Testing the accuracy of a PLC current input module - economical approach?
Obviously, the usual way is to simply buy a calibrator from a reputable company and have it regularly calibrated to a traceable reference (often by a 3rd party calibration firm).
A short-term stable voltage source in conjunction with a precision ammeter and a short-term stable resistor OR a precision resistor and a precision voltmeter can be used as well.
The accuracy of the voltmeter and resistor or the ammeter will determine your total accuracy so long as the voltage source is short-term stable and reasonably low noise.
Any method that makes assumptions about the precise value of the load resistor internal to the input module would be suspect.
Before you can talk about how exactly to do this, you need to know what the rated specifications are that you are testing to, and the recommended conditions (eg. ambient temperature, warm-up time, supply voltage, and so on). Compare that to what trusted measuring instruments you already have available.