connecting sensor to USB-6501 DIO

[Paul McK.]

I am hoping someone can provide an easy answer to this problem, I am fairly new to DAQ and have not seen this issue before. I have a USB 6501 DIO unit which is installed properly and which passes all self tests. I am trying to connect it to read an optical sensor. The sensor is driven by a 12V DC power supply and provides an output of either 0 or 8V referenced to the ground output of the power supply. It works fine and I can test it with an oscilloscope or a volt meter.

I connected the output of the sensor to an input line on my DIO device. After doing that I checked with a volt meter and there are zero volts between that line and the power supply ground. I then toggled the sensor high and there was 8 volts between that line and the power supply ground. However, there were also 8 volts between every other line in the channel and the power supply ground when the sensor was high (and zero when low). Further there was 5V between any line in the channel and the ground on the DIO device whenever the sensor was plugged in, regardless of the state of the sensor output.

So then, the mere fact of plugging in the output from my sensor, whether that output was high or low, set every line in the channel to a +5V condition with respect to the local ground. This behaviour is duplicated in all three channels on the DIO box.

I am hoping I am just doing something simple and dumb but I do not know what it is. I have not run into this issue with other DAW devices in the past.

Thanks in advance for any help.

Paul

[hoffiz]

Hi Paul McK.

Let’s first start with the specification of the card if you take a look at the card NI USB-6501 User Guide and Specifications an input voltage between 2.0 Volts and 5.8 Volts will be consider as an high, this is a TTL device so you should no be trying to read an 8 Volts signal, don't worry about damaging the card since the card has an over voltage protection of 30 Volts for one channel and 8.9 Volts per port and both are reference to ground. But if you configure a DIO line as an input, do not drive the line with voltages outside of its normal operating range. You will need to use some circuitry to attenuate your 8 Volts to 5 Volts. One solution I have for you can be found in this article: Basic Analog Circuits also always remember to skim the “User Guide” for maximum and minimum currents and voltages allowed.

I hope it helps