PC Form Factors
Computers come in different form factors. ATX is the most common. AT used to
be the standard but is now obsolete. NLX and LPX are two others. These forms
describe the shape and size of the motherboards, as well as the layout of the
components on the board. The form factor will also determine the type of case
you must buy, as the case is laid out differently and uses a different type of
power supply.
AT Form Factor
Within the AT form, we have regular AT and Baby AT. They basically differ in
size. An AT board is about 12" wide which means it can't fit in many of today's
cases. AT boards generally are the older boards, 386 or earlier. Working
inside the case was a lot more trouble with these because the size of the
motherboard overlapped drive bays and such.
Baby AT is the form used by many 486 and Pentium boards.. Many Socket 7
motherboards and a few Pentium II boards used this form factor. A Baby AT
board is roughly 8.5" wide and 13" long. The size varies a little from board to
board. This reduced size makes it easier to work inside the case simply
because there is more room. There are three rows of mounting holes to hold
the board in the case.
AT form boards share common traits. They all have serial and parallel ports
attached to the case in an expansion slot and connected to the board through
cables. They also have a single keyboard connector soldered onto the board
at the back of the board. The processor is still at the front of the board and
can sometimes get in the way of expansion cards. The SIMM slots are in
different places, although they are almost always at the top of the board.
There are some annoyances with the AT design. One is due to the layout.
Since all ports are attached to the case and then connected to the
motherboard via a cable, the board must have connectors for all of these:
COM 1, COM 2, printer port, USB, PS/2 mouse, etc. Often these connectors
are directly next to the IDE channel connectors and floppy drive connector.
This leads to a severe cramping problem and makes working inside the
computer more difficult. Secondly, the AT design is not conducive to efficient
cooling of the system. Air is not blown over the areas that need it, namely the
CPU. Also, the air flow draws in dust. Over time, the AT power supply will get
dusty and the inside of the system will be coated with a layer of dust. For this
reason, it is recommended you regularly remove the case and blow off the
interior of the case.
ATX
In 1995, Intel released the ATX form factor. It was slow to catch on due to the
long-time acceptance of the AT board, but slowly it gained popularity. Almost
all Pentium II and later boards are ATX, although there were some AT
Pentium II boards available. Pentium boards were still primarily AT, although
many manufacturers have released ATX versions. ATX is now THE form
factor, and it would be a bit difficult to buy anything else.
The ATX design is a vast improvement over AT. It fixes the annoyances of the
AT form. Since the AT form was so old, as new demands were placed on it
from new technology, little problems began to show up. These problems are
taken into account with ATX.
Some features are:
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Integrated I/O Connectors: While the AT uses headers on the board that
are attached to the actual ports on the back of the case, the ATX board
has the actual ports built right onto the board. This makes installation
easier and enhances reliability.
Integrated PS/2 Mouse Connector
Reduced Overlap Between Board and Drives: The ATX board looks
like it is rotated 90 degrees so that it does not overlap the drive bays in the
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front. This way, one can reach the entire board instead of having to reach
around a drive, or even remove the drive, in order to reach certain areas
of the motherboard. This also reduces heat.
Reduced Processor Interference with Cards: The processor is moved
from the front of the board near the slots to the back, top of the board,
near the power supply. This means that a user can install full-length
expansion cards in the slot without having to worry about hitting the CPU
or heat sink.
User-Friendly Power Connector: ATX uses one 20-pin connector to
attach to the motherboard. The ATX connector is also keyed so that it will
only go in the correct way. This is easier than the two separate connectors
of the AT form which look almost the same. It also gets rid of the problem
of frying the board due to misplacement of the connectors on the
motherboard.
Better Cooling Conditions: The ATX power supply blows air into the
case instead of out. This means that air blows out all the holes in the case
and thus keeps dust out.
3.3 Volt Power: The ATX motherboard is designed to accept 3.3 volt
power directly from the power supply. Since almost all modern processors
operate at 3.3 volts, this removes the need for a voltage regulator on the
motherboard to reduce the voltage from 5V to 3.3V. It must be noted,
though, that many processors don't use this voltage, and therefore must
use a voltage regulator anyway.
A Little Automation: The ATX power supply is capable of being
controlled through software and other means. This is because it always
has a slight voltage going through it. This gives the computer the ability to
turn itself on at specified times and perform some task given to it by the
software. Some ATX motherboards have the option of turning the system
on by pressing the space bar on your keyboard or being woken up by a
command sent down through the LAN. Lastly, the shut down procedure is
automated using ATX. When one uses Win95 and chooses "Shut Down",
the computer will perform all shut down tasks, then turn itself off. Not too
shabby.
NLX Form Factor
NLX is intended for low-profile PC's. NLX systems uses a riser board, similar
to the LPX form. The riser boards sits vertically in the case and is connected
directly to the power supply. Each of the expansion cards connect directly to
this riser board, as well as the HDD and FDD connectors that usually reside
on the motherboard itself. So, basically, the riser card is the hub of the NLX
system, with everything connecting to it. Even the NLX motherboard can
simply be removed from its slot on the riser card and replaced.
An NLX motherboard looks different than that of AT of ATX. The first thing
you'll notice is that it has no expansion card slots nor does it have any drive
connectors. As said before, all this has been moved to the riser card. Also,
you will notice a 340-pin connector on the side of the motherboard. This is
what interfaces with the riser card. Like ATX, the I/O connectors are on a
panel on the rear of the board, and like ATX, this part of the board sticks out
of the case so you can connect your USB, parallel and serial devices to the
NLX system.
The power supply looks similar to that of an ATX machine, and connect to the
motherboard via a 20-pin connector. It has a single exhaust fan which
happens to be also placed near where the CPU is placed on the motherboard,
so that the power fan can help cool the NLX system.
LPX Form Factor
LPX is like an older AT-version of the NLX form factor. LPX motherboards
have the same, if not smaller, dimensions as a baby-AT motherboard. They
also share the same AT-style power connectors: two 6-pin connectors usually
labeled P8 and P9. Like NLX, it uses a riser card as the hub of the system.
The external I/O ports on an LPX system follow the same format usually:
(from left to right) VGA, parallel port, 2 serial ports, PS/2 mouse, PS/2
keyboard. Some newer versions alter this layout to accomdate USB and
possibly LAN connections.
LPX, back in its day, was taken on by several manufacturers, and it was used
pretty much up until the Pentium II days. But, with the lack of standardization,
it did not go beyond that. LPX riser cards had to meet certain electrical
requirements, but beyond that, the arrangements of the various connectors
varied from manufacturer to manufacturer. This led to difficulty in upgrading
because you could not just swap out riser cards. Thus, it had a limited life.