[PLUG-TALK] oscilloscopes are fun!

Russell Senior seniorr at aracnet.com
Fri Mar 3 21:50:26 UTC 2006


A few months ago, Galen mentioned the degenerate behavior of a
particular USB-to-RS232 converter sending what was supposed to be a
BREAK as a series of NULs instead (if I remember correctly).  I've
been borrowing his oscilloscope, and have recently been looking at the
serial output of a couple USB-to-RS232 devices.  First was a cellphone
cable that I am using as a serial console on a Netgear WGT634U.  The
peculiar thing about these router devices is that their RS232
interfaces operate at TTL voltages, in this case nominally 3.3V,
rather than the normal higher voltages of an ordinary PC or modem
RS232 interface.  It was cool to watch the pattern of bits,
alternating from a binary zero value at about 3.6V to a binary one
value at ground.  

Yesterday, I got around to hooking up a USB-to-RS232 (for a soekris
board that uses normal RS232 signalling) and found something
interesting.  There was a substantial 30 kHz sawtooth signal on top of
the RS232.  The amplitude of the sawtooth is about 1.7-1.8V.  The idle
signal is down around -6.8V, a binary zero bit is about +6.8V, and the
binary one is -6.8V.

I mentioned the sawtooth to Galen last night, and I got to thinking
more about it afterwards, and it suddenly occurred to me: USB is only
supplying 5V.  The device (in a pretty small package) is having to
boost the voltage and smooth it out again.  Evidently, to get it all
fit into such a small package, a few engineering tradeoffs were
required and the sawtooth was one of the artifacts of the tradeoffs
they made.  The device works fine, but it is kinda funny looking.

Which leads me to my question of the day: how do DC-to-DC converters
work?  In the context of Power over Ethernet (PoE, 802.3af) devices,
I've been doing some reading that leads me to understand step-down
devices often seem to use Pulse Width Modulation (PWM), which I can
understand, but I don't grasp how that could work for boosting
voltage, so some kind of AC transformer (or something beyond my
elementary-level grasp of electronics) must be involved.

Any of you solder guys want to clue me in?


-- 
Russell Senior         ``I have nine fingers; you have ten.''
seniorr at aracnet.com



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