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 board.
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?
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.
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, too...
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.
That rather obviously depends on what peripherals you're going to use. If they don't need 12V, your main board doesn't either.
Some may. You buy them --- so _you_ get to choose.
Hans-Bernhard Broeker (email@example.com)
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