bqt at softjar.se
Tue Jun 2 12:29:22 CDT 2009
"Dave McGuire" <mcguire at neurotica.com> 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 softjar.se || Reading murder books
pdp is alive! || tryin' to stay hip" - B. Idol
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