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When the radar display is stable but the PLC value jumps, capture both signals at the same time before changing radar damping. The displayed value may be filtered, the current output may use another variable, or the PLC may substitute a default during a quality fault. Each explanation requires a different repair.
Confirm That the Two Displays Represent the Same Variable
Check whether the local screen shows distance, level, percentage or volume, and identify the variable assigned to the output. Review update and filtering settings on both sides. A steady screen sampled occasionally cannot prove that the underlying measurement remained steady throughout a short disturbance.
For the METRAVON MV-RD12, first identify whether the installation uses its ordered 4-20 mA/HART circuit or serial electronics. The investigation below follows the analogue path. A serial installation also needs the register and timeout checks in the Modbus integration guide.
Radar Display Is Stable but the PLC Value Jumps: Find the First Changing Value
| Evidence | Investigate | Avoid |
|---|---|---|
| Distance moves | Process movement, echo selection and the difference between raw and displayed data. | Assuming the radar is stable because a filtered display looks calm. |
| Current moves | Output assignment, fault state and loop power while the local measurement remains valid. | Blaming PLC arithmetic before confirming the received signal. |
| Raw input moves | Input configuration, circuit reference, channel diagnostics and test-point location. | Changing the tank datum to compensate for an electrical disturbance. |
| Scaled tag moves | Program conversion, substitutions, limits and the HMI's actual source tag. | Replacing a transmitter when the underlying input data is unchanged. |
These observations locate the next test; they are not conclusive diagnoses on their own. Record where current was measured and whether a barrier or isolator lies between that point and the PLC. A correct signal before an isolator does not prove the signal after it.
Divide the Circuit With a Controlled Test
Under the site's test procedure, a suitable current source can check the receiving channel independently of the field transmitter. Confirm the active or passive input arrangement before connecting it. Test several values and record the raw input, engineering conversion and displayed result.
If the receiver is stable with that source, extend the investigation to the field wiring and intervening devices. Where the radar supports output simulation, verify the complete circuit from the transmitter end. Simulation tests the output path; it does not establish that the radar is following the correct process surface.
Preserve the original settings and remove temporary forcing, simulation and overrides before release. A loop can appear repaired because a forced PLC value was never removed. Include an explicit return-to-measurement check in the record.
Reproduce the Operating Event
If the jump occurs during a motor start, save the instrument status, loop current, supply condition and PLC diagnostics during that event. Inspect the designed cable separation and grounding rather than adding unplanned shield connections. If the problem occurs only at high output, review the terminal-voltage margin using the 4-20 mA loop checks.
Repeat the relevant event at more than one signal level after the correction. Agree on the observation period and events that constitute a representative test. A quiet-cabinet check made with surrounding equipment stopped does not establish performance during production.
Troubleshooting Questions
Q1. Should the analogue input card be replaced first?
Not without evidence. Inspect its configuration and diagnostics and test it with a known input. A controlled comparison can distinguish a channel problem from wiring or program behaviour while avoiding unnecessary replacement of a working device.
Q2. Why might the HMI jump when the PLC's engineering tag is steady?
Check the HMI source address, communications quality and any display-side substitution or calculation. Also compare timestamps. The HMI may be reading another tag or applying its own fallback rather than showing the value being examined in the PLC.
Q3. What evidence makes remote support useful?
Provide one synchronised event record with local variable, measured current, raw input, scaled tag and equipment state. Add the loop drawing and exact input configuration. Avoid sending only a screenshot of the final displayed number, which leaves the fault location unresolved.
Summary
Find where the value first changes, test that part of the chain and repeat the event that originally exposed the problem. Send the signal trace and loop details for a diagnostic review. Keep radar geometry adjustments separate from repairs to electrical or software faults.
