cctalk Digest, Vol 70, Issue 3

Dave McGuire mcguire at
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 McGuire
Port Charlotte, FL

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