2 Jun
2009
2 Jun
'09
5:29 p.m.
"Dave McGuire" <mcguire at neurotica.com> wrote:
On Tue, June 2, 2009 4:45 am, Johnny Billquist wrote:
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
--
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
> >> 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! 
3 Jun
3 Jun
3:34 p.m.
On Jun 2, 2009, at 1:29 PM, Johnny Billquist wrote:
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
>> 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.
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