I can not read Analog input (a square wave)

[jhan]

I am programming with Labview 7.1 on FPGA. The hardware is PCI7831. With CB-68LP connector, I connect analog input (a square wave generator) to AI0+(68) and AIGND0(67). I try to read it with waveform chart. But I can't get a square wave on the screen. I did check the square wave generator's output. It is correct. The connector CB-68LP connected with PCI7831 on analog port.

I am runing the program on emulation, not on real FPGA.

Thanks

[mikeporter]

If you're not seeing a squarewave, what are you seeing?

What is the period of the squarewave?

What is your analog sample rate? (There's no fooling Mr Nyquist.)

Mike...

[jaja]

Thanks for your reply.

I got a straight line with some noise-like tone on the screen. When I changed amplitude, I saw the voltage level changed. I did try different frequency from 1 to 100K. There are some changes, but they are not square wave.

Is it because I am choosing emulator as target? With emulator I can not read and write to I/O port?

jason

[mikeporter]

Could it be that you are sampling so fast relative to the squarewave repetion frequency that all you're seeing is the "high" level? Also can you see the squarewave running the test panel in MAX?

Mike...

[jhan]

I got the square wave on the screen after I connected signal generator output to AI0- and signal generator ground to AI0+. I really do not understand. I assume that generator output should connect to AI0+ and generator ground should connect to AIGND0.

But I get the square wave. Does it make sense? Maybe the label on the terminaI is not correct?

Good day!.

jason

[mikeporter]

Doesn't make any sense that I can see right off hand. But if it's working... What do the connections look like at the signal-generator end of the wire, any chance that could be backwards?

Mike...

[jhan]

Signal generator I used is 3010 function generator for BK precision. It is an old instrument. I probed output signal from this generator on oscilloscope with signal generator output conected to oscilloscope probe (+) and signal generator ground to probe (groud). It is a square wave.

I suspect the teminal I used for I/O port to PCI7831. Because my coworkers installed the hardware for me, he is absent recently. The terminal is CB-68LP  from NI with numbers ( 1 to 68) to specify each connector on it. And I got a SCB-68 Quick Reference Label sheet on my desk to specify numbers correspanding I/O port (for example, 68 -- AI0+). I suspect if this SCB-68 Quick Reference Label sheet matches the terminal CB-68LP?

I will talk to my coworkers next week.

jason

[AnalogKid2DigitalMan]

Is the signal generator applying a negative DC offset to the squarewave? Is anolog input set up for unipolar operation? The combination of these two may explain why you only see the squarewave with the input connection polarity backward.