DIO/Counter Reading Errors

[Christopher Quijano]

(MY TEST IS CURRENTLY RUNNING AND I'M COLLECTING BAD DATA, PLEASE HELP)

I'm not sure what my problem is, so I will describe my setup and hopefully
someone can tell me if I'm doing something wrong.

I have a DAQCard AI-16XE-50 installed into a Toshiba Satellite laptop running
on battery power. The card is connected through a shielded cable to a SCB-68
connector box. I am trying to read a couple of thermocouples, strain gages,
and both counters. The analog signals read correct, and I get the same digital
results whether they are connected or not.

Counter 0 source is connected to the engine tach signal with a 0-10 volt
range. This signal is grounded to an auto battery through DGRD. Counter 1
source is connected to a magnetic pickup, with a signal which ranges from
0-10 volts. Again, this device is powered by an auto battery (DGRD connected
to neg auto battery). Both of these signals can be read from an external
tach without any problems.

The problem. Labview is reading 2 to 15 times the value depending on RPM
(Highter RPM is higher error). This occures on both channels.

In trying to debug the problem, I disconnected all wires from the SCB-68
and using a DMM, I found 4.7 volts between Chnl 1 Source and DGRD. Between
CHNL 0 Source and DGND, the voltage was 0.7. Between any analog channel
and ground the voltage was zero. If I connect up just the counters, I still
see this offset.

When I examine the low state of the magnetic pickup it reads 4.7V (the offset
voltage). When I make the signal high, I get 10 volts. This gives me a
zero count since the signal never seems to go low. The only way I am able
to get a signal is by shitching the high and low leads, which gives me a
signal from 4.7V to -10V.

The bad counts show up in Labview, and also using the NI-Daq test panel for
counters. In the test panel, the counter does count the 100KHz internal
signal correctly (I have a frequency generator on order).

If anyone has any idea of what is goeing on, please help me out. Also, thank
you for reading such a long post.

Christopher Quijano

[Jeremy]

The counter inputs on the DAQCard are TTL-logic compatible. This means that the maximum input range on these signals is 0 to 5 V. You say the tach provides 4.7 (low) and 10 (high). I would recommend looking at some signal conditioning. Either SCXI (probably larger and more elaborate than you need) or SCC modules from NI. I think that the SCC modules have some isolated digital inputs that may work. Look at the maximum frequency and switching thresholds.

If you are good with a soldering iron and feeling brave, use a zener diode (about 5 V) and a bias resistor to level shift the signal down to 0 to 5 V. I wouldn't recommend this route, however, because noise induced in the tach signal could continue giving you bad data.