Hello Chris and everybody else,

I called in today to Parallax with an oscilloscope problem. I have configured my chip to toggle TTL level outputs to be used in an induction heater application. Once the software was debugged, the application worked great. I used two LEDs as indicators to let me know the state of the output. I then used my new Parallax oscilloscope to hook up to the tech board - which promptly crashed everything, nothing worked after that. The LED's no longer lit, even when I switched chips. I was afraid that I blew out the SX key. After speaking with Chris, I discovered that rather than hooking up my oscilloscope's ground to the Vss on the tech board, I hooked the ground to the Vdd - thinking that it would amount to the same thing. It turned out that I reverse biased the LEDs and destroyed them. Once replaced, everything worked out great, no damage done - Thanks Chris ...

Patrick,
 I'm glad you got it working.  As per our conversation, and so everyone knows in tring to determine the voltage drop there are two ways this could be accomplished with our scope.  One is to measure the voltage on one side of the resistor (in relation to Vss/ground) and then the other side (again in relation to Vss).  The first signal could be saved as a reference wave form and then compared with the new reading.  You can use the cursors to measure the voltage drop by positioning them at each reading.  The other way is similar but instead you use both channels of the scope and again use the cursors.  The problem you had I believe was related to causing a ground loop since the ground lead was connected to a different spot on the circuit essentially changing the level of ground.  This can be a problem on any scope when the ground is at a level other than what it should be.  In theory you could cause a short through a virtual ground loop through the power lines.  Hard to explain but that can happen.

I usually find that it is the serial cable that is connecting the grounds together. (That is ground on the circuit to ground on the computer).

If you don't have anything connecting ground on your circuit to ground on the computer, you should be okay connecting the black lead to non-ground on the circuit.

Bean.