Yeah, I knew better and feel like a moron. I wanted to post here for two reasons, one to provide an example of what can happen and two, to inquire about how to protect the PC in the future.
I looked here: http://arduino.cc/forum/index.php/topic,124547.0.html and on a much older thread and didn't see a good solution.
I was working with this H-bridge motor controller:
The documentation isn't the best. It has four terminals on the output side. Two for power (B+ and gnd) and two for the motor (M+ and M-). There are six on the "input" side. B+, ground, enable, pwm1, pwm2, vt and ct. At the time of the incident I did not have a data sheet for the motor driver chip - it is a L6201P.
The datasheet is on this page: http://www.st.com/internet/analog/product/63228.jsp
I had a 24V, 5A external power supply for the output side. It has a circuit breaker on the 120V side and a 6A circuit breaker on the 24V size.
The Arduino was wired to the input side of the controller as follows:
5V to B+
Pin 4 to Enable
Pin 3 to Pwm1
Pin 5 to Pwm2
Gnd to Gnd
CT and VT not connected.
The Arduino was powered via USB from the laptop PC
So..... after everything was connected, I turned on the line-side cb and then the output cb and ..... poof! I shut off the cb (still had my hand on it - it doesn't take long) and disconnected the USB cable. The PC display was blank. I could smell a hint of released electronic smoke.
The PC came back to life after pushing the power button. It was like it went to sleep or something. (BTW - it is 6 month old, win7 64b, Dell precision something or other WORK computer).
The Arduino was dead, TX RX and power on LED's were lit. I checked voltage on 5V and it was 4.4V, 3.3V was 3.3V. I figured I fried both processors on the board.
Here's the moron part. B+ is B+. The 24V on the "output" side is connected to the first pin on the "input" side. I put 24V on the 5V Arduino pin.
So I broke out Arduino #2 and couldn't load a sketch. I don't recall the exact error, but it contained "synch". The IDE looked like it finished loading, but couldn't confirm? So, I figured I would reboot to get back to square one.
The PC would not reboot. It posted and then hung. It is still in that state. I installed Ubuntu Live CD to make sure all my data was there and the drives appear to be ok.
I have interfaced to other industrial stuff, like solenoids, VFD's etc... and I have used opto-couplers. In this case, I didn't think the pwm would work across that interface. I also thought I needed 5V for the driver chip (wrong) and don't have a 5V supply handy.
Anyway, other than "don't do THAT", what advice is there to avoid cooking a PC across the USB line.