character is specified (in millimeters) as character height (CH). W1re diameter and pitch are much smaller for 24-pin heads than for 9-pin heads. Not all wires can be used to form every character. For example, a 9-pin print head with a 2.5-mm CH might only use seven wires to form most characters. Wires 8 and 9 could be used to form characters with descenders. The concept of true descenders is illustrated in 3-9. When all nine wires are used to form characters, there is no room left for descenders, so characters can be printed with false descenders, also shown in 3-9. Overall character size might appear larger when all nine wires are used, but many people find false descenders awkward. The technique of DMI printing is every bit as straight forward as character printing, but the actual formation of each letter, number, or symbol is a bit more involved. Data sent from a host computer is interpreted by the printer's main logic and converted to a series of vertical dot patterns. Motor commands start the carriage (and print head) moving across the platen. Simultaneously, printer circuits will send each dot pattern to the print head in series. Each dot pattern fires the corresponding print wires through an inked ribbon to leave a permanent mark on the page. This is also called serial or moving-head operation. Figure 3-10 is a photo of a heavy-duty DMI print head.
Advantages and Disadvantages of DMI Printing
To this day, DMI print heads remain a cornerstone of commercial printing technology. They are flexible and inexpensive devices, capable of a wide variety of fonts and enhancements, as well as draft or NLQ performance and bit-mapped graphics. DMI heads are reasonably fast, so they can achieve speeds easily exceeding 160 CPS. They are reliable devices. Heavy-duty print heads can last through more than 30 million characters. Smaller, general-purpose heads can last for more than 100 million characters. Impact printing is mandatory for printing multicopy forms. Finally, they require very little maintenance except for periodic routine cleaning. However, impact printing is very noisy. The continuous drone of print wires striking paper can become quite annoying. Although DMI printers are now made with plastic coverings that baffle much of its noise, they do not quiet the printer completely. Limited dot resolution is another concern. You might not notice this for NLQ text, but you can see individual dots in draft or bit graphics modes. You can only achieve just so many dots per inch. Finally, head overheating can be a problem during long printouts, especially when printing graphics where many wires must flre repeatedly.
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