Erratic analog reading when onlne

[knightnurse]

We have a T100MD-1616+ with the LCD display, and are  the analog input points.  
The problem occurs when we try to go online, with some of our laptops. Once the cable from the controller plugs into the computer, the analog I/O becomes very erratic, jumping all over the map
We can still download the program, and the analog I/O is quite stable once the computer is disconnected.
We were hoping to have the computer permanently connected to the system for monitoring purposes using ExcelLink.
We have tried both serial and USB connectiions.
Any brillant solutions out there.
Thanks.

[support]

It sounds like the PC is imposing sufficient noise on the PLC to affect the ADC. This is very unusual. Is this a laptop or desktop computer? If it is laptop, does it make a different if only battery power is used? Not that we suggest you should run the PC without AC power, it is just curious to us to know how that can happen. If you try a different PC does it make any difference?

If you have the Auto485, try connecting the PC to the PLC via RS485 and see if it makes a difference. In this case the PC does not connect in common to the PLC's power supply and hence is less likely to impart any noise on the PLC's ground wire.

What kind of power supply is the PLC connected to? Do not connect the PLC's 0V to the AC ground as that could have adverse impact on the ADC.

[knightnurse]

We tried 4 different laptops,  and a desktop.  All presented the same problem and only one laptop, worked without a problem when on battery power.

The 0V of the power supply is floating, and the supply is an Idec PS5R switching power supply.

[support]

What is the reading range (for raw ADC(n) data)  that the ADC fluctuates when the PC is connected to it?  The ADC is permitted to fluctuate about +/- 3 LSB.

Did you have a chance to try a different power supply?  You can try with those wall-wart type power supply that gives up at least 12V to connect to the PLC to try to see if it has anything to do with your power supply.  

What about the source of the ADC input. You may want to try to connect some of the ADC input to a potentiometer that draws power from the on board 5V and 0V and see if the reading fluctuates in similar manner.