weak voltage

[NN Q]

I am sending digital signals to control several valves by using PCI_6025e (PA port). When my labview program is running, everything is fine. However, when exit from program or when the PC turns on, there is a weak voltage comes from the DAQ card which will open the valves. I detected the voltage using multi-meter. it is about 1.5v. Is there anyone know what is the reason and how to solve this problem to let the output to be 0v when the program is closed?

Thanks

[Lorne H.]

NN Q,

Most likely your problem is the cause of the 50 kohm (the resistor is not a high precision resistor so it can actually be anywhere between 17kohm and 100kohm) pullup resistors attached to all of the Digital lines on port 0(A) of the 6025E.  This resistor in conjunction with the resistance of your valves is probably creating a voltage divider which causes the 1.5 Volts between the output pin and ground.  One remedy is to connect a low resistance pulldown resistor between your output pins and ground.  A resistance low enough to pull the line low when not pin is not driven.

My guess is that the circuit you have connected has an input resistance of around 20kohm (assuming the pullup resistor is 50k).  A 5kohm pulldown resistor should make your output voltage (when not driven) around 0.5V (hopefully low enough to not open your valves).

Just make sure that whatever pulldown resistor you end up using, it needs to be high enough so that it does not draw too much current from your 6025E when you are driving the digital pin high.  The board will handle a maximum current of 13 mA on Port 0 (Ports 1 through 3 can only handle 2.5 mA), so do not pick a resistor lower than 5V / 13mA = 384 ohm (or 2kohms for ports 1 through 3).  If I were you I would avoid using any pulldown resistors below 2.5kohms to play it safe.

I hope this answers your question.

Lorne Hengst
Application Engineer
National Instruments