PC 104 or Small Embedded Boards

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Can anyone please tell me why some pc/104 boards or small embedded boards
need Dual voltage.
There are some boards on the market require two votages 5 volt and 12 volt.
They say the 12 volt is to operate other peripherals  connected  to the

The boards with single 5 volt operation have facility`s for connecting
peripherals, according to the manifacturers.

Is it neccesary to purchase a board with dual voltage?

Can peripherals operate on 12 volts?

Is this true.

Thanks Craig

Re: PC 104 or Small Embedded Boards
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Some peripherals may require a +12V rail. Many (most? practically
all??) do not. LCDs, for instance, may need 12V for the inverter. Or
not. If the board brings out +12V to an external jack for connection
to some standard inverter type, you'd better be feeding +12V in
someplace (I don't want wasted space, noise and cost with an extra
SMPS on the motherboard).

In some cases, the board itself may work better with +12V available.
For instance, some of the boards we use are jumperable to use +12V, if
available, for the audio amplifier.

Re: PC 104 or Small Embedded Boards

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Essentially because of the "PC" part of their name.  PC/104 is,
roughly said, a simplified plain old ISA PC in a different "form
factor".  To the best of my knowledge, PCs have had 12V supply lines
ever since they hit the street over 20 years ago.  And -12V and -5V,

Note that even today, all the power cables going to hard disks and
other drives supply both +5 and +12 Volts.

Usage of all those lines by PC mainboards has varied over time.  I
guess they were originally meant to drive the RS232 ports.  

Nowadays, the +12 V seem to be used primarily as input for voltage
regulators that feed the CPU and/or AGP bus drivers.  They use the 12V
lines because taking those 80 Watts from the 3.3V line would require
uncomfortably high currents --- cable resistance becomes a
non-negligible factor at 25+ Amperes.

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That rather obviously depends on what peripherals you're going to use.
If they don't need 12V, your main board doesn't either.

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Some may.  You buy them --- so _you_ get to choose.

Hans-Bernhard Broeker ( snipped-for-privacy@physik.rwth-aachen.de)
Even if all the snow were burnt, ashes would remain.

Re: PC 104 or Small Embedded Boards
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Most hard drives, at least, only use 5V though.

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On PC/104 boards, at least the ones I've used, 12V is mostly just
passthrough to peripherals.

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