Analog input reads full scale: where do you start?

Work from the field back, and check the cheap things first. Full scale on a 4 to 20 mA input almost always means the loop is open or the input is seeing more current than it should. In order: 1. Measure the current at the transmitter with a meter in series, or use its own test facility. If the transmitter is producing 4 mA and the card reads full scale, the fault is between them. 2. Check the loop is not open. An open 4 to 20 mA input on many cards reads high rather than low, which is the opposite of intuition. 3. Check the wiring type against the card configuration. A two wire transmitter on a channel configured for four wire will not behave, and the same is true in reverse. 4. Check the screen is earthed at one end only. Earthed at both ends it becomes a loop and picks up whatever is nearby. 5. Only then suspect scaling. If the raw count is at its maximum, scaling is not the problem. The single most useful habit is measuring the actual milliamps early. It splits the problem in half in about a minute and tells you which side of the terminal to look at.