TTL signal count problem: mistaken counting

[KXZ]

Hello,

I use a 0.2Hz TTL signal to synchronize two devices. I use a Usb 6210 card to count the TTL pulse. The counter is triggered when it detects rising edges. In my case, the counter was sometimes triggered at low level, which causes a false result (see the attached images, the TTL signal is sampled at 20Hz and the dots represent an increment of counter). How can I resolve this problem?

In addition, the cable which connects the TTL output was welded by myself, could it be a problem of poor contact?

Thanks a lot,

KX

[mikeporter]

Have you looked at the signal with either an analog input channel or a scope? Noise can cause false triggering.

Mike...

[KXZ]

Thanks Mike.

Yes, i've looked its analog waveform but it seems perfect... you can see the images attached to the first post.

[Bob_Schor · Accepted solution]

What is the nature of the device producing the TTL trigger signal?  Is it possible, for example, that it could produce a 1-microsecond pulse (that might not be visible on your plot of the signal) that would trigger the counter?  Are you really producing a digital pulse (i.e. is your circuit something that is either "on" or "off", as opposed to "produces an analog voltage in the range 0 to 5 volts")?  Are there other devices around that could be producing pulses that are being "picked up" by your counter?  Is the cable connecting the TTL pulses to the counter shielded?  Is the shield grounded at only one end?

These questions (and the previous one) suggest that the problem may be "electronics" rather than "LabVIEW" ...

[KXZ]

Thanks for your answer Bob.

And after shielded the cables, it seems that the problem is resolved!