Why does my PFI line(input trigger)kill the signal being acquired?

[bstarks]

I have an application set up in which a 556 timer is being used to generate a pulsing signal in order to flash some LEDs. This same signal is used as an input trigger(treated as a Digital A trigger) to my LabView program. The problem is that whenever I plug the PFI0/TRIG1 line into my circuit board(at the same place as the output to the LEDs) to acquire the signal, the pulse waveform dies.(goes to all zeroes) Do you know why this might be happening? Is there somewhere that I need to configure the PFI0 line? Am I maybe missing a ground for the PFI0 line?

Thanks,
Bryan

[rgharrison]

Hi Bryan,

I think the PFI0 line is not getting enough current. PFI0 is a special functional line used for analog and digital triggering and has a 10k ohm resistor between the line and digital ground. The other PFI lines do not have this 10k resistor in the path. What this means is that if your 5V source (556 timer) is only supplying .4mA of current (TTL Spec) then the LEDs would draw much of that current and if it draws more than half of the current then the line will be pulled low by that 10k resistor.

This means you need to either use a different PFI line or put at least a 10k resistor in the path of the LED so that the current division gives enough current such that the line is above 2V (TTL Spec). Hope that helps.

Ron