All I can suggest it recheck your probes and cables. And make sure the software is set for to correct probeZ. I'm pretty much at a loss at this point, particularly since you get the correct loop back result with or w/o probes. Which I could help more but it seems odd.
Two last things you might try.
1) Using only the Probe 1 input. First make a cal. Then with the 1k, 1k divider in place measure the voltage between the resistors (should be -6 dB) using the probe1 input. That is, obtain the impulse from probe1 using the Acquire, Transform, FFT, Cal manual sequence. The result should be -6dB.
2) The do the same thing using probe2.
With Probe2 connected at the ref point, do Acquire, Transform, FFT, Set as CAL. Then move the prove between the resistors and do Acquire, Transform FFT, cal. Should get -6 dB again.
Two last things you might try.
1) Using only the Probe 1 input. First make a cal. Then with the 1k, 1k divider in place measure the voltage between the resistors (should be -6 dB) using the probe1 input. That is, obtain the impulse from probe1 using the Acquire, Transform, FFT, Cal manual sequence. The result should be -6dB.
2) The do the same thing using probe2.
With Probe2 connected at the ref point, do Acquire, Transform, FFT, Set as CAL. Then move the prove between the resistors and do Acquire, Transform FFT, cal. Should get -6 dB again.
john k,
The fact that the level was half what it should be caused me to check that the LSB was there. Sure enough, after making up a breakout lead I find line 10 in the Centronics lead is open circuit. This means that 2 bits are corrupt in the data passed to the PC, which explains the erratic readings.
Many thanks again for your diagnostic advice, which was what pointed me in the right direction. All I have to do now is find another printer lead.
The fact that the level was half what it should be caused me to check that the LSB was there. Sure enough, after making up a breakout lead I find line 10 in the Centronics lead is open circuit. This means that 2 bits are corrupt in the data passed to the PC, which explains the erratic readings.
Many thanks again for your diagnostic advice, which was what pointed me in the right direction. All I have to do now is find another printer lead.
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