Measurement Computing DataShuttle - Manual (Page 29)

DataShuttle and DynaRes 11-13-01 DataShuttle - Technical Notes 2-11
Troubleshooting: Operation
Hints
It’s essential to have exactly one ground reference per channel. This single connection to ground makes sure
you don’t exceed the common mode range of input.
(More than one connection per channel can lead to ground loops, causing errors or erratic readings.
Connecting the – to the Com terminal provides a single ground. Your sensor might also provide another
ground. If you’re not sure that your sensor is grounded, try connecting the – to Com, and not, and see which
works best.)
Remember that if speed is not critical, selecting the “low noise mode” in software always provides the best
accuracy and resolution with your data acquisition unit.
Operating Qs and As
1. Problem: My unit reads analog inputs wrong.
Action: Make sure the calibration numbers in the non-volatile memory are non-zero
(run EDITCAL to check this). If the figures are correct, try shorting + to – to Com: the unit should
return a reading around 0V.
2. Problem: My unit reads the thermocouple as a very negative temperature.
Action: Verify the connections are secure. Opens read as negative temperature. Make sure use
lowest channel numbers first and scan only connected channels.
3. Problem: My board reads thermocouple input wrong.
Action: Confirm that the CCAL number in the EEPROM matches the number on the terminal panel’s
calibration label.
(If you need to use a general purpose terminal panel – one that doesn’t have a factory CCAL
number – with a thermocouple, you can achieve approximate results by entering a CCAL of 22000.
Then change the figure by about 75 points for each degree C. that you want to raise the temperature.)
4. Problem: My readings are noisy.
Action: Try using the low noise mode (see your software manual for more information).
5. Problem: My digital inputs are “high” with nothing connected to them.
Action: This is normal. The digital inputs are pulled up to about 1.5 volts by leakage from the
LS7407 chips. This is enough to read logic high. You can pull them low with a 470 ohm resistor to
ground.
6. Problem: Can I get any output signal from the digital input terminals?
Action: Yes, you can use an input terminal to drive light loads such as a TTL input.
7. Problem: What should I do with the jumpers connecting the analog input to COM?
Action: In general, it is best to leave them connected as shipped.
8. Problem: My 5V terminal reads only 4.6V.
Action: This is normal. Tolerance is 4.5 to 5V.
9. Problem: There is significant crosstalk
Action: High impedence, greater than 1K, inputs, such as accelerometers require signal conditioning.
10. Problem: My analog output accuracy is poor.
Action: From the DOS command line, or in GO.BAT, run GFIND -C. This calibrates analog outputs.
Note, however, that calibration sends full scale outputs, so it may be necessary to remove any
instruments connected to the outputs.
11. Problem: Errors in Thermocouples on multiple units.
Action: If thermocouples are connected to metal, object, remove
− to common strap on terminals.