FMD88 D/A#3 and D/A#4

[ccdubs]

Hi, I have had to swap an old T100MD888 with a new FMD88 but had an issue with the analogue input values I was reading. From what I can see from the connection wiring the only difference is that our application connects the future D/A's #3 and #4(pins 10 and 11)  to 0V as that is what these pins were on the T100MD888.

Would this be the cause of the spurious analogue input readings I now get? If not, are there other differences (software and hardware) that I should be made aware of when swapping a MD888 with a FMD88?

Thanks.

[garysdickinson]

Please be aware that the ADC on the FMD/Fx/Nano PLCs is a 12-bit ADC.  The older T100MD had a 10-bit ADC.

If your ADC readings with the FMD are about 4x too high, the then differences in the ADC circuitry are the root cause. You will need to modify the CF that read the ADCs to accommodate the extra 2 bits of resolution.  You could just divide the new readings by 4 and then just use the old code from that point forward.

As I am sure you have noticed, the DB15 connector is wired a bit differently between the generations of PLCs.  If your analog circuitry is dependent on ground reference or 5V ADC reference voltages you have a good chance of being wrong with your old cable.  There is a only one pin for each signal on the FMD vs. 3 pins for each on the T100MD88.

TRI has a PDF on differences between the old T100MD and the FMD boards here:

http://www.triplc.com/documents/FMD1616-10_Upgrade_Notes.pdf

The other differences have to do with the amount of DM memory and non volatile memory.  There is less of each on the newer FMD boards. If this poses a problem, the add-on RTC-FRAM module is what I use to get back these resources.

Best regards,

Gary D*ckinson

[ccdubs]

Thanks Gary. Now I'm really confused as I have used these PLC's a lot and haven't changed any of my software between changing from T100MD to FMD88 and the analogues have always read fine in my SCADA with a max scale of 4095?

Regarding the wiring, the only difference with the way I have the connections configured is that the pins allocated "future #D/A3 and #D/A4" are connected to 0V all other pins (apart from A/D#1-4) are not connected.

[ccdubs]

I have my head around this now. 4095 is 12bit resolution but I now remember that the T100MD had 10 bit but the plc firmware scaled it to a 12 bit equivalent. So this is why I don't have a problem between PLC versions.

Still don't know about the wiring though.

[support]

If you wire the DA3 and DA4 (for FMD PLCs) to GND then as long as you don't turn on the DA3 and DA4 outputs they should not have effect on your ADC readings. But if for some reason the program runs the SETDAC on channel 3 or 4 you can cause a short circuit to the DAC output and that can affect the 5V reference voltage so we recommend that you do not connect them to 0V.

[garysdickinson]

Excellent on 10/12 bit behavior on older PLCS.  

I used an external 16-bit  ADC on my T100MD projects after testing the PLC's DAC cabibilities and determining that 10-bits was not quite enough. I'd forgotten that fact.

The analog ground and 5V reference wiring has a high probablity of giving you odd ADC values.

Good hunting,

Gary D