FMD88 Analogue Inputs

[ccdubs]

I have started using the FMD88 PLC for the first time and have experienced some anomalies with the analogue inputs. After some fault finding, it looks like some of the channels may be using a different averaging constant. I indentified this as I need to sample AI channels only once after an event where the AI value from the sensor is latched, read by the PLC and then cleared back to 0. When the PLC reads the value I noticed that the values of ADC 4 and 6 were half of ADC3 and 5 respectively when they should be the same and the voltage in to the PLC was correct.

I think this can be explained by the averaging constant being set. This PLC is fresh out of the box and after looking at the AI calibration it looks like values the ADC Calib and the avg constant have been set. What should these settings be when shipped from the factory?

It doesn't look like the averaging constant can be set per channel so I can't understand why some readings are correct and others aren't.

[support]

The moving average is normally set to 3 when shipped from factory.

The calibration constant is what the tester uses to try to adjust the readings to an applied reference voltage to each channel.  If are normally within +/-200. If it runs into thousands then it may be wrong.

You could also set all calibration constant to zero first and check if the readings roughly match the reference voltage. If so then you could do a more accurate calibration using the calibration technique explained in the User's Manual.  

[ccdubs]

I like the idea of leaving the avg const at 3 but it doesn't work when I need to do my one off reading from the sample and hold system I have set up.

Can I trick the PLC by reading the ADC channel 3 times to essentially give me a one off reading? Do I have to use the REFRESH command after each read?

E.g.,
REFRESH
DM[103] = ADC(3)
REFRESH
DM[103] = ADC(3)
REFRESH
DM[103] = ADC(3)

[support]

You can change the moving average to 1 which will not do any averaging.

You certainly also can read the same ADC channels 3 or more times to remove the initial zero value when the PLC is first started up.

[ssiewert]

So the documentation mentions the ADC's are 10 bit x 4 to get 12 bit values.  Funny thing is, when i watch 12 bit ADC channels, the numbers don't jump around by increments of 4.   Is this due to averaging of the 12 bit values or what exactly is going on in the background?

[support]

The older generation of T100MD+ and T100MX+ have 10-bit analog inputs normalized to 12-bit.

All the newer Super PLCs (Nano-10, FMD88-10, FMD1616-10, F1616-BA and F2424) have built-in 12-bit ADC so the increment is by 1 and not by 4.