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
To keep the paper from clinging
to the drum, it is discharged by
the detac corona wire
immediately after picking up the
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
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 halfduplex, 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)
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
• And, bottom line, how much does it cost?
CISC vs. RISC architecture
• Computing architecture affects both the speed and cost of laser
• 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-ona-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 ultralow-power 32-bit cores to 64-bit 1-GHz CPUs,
anything is possible.
Thank you
Ramiz Bleibel
Anton Petrosyan
Mohamad Ghuneim
Bertha Sierra

Laser Printers - UH Cullen College of Engineering