Byron A Jeff wrote:

> The amperage is fine.

  120mA for the keyboard connector is nominal.  Except for rare machines
which are fused (like the NEC SX16 I just cleaned), it could deliver up
to 5 amps if you trusted the connector.  It is *limited* only by the
main SMPSU.  If you want 250mA for example, that is almost always
doable.  Similar for games port.

> I wouldn't dare comment on the noise though as it is the main +5
> supply for the PC.

  I'd reckon it pretty horrendous.

> Which is why folks have suggested regulating the 12V supply instead
> of using the 5V.

  But we were talking of externally available supplies.

> That's the idea. However if it turns out that the line isn't clean
> enough you can always resort to the hybrid method where you supply the
> ADC from a battery generated, regulated supply and power the rest from
> the keyboard.

  Batteries?  Ugh!  I hate batteries.  If sub-regulation to a 2.5V
reference is appropriate, that would be one option.  Otherwise a MAX-232
to generate 10V and regulate that back to 5 for the reference.  *DO*
pay attention to board design, lead dress and as was mentioned,
shielding if that helps.

> I did exactly this for my programmer because I needed 13V which I
> regulate down from 2x9V batteries.

  I hate batteries.  Worst of all for something you use only very
occasionally.  There is a chip called a TL497 which isn't particularly
expensive, used in most serious programmers.  Once you are used to using
it, no other way is worth doing (hint; voltage can be *programmed* using
a 74HC05).

Steve Blackmore wrote:

> I haven't used parallel port, but have powered adc via serial port 5v
> - seems to work fine on desktop machines but not on all laptops.

  Well, except for the laptops, the serial port can be regarded as a
current source from somewhat *greater* than 5V, to which you use a shunt
regulator, so it should work quite well for ADC.
--
  Cheers,
        Paul B.