Common Faults in Analog Signal Output

Common faults in analog signal output primarily include issues such as signal anomalies, output instability, module errors, and external interference. Among these, the most typical issues are a complete lack of output (for 4-20mA or 0-10V signals), signal drift, short-circuit damage, and poor immunity to interference.

I. Analysis of Common Fault Types and Causes

1. No Output Signal (0mA or 0V)

Possible Causes:

The output module is not powered, or the power supply wiring is loose.

Incorrect value assignment in the program, or an address access error (e.g., using PQW instead of QW).

Damaged DAC chip or internal circuit failure.

Open-circuit load or loose wiring terminals resulting in a broken loop.

Troubleshooting Suggestions: First, use a multimeter to measure whether voltage/current is present at the output terminals; then, check if the PLC program's output address is being written to correctly.

2. Abnormal Output Signal (Too High, Too Low, or Negative)

Typical Manifestations:

Actual output is -20mA instead of +20mA (commonly seen with incorrect range settings or improper DIP switch configuration).

Output value deviates from the setpoint (e.g., an expected output of 12mA is measured as 8mA).

Possible Causes:

Incorrect module hardware configuration (e.g., wrong selection of current or voltage mode).

Mismatched signal range parameters (e.g., incorrect settings for P0776/P0777).

External load impedance exceeds the allowable range, leading to signal attenuation.

3. Unstable Signal Output (Fluctuation, Spikes, Drift)

Typical Symptoms: Values fluctuate frequently, resembling a "Parkinson's" phenomenon.

Main Triggers:

Strong electromagnetic interference (e.g., signal wiring routed too close to variable frequency drives or high-power motors).

Poor grounding of the shielding layer, or failure to implement single-point grounding.

Power supply fluctuations or poor power quality.

Poor heat dissipation within the module leading to temperature drift.

Case Study: At a factory, a PLC failed to output a 4-20mA signal to control a variable frequency drive. It was ultimately discovered that interference signals were feeding back into the system through the signal lines; normal operation was restored after installing a TA3012 signal isolation module.

4. Module Short Circuit Leading to Burnout or SF Red Light Alarm

Fault Symptoms:

After a short circuit occurs at the output port, the module overheats, generates an error, or even burns out.

The SF (System Fault) light on analog output (AO) modules-such as those in the S7-1200 series PLCs-flashes, diagnosing a short-circuit fault.

Three Major Risks:

Module Burnout: Short-circuit currents surge, causing the module's temperature to skyrocket; repair costs can exceed 30% of the total PLC price.

System-wide Interference: Transient currents trigger system malfunctions-such as valves opening erroneously or pressure gauges displaying garbled data.

Equipment Loss of Control: If controlling motor speed, a fault could cause the motor to overspeed, potentially leading to safety accidents.

Preventive Measures: Avoid short-circuiting the output terminals; use modules equipped with short-circuit protection features; route signal wiring away from power cables.

5. Signal Attenuation or Insufficient Load Capacity

Manifestation: A 20mA signal is output, but the downstream device receives only 16mA.

Cause Analysis:

The input impedance of the downstream device is excessively high, exceeding the isolator's load capacity (e.g., >350Ω).

The power supply lacks sufficient capacity to drive the connected load.

Signal cables are excessively long and unshielded twisted-pair cables were not used, resulting in voltage drop.

II. Key Protection and Optimization Measures

Expert Tip: In systems where a PLC shares a common power supply with a Variable Frequency Drive (VFD), it is mandatory to install isolation modules on the analog channels; otherwise, interference is highly likely to cause control failures.