Question: I’m using the XT1221 to measure differential voltage signals with a range of 0-5VDC. I’ve set up my PLC scaling such that a Modbus count of 0 = 0V, and a Modbus count of 30000 = 5VDC. Our technician recently installed new XT1221 units, and now the values we are seeing in our PLC are about 2/3 of the actual signal values. What could be causing this?
Answer: All XT series analog Modbus modules have built-in legacy support, which is enabled by default. With legacy support turned on (YES), bipolar analog input ranges are normalized to ±20000 (unipolar is 0-20000). When legacy support is turned off (NO), bipolar analog input ranges are normalized to ±30000 (unipolar is 0-30000). Therefore, you must disable legacy support via the USB program of your XT unit so that the Modbus counts are scaled correctly in your PLC. This explains why you are seeing 2/3 of the actual value in your PLC.