Troubleshooting Siemens S7-200 EM232 Analog Output to VFD

The SIMATIC S7-200 PLC paired with a SINAMICS V20 (or similar inverter) over an EM 232 analog output module is one of the most common low-cost variable-speed control architectures in small machine OEM designs. A single field failure—an output stuck at 10 V, the drive refusing to follow the speed command, or LEDs extinguishing on the analog card—can halt a conveyor or pump line in minutes. This reference covers the diagnostic procedure to isolate the failure between the EM 232 module, the field wiring, and the VFD analog input, then restore correct operation after a module swap.

1. Problem Description and Symptoms

When the EM 232 output to a VFD fails in service, technicians typically observe one of the following symptoms on a system that previously ran correctly for 2+ years:

• Output stuck at maximum: The VFD reports maximum frequency (50/60 Hz) regardless of the PLC's speed command.
• Output stuck at zero: Motor remains at 0 Hz; VFD does not run.
• Output pinned at +10 V: Wiring checks confirm +10 VDC across the output terminals even when the PLC program forces the analog word to 0.
• LED indication lost: Only the green 24 VDC power LED is illuminated on the EM 232; previously-lit per-channel status LEDs are off.
• HMI / programming port failure: STEP 7-Micro/WIN cannot connect via PPI/USB, or the analog output word (AQW) is non-responsive in the status chart.

2. Hardware Identification

Before troubleshooting, confirm the exact catalog numbers on the system nameplate. S7-200 EM 232 variants differ in output range, channel count, and DIP switch layout.

The 0–10 V range on the EM 232 is unipolar voltage (0 to +10 VDC). The current loop is 0 to 20 mA. The module does not natively produce 4–20 mA; if 4–20 mA is required, scale the PLC output word so that 0 mA = 4 mA and 20 mA = 20 mA by clipping the lower 4 mA in the program, or use an external 500 Ω resistor to convert 0–20 mA to 0–10 V and re-scale.

For the VFD side, this article references the SINAMICS V20 application note (Siemens ID 65688720). Although that document focuses on USS digital control, the analog input terminals described below are identical for 0–10 V speed command mode.

3. Root Cause: Output Stage Failure of EM 232

The most common failure mode on the EM 232 analog output stage is:

1. Open output op-amp: Internal op-amp U1/U2 fails open, forcing the output to the upper rail (+10 V) regardless of DAC value. Result: VFD receives full speed command.
2. Short between DAC and output: Output stage is shorted to +10 V reference. Same symptom as above but with measurable voltage on the field terminals.
3. Loss of channel enable / calibration data: Non-volatile memory corruption on the calibration constants can result in output pinned at 0 V or full scale on one or both channels.
4. 24 V supply dropout: Field power supply collapses under load, causing the green "24 V" LED to remain on while internal logic resets (only the power LED is visible). Always measure 24 VDC at the EM 232 terminals before condemning the module.

Confirmation that the EM 232 (and not the VFD, wiring, or program) is at fault requires a 4-step isolation procedure.

4. Step-by-Step Diagnostic Procedure

4.1 Prerequisites

• STEP 7-Micro/WIN V4.0 SP9 (or current Micro/WIN SMART if using a S7-200 SMART — note the S7-200 classic and SMART have different analog output mappings)
• Digital multimeter (DMM) with 0–20 VDC and 0–20 mA ranges, 0.5 % accuracy or better
• 4.7 kΩ linear potentiometer (for VFD input test)
• Small flat-blade screwdriver for VFD DIP switches and terminals
• Wiring diagram for the specific machine

4.2 Step 1 — Disconnect Field Wiring and Verify the Output

With the PLC in STOP or in a controlled RUN state, isolate the EM 232 from the VFD by disconnecting the two output wires at the EM 232 terminals (typically labeled M0, V0 for channel 0 and M1, V1 for channel 1). The M terminal is the analog common (return) and the V terminal is the voltage output.

Force the PLC program to write specific values to the analog output word (AQW0 for the first output on the EM 232). In STEP 7-Micro/WIN, open the Status Chart and use the force function:

AIW0: 0      (read-only input, informational)
AQW0: 0       (force to 0 VDC output)
AQW0: 32000   (force to +10 VDC output)
AQW0: 16000   (force to +5 VDC output)

Measure across the EM 232 output terminals with a DMM:

If the output reads +10 V regardless of the forced value, the EM 232 output stage is shorted to the +10 V rail. Replace the module. If the output correctly tracks each forced value, proceed to Step 2.

4.3 Step 2 — Verify the VFD Analog Input

With the EM 232 disconnected, manually exercise the VFD's analog speed reference input using a 4.7 kΩ potentiometer wired as follows:

• One end of the pot to the VFD's +10V reference terminal
• The other end of the pot to the VFD's ACM (analog common) terminal
• The wiper of the pot to the VFD's AVI (analog voltage input) terminal

For a SINAMICS V20, these terminals are on the control board strip and are labeled +10V, AVI, ACM. Set the VFD's analog input selection DIP switch (SW2 on V20) to AVI (0–10 V) mode. Run the drive in Hand (local) mode or with a small fixed frequency command, then sweep the pot. The motor speed should change smoothly from 0 to 50/60 Hz as the wiper moves from ACM to +10V. This confirms the VFD input is healthy and not the source of the failure.

4.4 Step 3 — Re-wire and Verify with PLC

Reconnect the EM 232 output to the VFD. For 0–10 V voltage mode:

Re-force the PLC to several AQW values and measure voltage at the VFD AVI terminal, then command a run and verify the motor follows. If the VFD reads maximum frequency immediately, swap M0 and V0 wiring — a common installation error is to land V0 on ACM and M0 on AVI, which produces either a 0 V or a 10 V reading depending on internal bias networks.

4.5 Step 4 — Verify Output Status LEDs

On a healthy EM 232, the green 24 V power LED is on, and on most firmware revisions the channel output indicators (or the input/active LEDs) reflect program activity. If only the 24 V LED is illuminated and the module does not respond to AQW writes, the internal microcontroller is not running — likely a failed module or a missing 24 V supply. Measure the supply at the EM 232's L+ and M terminals with the PLC under load.

5. Replacement Module Configuration

When installing a replacement EM 232, three items must be verified before powering the system:

5.1 DIP Switch Settings (Voltage vs Current)

The EM 232 has a small DIP switch block on the bottom of the module that selects voltage or current mode per channel. Switch positions:

For 0–10 V control of the VFD, both switches must be OFF. For 4–20 mA (or 0–20 mA) control, both must be ON. The same physical output terminals M0/V0 are used for both modes — current flows out of V0 and returns on M0.

5.2 Module Placement and Addressing

The EM 232 must be plugged into the S7-200 I/O bus to the right of the CPU (or another expansion module). The analog output address is determined by the slot position. Typical mappings:

If the replacement EM 232 is placed in the same slot as the failed module, the program addresses remain identical and no PLC program changes are required.

5.3 PLC Program Changes

Replacing an EM 232 CN with a genuine Siemens EM 232 of the same catalog family generally requires no program changes when:

• The slot position is unchanged
• The output range is unchanged (0–10 V vs 0–20 mA)
• The scaling is the same (raw 0–32000 to 0–10 V vs 0–20 mA)

If the replacement module has a different resolution (e.g., moving from a 10-bit clone to a 12-bit Siemens module), small scaling adjustments may be required for tight accuracy but the existing program will continue to operate. Verify the scaling math in the program:

// Example scaling: 0–100 % PLC speed reference -> 0–10 V at AQW0
// V20: 100 % = 50.0 Hz (P2000 default)
// 12-bit unipolar: 0 = 0 V, 32000 = 10 V
VD100 := SpeedRef * 320.0;          // SpeedRef in percent (0.0–100.0)
AQW0   := DINT_TO_INT(VD100);       // Output to EM 232

6. Common Wiring Errors and Field Lessons

The following installation errors are repeatedly observed in field service. Use this list as a verification checklist after any analog output wiring change.

1. V0 and M0 swapped at the VFD: Results in a 0 V or 10 V stuck command, or unstable speed. Always M → ACM, V → AVI.
2. +10V reference shorted to AVI: If the +10V terminal of the VFD is accidentally tied to AVI in addition to the pot, the input is always 10 V. With the PLC driving the input directly, +10V should not be connected to AVI.
3. Analog common (ACM) earthed at the VFD: If ACM is bonded to PE inside the drive and the EM 232's M terminal is also earthed at the PLC cabinet, ground loops inject 50/60 Hz noise that biases the speed command. Keep ACM isolated from PE and use a single-point ground at the cabinet.
4. SW2 (V20) set to 4–20 mA while wiring is 0–10 V: Drive reads 0 mA (4 mA threshold = minimum) and refuses to start, or reads 0 mA = 0 % = 0 Hz.
5. Shielded cable not grounded: Use shielded twisted pair, ground the shield at the cabinet end only, do not ground at the VFD end. This avoids induced noise from the VFD output stage appearing on the speed command.

7. Choosing 0–10 V vs 4–20 mA

Most industrial sites standardise on 4–20 mA for analog signaling because of the following practical advantages over 0–10 V:

• Live-zero detection: 4 mA = loop healthy; 0 mA = open wire, fault
• Noise immunity: Current loop is unaffected by IR drop and ground potential differences
• Long cable runs: Up to 300 m on 4–20 mA vs 10–20 m typical for 0–10 V

If 4–20 mA is required, the EM 232 outputs 0–20 mA. To convert to 4–20 mA, either:

• Scale in the PLC: Map the engineering range so that 0 % = 4 mA and 100 % = 20 mA by clipping the lower 20 % in the program. AQW0 := (SpeedRef − 20) × 400 for SpeedRef in 20–100 %.
• Use an external 250 Ω resistor: Convert 4–20 mA to 1–5 V at the VFD AVI input. The V20's AVI can be set to 0–10 V or 0–20 mA via SW2; some firmware revisions also accept 4–20 mA directly when SW2 = ACI and the ACI terminal is used instead of AVI.

For most new installations, 4–20 mA is preferred. For retrofit / replacement only, 0–10 V is acceptable when cable runs are short and shielding is correct.

8. Verification Procedure After Module Replacement

After installing a replacement EM 232, perform the following 10-point verification before returning the system to production:

1. Confirm 24 VDC at L+ and M terminals of the EM 232 (24.0 ± 0.5 V).
2. Confirm green 24 V LED is on; other module LEDs respond to AQW writes.
3. Force AQW0 = 0 in Micro/WIN. Measure 0.0 ± 0.05 VDC at V0–M0.
4. Force AQW0 = 16000. Measure +5.0 ± 0.15 VDC at V0–M0.
5. Force AQW0 = 32000. Measure +10.0 ± 0.20 VDC at V0–M0.
6. Set VFD SW2 (V20) to AVI (0–10 V). Verify ACM is not earthed at the VFD.
7. With VFD in local/Hand mode, wire the 4.7 kΩ pot to +10V / ACM / AVI and confirm motor speed varies smoothly.
8. Restore EM 232 → VFD wiring (M0 → ACM, V0 → AVI).
9. In PLC program, command 25 %, 50 %, 75 %, 100 % speed. Verify motor speed matches the VFD display (Hz or RPM).
10. Test run-stop sequencing: command 0 Hz from PLC, confirm drive ramps to 0; command full speed, confirm drive ramps to setpoint without overshoot.

9. Troubleshooting Matrix

10. Field Best Practices

• Specify Siemens OEM modules only for new builds. The cost difference versus a clone is recovered in 2–3 years of avoided downtime.
• Document AQW addresses and DIP switch positions on the cabinet drawing. This avoids miswiring during future maintenance.
• Use 4–20 mA for runs > 5 m between the PLC cabinet and the VFD. 0–10 V is fine for panel-integrated drives.
• Install a 250 Ω / 0.1 % precision resistor at the VFD input when converting mA to V. Do not use a generic 1 % carbon film; the speed accuracy depends on it.
• Add a hardware interlock for the VFD enable signal via a digital output of the S7-200 — never rely solely on the analog command going to 0 V as a stop command. A broken wire at 0 V looks identical to a commanded stop.
• Keep a spare EM 232 and S7-200 CPU on the shelf for any line using a clone module. Mean time to repair drops from days to minutes.
• Annual calibration check: Force 0 %, 50 %, 100 % from Micro/WIN and record the VFD display. Drift > 1 % indicates a degraded module or a wiring issue.

11. Frequently Asked Questions