Printers
This is a primary
accessory that a computer uses that you will be required to
service as a technician. You MUST be able to state the
parts, the inner workings, and the different terminology.
Connections
You can assume that
all printers hook up to the computer through either the serial
or parallel port. (Printers also can hook up through
networking cables, and some older printers used proprietary
ports.) A serial connector is a connection that sends
information one bit at a time, where a parallel connection
sends multiple bits at a single time. Most current
printers use the parallel port, or the LPT port.
LPT ports have
specific I/O addresses and IRQ values. LPT 1 (The
default LPT port) uses I/O 378-37Fh and IRQ 7. LPT 2
uses I/O 278-27Fh and IRQ 5. LPT 3 uses I/O 3BC-3BFh and
either IRQ 5 or 7. Notice that you can not use LPT 3 and
another LPT port set to the same IRQ value at the same time,
or the computer will create a conflict and freeze.
Although printers don't often use the IRQ values, devices like
Tape back-up drives and external storage devices often
will. If you have a printer and an external drive
conflicting, check the IRQ's.
Parallel ports use
two different kinds of connectors. The port on the
computer itself is generally a 25-pin DB female
connector. The cable will have a 25-pin DB male
connector that fits over this connection.
The printer itself
will generally have a 36-pin Centronics connector.
Centronics connectors look different that DB connectors, as
they connect in a much different way. The male
Centronics connector consists of two rows of metal contacts
anchored by a center strip down the middle of the
connector . The female connector has the exact
opposite center strip, with metal connectors attached so they
will contact with the pins on the male connector. (This
is hard to explain, but easy to see. Disconnect your
printer from your computer and look at the two ends. The
one at the printer is the Centronics.)
Important Notes
about Printer Cables and Accessories
The first deals with
cables. Never use a cable longer than 15 feet on a
printer, as the signal degrades quickly over longer distances.
The second is switchboxes
or printer data switch and Laser printers. Just
remember that you NEVER connect a laser printer to a
switchbox. When a switchbox throws a switch, it sends a
spike of electricity through the box that can damage the
printer.
The last is the IEEE 1284 Parallel port standard. The
IEEE 1284 Parallel Port Standard deals with bi-directional
communications over parallel ports, and is made up of three
standards;
-
Standard Parallel
Port (SPP) - One-Way transmission of Data
-
Enhanced Parallel
Port (EPP) - Allows data to transmit both ways over the
cable, but only in one direction at a time.
-
Enhanced
Capabilities Port (ECP) - Allows full bi-directional
simultaneous communications, but only when used with an ECP
(IEEE1284) cable.
Impact and
Jets
There are two basic
variations of the common printer. They are distinguished
by the method used to transfer the mark from the printer to
the paper. Any printer that uses a print head that
strikes a mark on a printer is called an Impact Printer.
These include Daisy Wheel printers and Dot-Matrix
Printers. All impact printers use a ribbon to transfer
the image on to the paper.
Any printer that
doesn't use a ribbon is called a non-impact printer. Ink
Jet, Bubble Jet, laser, and LED printers are all examples of
non-impact printers. They use a storage medium to store
the ink or toner until it is attached to the printer. In
the next section we will deal with exactly how this transfer
takes place.
Impact
Printers
As stated before,
impact printers use a ribbon that is struck by a print head in
order to transfer an image onto the paper. The oldest
type of Impact printer is the Daisy Wheel Printer. The
print head on a Daisy wheel printer is nothing more than a
metal wheel with each possible character the printer can
reproduce engraved on it. The wheel spins to strike the
ribbon with the appropriate character facing the ribbon.
Because the quality of the print is dependant on the quality
of the engraving, it is called letter quality printing.
(LQ)
Dot Matrix printers
also use a ribbon to create characters, but use a much
different impact system to strike the ribbon. Dot Matrix
printers do exactly what their name says; they print out a
matrix of dots to create letters. This is done with a
series of wires attached to pins that are controlled by the
print controller. There can be 9, 18, or 24 print wires
and pins on a dot matrix printer, with 24 pin printers capable
of producing Near-Letter Quality printing (NLQ) and 9 and 18
pin printers producing Draft Quality printing.
In order to make it's
impact, a dot matrix printer's print head pulls on the print
wires in a format to produce a specific character. A pin
in front of each print wire extends out from the print head,
and strikes the ribbon against the paper. A spring then pulls
the pin back into the print head after the mark is
made.
Dot Matrix printers
are measured by their resolution, or how many dots per inch
(DPI) they can create. A low-resolution printer will
have large spaces in between each dot, where a higher
resolution printer will overlap the dots to reduce these
spaces. Because they are an impact printer, they also
are used in situations where carbon is used to make multiple
copies. (A non-impact printer can't push on the carbon
to make a second copy, and thus can't be used for invoice
printers where multiple copies are made on one
sheet)
There are also two
different paper feeding mechanisms used by Dot Matrix
printers. A large rubber roller, called a platen, is
used is form feed mode to pull a sheet of paper through the
printer and past the print head. Form Feed uses single
sheets at a time. Tractor Feed mode uses rubber or
plastic pins that fit holes cut into the side of the paper to
push the paper through the printer. This allows pages to
be linked together in a chain so that a continuous stream of
paper can be fed into the printer without having to insert a
new page each time.
Two important aspects
of Dot Matrix printers; Never clean a dot matrix print head,
as they are not user-serviceable. And Print heads get
extremely hot during printing, and should not be handled when
in use.
Non-Impact
Printers
Inkjet printers, also
known as Bubble Jet and Desk Jet printers, are the most
popular printers used today. They are cheap, and they
are quiet, so they are perfect for almost every home and some
office uses.
Inkjet printers use
nozzles to "jet" ink from a reservoir onto a page. Here
is how the process occurs;
-
A nozzle has an
opening inside that is filled with ink from the ink
storage.
-
Either heat
(thermal) or an electric current (Piezo-electric) is passed
through the nozzle, causing the ink to expand.
-
When the ink
expands, the excess ink is sprayed out on to the
page.
-
When the ink
returns to normal size, it draws more ink from the reservoir
to fill the nozzle again.
There can be upwards
of 100 nozzles on an inkjet print head. The more nozzles
there are, the better the resolution the printer can
create. Inkjet printers are rating in Dots Per Inch
(DPI) like dot matrix printers, but range to upwards of 2400
DPI for photo-quality printers. Inkjet printers are also
rated on speed, counted in Pages Per Minute. (PPM)
The difference
between Piezo-Electric nozzles and Thermal nozzles is
significant. Thermal nozzles create a set temperature
inside the nozzle, forcing a set amount of ink out each
time. This means that the printer can only push out a
certain size dot. Piezo-Electric printers send an
electric current through the ink. By varying this
current, they can push more or less ink out, creating
different sized dots. (Because heat isn't used in
Piezo-Electric nozzles, they require less maintenance as well)
Therefore, Piezo-Electric Inkjet Printers are more expensive
than thermal Inkjet printers.
Lasers
Laser printers and
Light-Emitting Diode (LED) printers are the most important
printing format to understand. Since they are mostly
used in commercial offices, they are the most likely printer
format you will see when working in the field. There are 6 processes
that go into a laser printer producing an image on a sheet of
paper.
-
Cleaning -
The first step in printing a page is to clean off all the
elements of the last page. A rubber blade called a
"Doctor" blade cleans all the remaining toner off the drum,
and a fluorescent lamp removes all electrical charges that
may remain on the drum. Left-Over toned is not reused,
as it has already been exposed to the fusing process.
-
Conditioning
- The entire drum is given an equal -600 Volt charge by a
corona wire.
-
Writing -
Using a series of mirrors, the laser "draws" the image onto
the drum. Where the laser hits the drum, the charge is
reduced by about -100 Volts.
-
Developing -
A magnet inside a roller called the "Developer Roller"
attracts toner from inside the toner cartridge. The
developing roller is then passed across the drum, where the
ink sticks to the parts of the drum that have been exposed
to the laser.
-
Transferring
- The back side of the paper the image is to be produced on
receives a positive charge. This is done by a
registration roller, and it attracts the negatively-charged
toner to the front of the sheet.
-
Fusing - The
toner up to this point is held on through magnetism, and the
plastic elements in the toner must be melted to the
paper. The Fusing roller heats the paper and toner to
about 180 Degrees Celsius, and fuses the toner right to the
paper.
It is important to
realize that the drum is a constantly revolving mechanism, and
does not carry a charge for an entire sheet of paper at a
time. While the last part of the drum may be in the
cleaning process, the images from the middle of the page may
still be fusing or transferring. A laser printer does
everything line-by-line. This means the entire process
can be occurring all at the same time during a single
page.
There are also two
formats used during writing and developing. Example where the image is written to the drum,
and you should remember it as I have written it above.
Some printers however write the opposite of what is to be
printed on the drum, like a picture negative. They then
attract toner to the parts of the drum NOT exposed to the
laser.
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