A laser printer does a LOT of work internally, and thus actually requires inside itself a small computing system including a microprocessor, BIOS, RAM, network interface, I/O ports, etc. For example, my Lexmark C543dn Colour Laser printer has a 417 MHz microprocessor, 128 MB of RAM and USB2 and 100 Mb/s Ethernet communication ports. The device driver supplied with the printer, which installs in the host computer(s) using it, sends material to be printed as a series of bytes, of course, along with non-printing codes for what those bytes contain and thus how they need to be processed. For example, a document may contain several type fonts, and in preparation for printing that the computer and printer need to agree on what fonts are required, what ones already are in the printer, and what ones are missing and need to be downloaded and stored in the printer's RAM ready for use. A document may also contain graphic elements as bit maps, rather than character codes, and those need different handling.
A laser printer prints one horizontal line of dots at a time. Everything to be printed is broken up into patterns of tiny dots. The printer treats the final printed page as an array of dots at its current resolution (say, 300 dpi) in each direction, so the printing on an 8½" x 11" page with ¼" non-printable margins will cover an array of 8" x 10½", or 3,150 horizontal lines of 2,400 dots each. My printer does things at 600 dpi by default and can do higher, so that's at least four times the dot count of the 300 dpi option. Once the whole page is "laid out" in RAM, the printer must then calculate and print each of those dots, line by line, as the paper moves through the printer. Then it must re-print the same page if that is what it was told to do, or wipe that out of RAM and load a new page coming from the computer and go though the process again. In the meantime it also must manage paper flow, check its own operations for failures and low levels of consumable supplies, manage its heating andcooling systems, etc.
So, all of those tasks are carried out by the microprocessor-based system inside the printer, most of that mounted on the logic board.
By the way, in case you do not yet now how each little dot is printed, here's an outline. Laser printers all use a process called Xerography, marketed originally by Xerox. See
https://en.wikipedia.org/wiki/Xerography
As compared to the original photocopying system, a laser printer does the light part differently. It does not scan the document using a light source. Instead the laser light source beam, switched on and off rapidly according to where the light is required, is moved across the surface of a photosensitive drum by a moving optical system. In some recent designs, the light actually is generated instead by a fixed-position array of tiny LED's switched on and off and located just over the drum. The drum has a special photosensitive coating that works on electrostatic charges. As the drum rotates, in turn it is "spayed" with an electrostatic charge, and then the light from the laser or LED units is applied to dissipate the charge in areas that do NOT want to be printed, leaving residual charge where the printing dots are needed. Then the drum surface passes close to a tray containing the toner particles of opposite charge, and it picks up particles wherever the charge is on the drum. Then these particles are transferred to the surface of the paper pressed against the drum by a roller, and lastly the paper goes past a very hot heater. The toner particles of pigment are lightly coated with an adhesive that melts when heated, and bonds to the paper surface as it cools, thus anchoring the toner to the paper to make the printed dots. In a colour laser printer, the work is done with four different toner colours - Cyan (a blue-turquoise), Magenta (slightly purple deep red), Yellow, and Black - each applied to the paper sheet by separate printing drum units before the final heat setting stage. So in that case the microprocessor system has more than four times a much work to do, because it has to figure out how to produce four different coloured versions of the final page so that the final colours all look as they should.
Facebook
Twitter