cctalk Digest, Vol 70, Issue 3

[Johnny Billquist]

cctalk-request at classiccmp.org wrote:
> Message: 21 Date: Mon, 01 Jun 2009 20:16:20 -0500 From: John Foust <jfoust at threedee.com> Subject: Re: 1964 Antique MODEM Live Demo To: <cctalk at classiccmp.org> Message-ID: <6.2.3.4.2.20090601201242.03e96008 at mail.threedee.com> Content-Type: text/plain; charset="us-ascii" At 03:24 PM 6/1/2009, Gordon JC Pearce wrote:
>> >Okay, let's try again.  I have a circa-70s 300-baud modem, that will
>> >generate incorrect tones when it's plugged into a USB-to-serial
>> >converter, but not when it's plugged into a real serial port.
>> >Why might this happen?
> 
> 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.

The problem description is a bit vague. How do you know it is generating 
incorrect tones? Have you used a frequency analyser to check the tone 
frequencies? Or might it just be that what has been observed is that the 
modem don't work when used with the USB-to-serial? If so, then we have a 
number of possible problems, none of which neccesarily is a problem with 
the modem. Or is it just a case of listening to the modem while plugging 
in the cable, and not the same thing happening on the different serial 
ports. Might the USB-to-serial not start driving signals until something 
talks to it on the software side?

I'd start by checking that the USB-to-serial really delivers correct 
voltages. Break out the oscilloscope.
Next, I'd check if the USB-to-serial is happy without all kind of signal 
"right", and if the modem supplies all the "right" signals. Also check 
cables.

	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