Numerical plc IO module reads always changing, what is the reason

Roughly divided into the digital input DI, digital output DO, analog input AI, analog output AO. Digital inputs and outputs are generally of 24V. Class 2 4 kinds of analog quantity, category 2 are voltage-and current-mode. There are 0-10V Voltage and -10 to 10V. Current  plc communication module type mainly 4-20MA ,0-20MA. plc IO module and the module through the A / D converter to convert analog into digital +32767 to -32768 between then under control. If there is an alarm relay on the meter, then set the alarm temperature on the instrument, the output relay is directly connected to the digital inputs plc and public clients. If there is no alarm relay on the meter (meter is no such general alarm relay), direct access to analog values ​​plc to make their  plc analog input module own alarm in the program. And direct access to the analog case plc, and because plc and instrumentation wiring different models differ, analog signal inputs can be 4-20mA, it can be 0 - (5) 10V, can also be thermal resistance and the like, the situation is more, the need for specific models can be. Potentiometer is actually sliding rheostat, string a fixed value resistor, calculate resistance. If you do the experiment, find a 24v power supply, and then take analog output module  the fixed value resistor voltage or voltage across the potentiometer input into the AI, they have to be considered a good range. I remember before the first experimental range card transferred to the correct location. The best add isolation (barrier like), to protect AI point.