The PC's operating system or software sends signals to the laser printer to determine where each dot of ink should be placed on the paper. The signals can either be ASCII code or page description language. The printer processor then instructs a device to rapidly turn a laser beam on and off. The laser is always aimed at a spinning mirror which deflects the laser, forming a horizontal line across the surface of the organic photoconducting cartridge (OPC or can be referred to as the drum). Since the beam is turning on and off and the beam moves horizontally across the cylinder, there are tiny points hitting along a line across the surface of the drum. When the laser finishes flashing points across the entire width of the OPC, the drum rotates 1/300th (300 dots per square inch printers) or 1/600 (one hundred dots per square inch printer) of an inch, and the laser beam starts working on the next line of dots. While the drum is rotating, a system of gears and rollers pull a sheet of paper into the print engine along a path called the paper train. The paper is pulled past an electrically charged wire which gives the paper a charge and given a positive charge. Wherever the light hit the drum, a negatively charged film changes its charge to positive. Each of the positive charges on the film represent the black areas in the hard copy. The areas of the drum untouched by the laser will show up white on the hard copy.
When the drum has rotated about 180 degrees, it comes in contact with a bin containing black toner. The toner has a negative charge, which is the opposite charge of the areas on the drum touched by the laser. Since opposite charges attract, the toner sticks to the small dots wherever the laser created a charge. Then, the drum eventually presses against the sheet of paper and transfers the negatively charged toner to the positively charged paper (even though the drum and paper have the same electrical charge, the toner sticks to the paper since its electrical charge is much stronger). The rotation of the drum eventually reaches a corona, which returns the surface of the drum to its original negative charge so it can be ready for the next sheet of paper. Another pair of rollers pulls the paper through the fusing system. Pressure and heat cause a wax, which is part of the toner, to melt and cause the toner to be permanently bound to the paper. Finally, the paper comes out of the printer.
An ink-jet printer has an ink-filled cartridge attached to the ink-jet's print head. The head moves sideways across the width of the paper which is fed through the printer below the print head. The print head consists of four ink cartridges, one for magenta, one for cyan, one for yellow, and one for black. Each cartridge is made up of 50 ink filled chambers. An electrical pulse flows through the thin resistors at the bottom of the chambers which the printer will use to form a character. The resistor heats up to 900 degrees Fahrenheit for a few milli-seconds, causing the ink to boil and for a bubble of vapor. Ink is pushed through the nozzle as the bubble expands. The droplet overcomes the surface tension of the ink and is pushed out by the pressure of the vapor bubble. The volume of the is one millionth the volume of a drop of water ejected from an eyedropper. When the resistor cools, the bubble collapses and forms a sort of vacuum, pulling the fresh ink from the reservoir into the firing chamber.
Dot Matrix printers have been with us for quit a while, and it will probably be with us for the next couple of years. A good dot matrix printer costs a few hundred dollars, unlike laser printers which cost from $600-$1,000 dollars. The ribbons for dot matrix printers are relatively cheap when compared to the expensive laser cartridges. Today's 24-pin dot-matrix printer increases the printer's speed and quality of the type.
A series of hexadecimal ASCII codes are sent by the PC to the printer. These codes represent characters, punctuation marks, and printer movements. The printer stores the ASCII codes in a buffer, which is a special section of its RAM. Since the dot-matrix printer is slow in printing, the printer's own buffer frees up the PC to do other tasks. The buffer has a 7k to 8k capacity, and when it is full, the printer sends an XOFF control code to the computer to tell it to stop sending data. When the buffer empties, by sending the characters to the processor, the printer sends an XON code to the PC, telling it to resume sending its data. There are some commands which tell the printer to use a certain font's bitmap table. This table is contained in the printer's read-only memory chips, and it tells the pattern of dots the printer should use to create different fonts. For example, even though this word has the same hexadecimal ASCII code as this word, the two different words have different bitmap tables. The bitmap table sends the processor an entire line of type, and the processor calculates the most efficient way for the head to travel. The print head contains 9 to 24 wires, or pins, and each one has a solenoid (electromagnet) located behind it. Amplified signals from the processor travel to the appropriate solenoid, causing it to generate a magnetic field. This magnetic field repels the magnet on the end of the pin, and the pin moves towards the paper. The pin then hits an ink ribbon which transfers a dot of ink to the paper. After the pin fires, it is pulled back up by a spring.
