PLC analogue output not driving correctly
A PLC analogue output (to a valve positioner, VSD reference, etc.) isn't producing the right signal — the device doesn't follow the command, or the output sits at zero/full scale.
Safety first
A wrong analogue output can drive a valve or drive to an unexpected position/speed. Confirm safe operating range before testing. Isolate before wiring work.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Scaling/range mismatch between PLC and device
Most likelyThe output scaling doesn't match the receiving device's range, so the device misinterprets it.
- 2
Wiring fault on the output loop
#2An open or short on the analogue loop, or wrong polarity, gives a wrong/zero signal.
- 3
Program not updating the output value
#3The logic isn't writing the expected value (held, disabled, or wrong tag).
- 4
Load/impedance mismatch
#4Too many devices on one output, or an impedance outside the module's capability, distorts the signal.
- 5
Failed analogue output channel
Least likelyThe output channel itself has failed.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Measure the actual output signal (mA/V) and compare to the value the program is commanding.
Measured output matches the commanded value.
Output is correct — the issue is scaling at the device or the device itself.
Output doesn't match command — check program value, wiring, then the channel.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Analogue output wrong
→ step 2 - 2decision
Does the measured output match the commanded value?
Yes→ step 3No→ step 4 - 3result
Output correct — check device scaling/range or the device itself.
- 4decision
Is the program writing the right value and the loop wiring sound?
Yes→ step 5No→ step 6 - 5decision
Are scaling and loop load within spec?
Yes→ step 7No→ step 8 - 6result
Wrong value or wiring fault — correct it.
- 7result
Suspect a failed channel — replace per procedure.
- 8result
Scaling mismatch or load issue — correct it.
Common mistakes apprentices make
- Mismatched scaling between the PLC output and the receiving device.
- Not measuring the actual signal vs the commanded value.
- Overloading one output with multiple devices.
- Forgetting the program may be holding/disabling the output.
When to stop & escalate
Scaling/configuration and channel replacement follow site controller procedures with backups. Loop-impedance/design issues should be reviewed for the installation.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
Analogue sensor (4-20mA / 0-10V) reading wrong
An analogue sensor (pressure, level, temperature) gives a wrong or fixed reading — stuck at zero, pinned at full scale, or simply not matching reality.
PLC output LED is on but the device doesn't work
The PLC output indicator says the output is energised, but the connected device (valve, contactor, lamp, motor starter) does nothing. The program thinks everything is fine.
VSD powered up but won't start the motor
The drive is energised and the display is alive, but it won't run the motor. No fault may be shown — it just sits in 'ready' or 'stopped' and ignores the start command.