An FMR analog input does not have one universal display value for 10 V in SMLogix. The evidence identifies two representations: an unscaled value expressed in millivolts, where 10 V corresponds to 10000 mV, and a scaled value expressed in volts, where it corresponds to 10. Confirm the analog-input configuration and variable selection before interpreting the displayed number.
Determine Whether the Value Is Raw or Scaled
| Observed representation | Value at 10 V | Interpretation |
|---|---|---|
| Millivolts | 10000 | Unscaled electrical value |
| Volts | 10 | Scaling or conversion is enabled |
If data is obtained through Modbus, verify that the selected variable is the raw value before assuming that 10000 is expected. A converted variable can report volts directly.
Diagnose the Loaded Feedback Signal
The actuator feedback measured 10 V with the AIN disconnected but fell to 6.2 V when connected. At the low end, 2 V fell only to 1.88 V. These are loaded drops of 3.8 V and 0.12 V respectively. Because the error changes sharply with signal level, treat this as an electrical loading or device fault rather than correcting it with a linear software scale.
- Confirm that the AIN is configured for the voltage signal. The evidence specifically identifies an incorrect RT1000 input selection as a configuration condition to exclude.
- Measure the actuator feedback with the AIN disconnected and connected, recording both readings at the same commanded position.
- Check the FMR documentation for the specified AIN input resistance and compare the resulting load with the actuator output capability.
- As used in the reported diagnosis, connect 15-20 kohm in series and measure the resulting input voltage. A result in the approximate 1-5 V range was proposed as evidence that the output cannot supply the load.
Separate Scaling Errors from Electrical Faults
Scaling changes only how SMLogix represents the acquired signal; it cannot explain the physical terminal voltage falling from 10 V to 6.2 V when the input is connected. First correct or confirm the input mode, then isolate the source and load by voltage measurements. In the reported case, the series-resistance test led to the conclusion that the actuator feedback output was defective.
Verification
After correcting the electrical fault, repeat unloaded and loaded measurements across the operating range. The loaded signal should no longer exhibit the reported nonlinear collapse. Then verify that the software value matches the configured representation: 10000 for 10 V when expressed in millivolts, or 10 when scaling reports volts.
FAQ
What value should SMLogix show for 10 V on an FMR AIN?
Expect 10000 when the value is expressed in millivolts or 10 when conversion reports volts. Check the input scaling and whether the selected Modbus variable is raw.
Why does a 10 V actuator signal fall to 6.2 V on the analog input?
The measured 3.8 V loaded drop indicates an input-mode, loading, or actuator-output fault. Confirm voltage-input configuration, exclude an RT1000 selection, and compare AIN resistance with the output capability.
Can software scaling fix nonlinear analog voltage drop?
No. Scaling cannot correct a terminal voltage that changes under load; diagnose the electrical interface first. The reported 15-20 kohm series test identified a defective actuator output.
