cctalk Digest, Vol 70, Issue 3

Dave McGuire mcguire at neurotica.com
Tue Jun 2 10:39:26 CDT 2009


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!

                   -Dave

-- 
Dave McGuire
Port Charlotte, FL





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