USB-6009 Digital inputs always High even when not connected.

[I_Need_Serious_Help]

I use a USB 6009 to aquire some trigger pulses. these pulses are at base 25 V which i have stepped down to 5 V. i have wired up the ground correctly and use ports 0.0, 0.1, 0.2 to aquire the 5 V signal. i have only recieved high signals while expecting a low signal. even during the pulses the signal remained unchanges. i disconnected all wires and measured the voltage between ground and the ports and got 5 V. even with nothing connected i always get a high signal. this has not always been the case, a few days ago it worked just fine. can any one of you point me in the right direction?

[I_Need_Serious_Help]

ok, aparently when i short those pins to ground the signals return to low. what i want is to get a low continuously and once i apply 5v i get a high.

[crossrulz · Accepted solution]

Per the specification, the USB-6009 digital lines have a 4.7kOhm pull up resistor to 5V. My guess is that you are not driving the signal low hard enough at the input to overcome that pull up.

Here are a few of options:

1. Find an opto-coupler that can handle the 25V on the input size and have that drive a signal to ground. You may need to also insert a NOT gate to get the polarity you want.

2. Use a comparator that can handle 25V input that can drive into your DIO. You will need to do some circuitry to set your threshold level.

3. Use an op-amp to drive your DIO.