2 Jun
2009
2 Jun
'09
5:29 p.m.
"Dave McGuire" <mcguire at neurotica.com> wrote:
On Tue, June 2, 2009 4:45 am, Johnny Billquist wrote:
No, but close enough to not make much difference. If the modem would
draw anywhere near 100mA over RS-232 just to get the levels, something
is seriously wrong.
Johnny
--
Johnny Billquist || "I'm on a bus
|| on a psychedelic trip
email: bqt at softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
> >> USB only guarantees to supply 100
milliamps. Up to 500 mA can be
> >> negotiated with the controller, depending. Might your older device
> >> depend on more amps than the USB adapter can supply?
>
> Huh? The RS-232-port isn't supposed to deliver any power at all. Some
> devices admittedly abused the RS-232 by using something like DTR to
> actually supply the power needed to drive the thing, but that is abuse.
>
> But I'd be surprised if a circa-70s modem was ever designed to use the
> power from the RS-232 port to drive the modem itself. I'd expect it to
> have an external power supply.
He's not talking about powering the device, he's talking about driving
the signal lines. Their input impedance isn't infinite, you know!