Archived from groups: alt.comp.periphs.mainboard.asus (
More info?)
In article <DG6Qc.184$S%2.148@newsfe2-gui.ntli.net>, "Roger Hamlett"
<rogerspamignored@ttelmah.demon.co.uk> wrote:
> "Herbert Chan" <herbert@chan.com> wrote in message
> news:4110d693$1_1@rain.i-cable.com...
> > Hello,
> >
> > What is it for? I think CPU fan runs on +12V. How about -12V? My
> computer
> > always reports that there is no -12V at startup. There will be -12V
> (like at
> > this moment, it is -12.27V) again when the computer has been on for a
> while.
> >
> > I want to know what components actually run on this voltage. Does it
> mean
> > that if there is no -12V, those components won't run during startup
> until
> > there is a voltage?
> The -12v rail, was historically used for two things. On early machines
> some memories required a -ve bias (hasn't applied for perhaps 10
> years...). The other user, is the RS232 ports. A few PCI cards use the
> supply too, but these are now very rare.
> Many modern boards use a chip like the MAX232 to supply the RS232, and no
> longer use this rail at all.
> Even if the rail is used by such drivers, they will still wake up and
> 'run', but just be unable to pull the line properly 'down' when sending
> RS232 data.
>
> Best Wishes
I have just discovered to my shock and horror, that my P4C800-E Deluxe
uses a TI GD75232 for at least the COM2 port, and that is not a MAX
type device. The MAX232 style has a charge pump built in, and converts
a motherboard voltage to make the necessary negative voltage. The charge
pump uses external capacitors to work. It seems the GD75232 is one
of the old style parts, running off +12 and -12V. So, there are
still boards that need the -12V supply.
http://www-s.ti.com/sc/ds/gd75232.pdf
For the following, see pg.17 and C1, C2, C3, C4, which are parts of
the voltage doubler and voltage inverter. +5V makes something like
+10 and -10V.
http://pdfserv.maxim-ic.com/en/ds/MAX220-MAX249.pdf
When a GD75232 costs $0.50 and a MAX232 type costs $1.85, it is not
difficult to see which they will buy.
Paul