Laser Printers Using MIPS

Path of paper through laser printer The primary principle at work in a laser printer is static electricity. Static electricity is simply an electrical charge built up on an insulated object. Since oppositely charged atoms are attracted to each other, objects with opposite static electricity fields cling together. A laser printer uses this phenomenon as a sort of "temporary glue." The core component of this system is the photoreceptor, typically a revolving drum or cylinder. This drum assembly is made out of highly photoconductive material that is discharged by light photons.

Initially, the drum is given a total positive charge by the charge corona wire, a wire with an electrical current running through it. As the drum revolves, the printer shines a tiny laser beam across the surface to discharge certain points. In this way, the laser "draws" the letters and images to be printed as a pattern of electrical charges -- an electrostatic image. After the pattern is set, the printer coats the drum with positively charged toner -- a fine, black powder. Since it has a positive charge, the toner clings to the negative discharged areas of the drum, but not to the positively charged "background."

With the powder pattern affixed, the drum rolls over a sheet of paper, which is moving along a belt below. Before the paper rolls under the drum, it is given a negative charge by the transfer corona wire (charged roller). This charge is stronger than the negative charge of the electrostatic image, so the paper can pull the toner powder away. Since it is moving at the same speed as the drum, the paper picks up the image pattern exactly. To keep the paper from clinging to the drum, it is discharged by the detac corona wire immediately after picking up the toner.

Finally, the printer passes the paper through the fuser, a pair of heated rollers. As the paper passes through these rollers, the loose toner powder melts, fusing with the fibers in the paper. The fuser rolls the paper to the output tray, and you have your finished page. The fuser also heats up the paper itself, of course, which is why pages are always hot when they come out of a laser printer. After depositing toner on the paper, the drum surface passes the discharge lamp. This bright light exposes the entire photoreceptor surface, erasing the electrical image. The drum surface then passes the charge corona wire, which reapplies the positive charge.

The Controller Before a laser printer can do anything else, it needs to receive the page data and figure out how it's going to put everything on the paper. This is the job of the printer controller. The printer controller is the laser printer's main onboard computer. It talks to the host computer (for example, your PC) through a communications port, such as a parallel port. At the start of the printing job, the laser printer establishes with the host computer how they will exchange data. The controller may have to start and stop the host computer periodically to process the information it has received. Printer Controller Inputs

Parallel Port The original specification for parallel ports was unidirectional, meaning that data only traveled in one direction for each pin. With the introduction of the PS/2 in 1987, IBM offered a new bidirectional parallel port design. This mode is commonly known as Standard Parallel Port (SPP) and has completely replaced the original design. Bidirectional communication allows each device to receive data as well as transmit it. Many devices use the eight pins (2 through 9) originally designated for data. Using the same eight pins limits communication to half-duplex, meaning that information can only travel in one direction at a time. But pins 18 through 25, originally just used as grounds, can be used as data pins also. This allows for full-duplex (both directions at the same time) communication.

The Controller Language For the printer controller and the host computer to communicate, they need to speak the same page description language. The primary printer languages these days are Hewlett Packard's Printer Command Language (PCL) and Adobe's Postscript. Both of these languages describe the page in vector form -- that is, as mathematical values of geometric shapes, rather than as a series of dots (a bitmap image). The printer itself takes the vector images and converts them into a bitmap page. With this system, the printer can receive elaborate, complex pages, featuring any sort of font or image. Also, since the printer creates the bitmap image itself, it can use its maximum printer resolution.

The Controller Language - continued Some printers use a graphical device interface (GDI) format instead of a standard PCL. In this system, the host computer creates the dot array itself, so the controller doesn't have to process anything -- it just sends the dot instructions on to the laser. But in most laser printers, the controller must organize all of the data it receives from the host computer. This includes all of the commands that tell the printer what to do -- what paper to use, how to format the page, how to handle the font, etc. For the controller to work with this data, it has to get it in the right order. In most laser printers, the controller saves all print-job data in its own memory. This lets the controller put different printing jobs into a queue so it can work through them one at a time. It also saves time when printing multiple copies of a document, since the host computer only has to send the data once.

Printer Speed It may seem perfectly natural, when judging the performance of a high-speed laser printer, to look at the clock speed of the processor that's driving it. The more megahertz, the better, right? Not necessarily. Clock speed - an indication of how many instructions per second a processor can execute - as the measure of performance in PCs. And many consider it the driving force behind printer speed, which is the number of pages per minute a printer can generate. There's more to print speed than clock speed: Is the processor RISC or CISC? How fast does it process large graphics files and the very long algorithms characteristic of PCL and Adobe PostScript printer languages? And, bottom line, how much does it cost?

CISC vs. RISC architecture Computing architecture affects both the speed and cost of laser printers. CISC (complex instruction set computer) have a much lower effective speed in an embedded application like a laser printer. That's because the CISC architecture was designed for computers. CISC chips are burdened by multi-cycle, micro-coded, complex instructions - a legacy of 1970s development -- many of which are not required in embedded applications. Aside from performance, it can negatively impact the cost of other system components, including the electronics, power supply and pin count - a detriment to cost-sensitive embedded applications like laser printers. RISC (reduced instruction set computer) architecture was developed in the 1980s as a simpler, faster, superior alternative to CISC. It offers easier decoding and pipelining, and typically executes at least one instruction per clock cycle, as opposed to CISC, which often does less.

MIPS architecture Of the current RISC architectures, the MIPS architecture is the only one in the embedded systems industry generally available for licensing. They range from ultra-low-power 32-bit CPU cores occupying less than a half-millimeter of silicon, to 64-bit dual-core processors running at 1 GHz. Cores are designed for easy integration into system-on-a-chip designs, which offer additional performance advantages in embedded applications, such as lower power and fewer components for higher reliability and out-of-the-box functionality.

Example: HP LaserJet 9500 PMC-Sierra is the manufacturer of MIPS-based processors used in Hewlett-Packard laser printers. PMC-Sierra’s latest processor, 64-Bit MIPS RISC Microprocessor with integrated L2 Cache, is used in the latest network printers from HP. The processor features 600MHz operating frequency, 2 levels of cache: 1st level 16KB 4-way set associative 32-byte line size Instruction and Data caches, 2nd level 256 KB 4-way set associative 32-byte line size, Also, an L3 external cache (off chip) 512KB-8MB direct-mapped, 32-byte line size

Example: HP LaserJet 9500 64-Bit Processor

Will Printers become faster ? Future generations of workgroup printers will continue to offer increasingly higher speeds. Given the uniquely broad range of processors being designed by MIPS licensees, from ultra-low-power 32-bit cores to 64-bit 1-GHz CPUs, anything is possible.

Thank you Questions? Ramiz Bleibel Anton Petrosyan Mohamad Ghuneim Bertha Sierra