Choosing FX3U Analog I/O: Separate 4–20 mA Inputs from 0–10 V Outputs
Choosing an analog module by channel count alone is not enough. Start with the sensor signal, actuator reference and the accuracy and update speed the control loop actually needs.
Write AI and AO requirements separately
A load-cell transmitter may send a 4–20 mA input to the PLC while a VFD expects a 0–10 V output. Calling this simply “two analog points” hides the important electrical difference.
Resolution alone does not define accuracy
A large raw-count range does not guarantee a stable control result. Conversion time, transmitter accuracy, wiring noise, filtering and scaling all matter. Short-window kg/h calculations can magnify one-count movement into a much larger apparent flow change.
A useful selection order
- Separate required AI and AO channel counts.
- Confirm voltage/current range for each channel.
- Calculate needed resolution and control-loop update rate.
- Check isolation, filtering and wiring requirements.
- Confirm expansion method, power and PLC memory/address mapping.
Purchase last
Do not lock the hardware simply because a familiar module looks close enough. Match the signal table and error budget first, then choose the part. The MACHCTRL dosing example keeps the final analog hardware open until the accuracy and response requirements are verified.