modem help

From: Ross Archer <archer_at_topnow.com>
Date: Fri Oct 25 21:47:00 2002

Tony Duell wrote:
>
> > That sportster needs 9VAC. Older sportsters need 12VAC.
> > I don't know about the Motorola.
>
> FWIW, the 'AC' is important. You can't use a DC-output adapter. Many
> modems (and other devices with RS232 ports) produce both a +ve and -ve
> supply from the AC input for the RS232 buffers (and maybe the analogue
> circuitry too).
>
> -tony

Always thought that was rather clever
.
Two half-wave circuits with diodes conducting in opposite
cycles has to be the cheapest way to get bipolar supplies.

These days, it seems everyone's using MAX232s instead to get
the required negative supply. Which is good since the other
solution rules out battery ops. And everything cool is
goin'
portable. Look at the SX-64 :)

-- Ross
Received on Fri Oct 25 2002 - 21:47:00 BST