RS-232

[Dave McGuire]

On Jun 2, 2009, at 1:29 PM, 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.

   Well yes...100mA is excessive.  I'm not sure it's reasonable,  
though, to assume that there'd be 100mA (or even 10% of that) of  
drive capability available on the TxD pin from a USB<->serial  
bridge.  The suggestion was that the drive capability (whatever it  
may be) is insufficient, that everything will require a finite amount  
of current there, and perhaps the ancient modem just wants too much.

               -Dave

-- 
Dave McGuire
Port Charlotte, FL