Blew up my logic analyzer and device - ground issues?

Other Miscellaneous
1

Trying to figure out the protocol the device is using. We guess SPI or I2C, but maybe serial.

Setup:

USB Logic Analyzer - 25MHz/8-Channel

Device that is deemed to use 3.3v using voltmeter between GND to PWR and 2 datalines

We plugged in the logic analyzer to the computer and it showed up fine.

We then plugged in the analyzer to the device we are testing. GND to GND, and D0/D1 to the two signal wires that were previously measured at 3.2v (PWR was 3.3v).

We then power up the device. It is powered by 110v but again, we measured the four data cables to be 3.3v against its ground.

POP! The device and the logic analyzer are fried.

Was the ground too different between the analyzer and the device? Suspecting that the ground from the computer+USB was not close enough to the ground of the device so the initial surge fried some circuits.

Any ideas? What should we do in the future? The device was $70 and the analyzer was $20. WOuld a more expensive analyzer prevent this?

Cheers,

Mattias

2

Is this one of the logic clones? I read that some of them do have protection.

I have one I use when I am afraid to destroy the real ($500) one.

I think the one I have has some series resistors and some clamping diodes.

I have put it through a lot and surprisingly it still works!

Did you try the logic analyzer on another device? Does the computer recognize the logic analyzer now?

It might not be blown, it might just need the eeprom reprogrammed.

There is a lot of info if you search on how to reprogram it.

As for your device, when you say “fried”, does that mean like smoke or popping sound when

you plugged it in with the logic analyzer?

I am guessing it no longer works.

In case anyone is interested here is a protection circuit for an 8 CH logic analyzer:

https://oshpark.com/shared_projects/e0yYgGOM

3

What’s the device? It sounds like the device did not have an isolated power supply. If that’s the case, be careful since it could be lethal.

I would carefully measure the voltage between the device’s ground and the computer’s ground, using an AC voltmeter. If it is not less than a volt or so, DO NOT connect them. You can have 3.3v between Vcc and ground on the device, while having 120v AC between the device’s ground and the PC’s ground on a non-isolated device.

In the past, I have used a USB isolator when trying to connect a non-isolated line powered device to a computer. An isolation transformer could also help make it safer.

/mike

4

Thanks for the replies and ideas!

We did try the analyzer on another computer but it doesn’t light up its LED and shows up as an invalid USB device. I’ll look for the reprogramming articles, thank you.

We heard a loud pop when plugging in the device (it’s an RF controllable AC switch) and it doesn’t work at all now. There are also black marks on the power strip where we plugged it in.

The isolated power supply sounds like the best bet. We have another one and we will test the two grounds for both AC and DC voltage.

The USB isolator sounds like a perfect addition here. We wish SparkFun was carrying one :slight_smile: Would a cheap one from eBay be ok you think or do we need a $100+ bigger box?

Cheers,

Mattias