Serial port changes?

Hi, I’m developing with ScadaBR. I have a datasource with alot (ALOT) of datapoints. More than 1000. Well, the thing is that at some point the serial port of the datasource is changing… alone! Then, all the datapoints stops to reach values. Why is this happening? Is really strange.

I mean is changing from this value:
/dev/ttyS0

To this:
/dev/ttyS1

Without human intervention.