Johnny Billquist bqt at
Tue Jun 2 12:29:22 CDT 2009

"Dave McGuire" <mcguire at> wrote:

> On Tue, June 2, 2009 4:45 am, Johnny Billquist wrote:
>>> >> 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!

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 Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol

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