Troubleshooting S7-1215C DC/DC/DC Analog Outputs: Wiring, Scaling, and Configuration

The SIMATIC S7-1215C DC/DC/DC integrates two on-board analog outputs (AQ0 and AQ1) that engineers frequently mis-wire or mis-configure when scaling a 0-20 mA current loop. Typical field reports show the output tag driving correctly (0 at 0 %, 27648 at 100 %) in the watch table, yet the measured loop current is wrong — for example, 0.17 mA at 50 %, 0.38 mA at 100 %, or an anomalous 18.54 V DC at the terminals. This reference isolates those failures, walks through the official Siemens diagnostic procedure, and provides the verification steps needed to confirm a healthy output channel.

1. Symptom Summary

When a field report mentions a CPU 1215C DC/DC/DC with the symptoms below, the most likely root causes are wiring, output-type configuration, or an open 4-wire loop. Use the symptom table as a triage matrix.

2. S7-1215C DC/DC/DC Analog Output Hardware Specifications

Confirm the channel ratings before drawing current. The values below are taken from the official SIMATIC S7-1200 manual collection, current-output section for the CPU 1215C.

Source: Measurement ranges of the analog outputs for current (CPU 1215C and CPU 1217C) in the S7-1200 manual collection on docs.tia.siemens.cloud.

3. Default Tag Addresses and I/O Memory Map

On a CPU 1215C DC/DC/DC with no signal module (SM) and no signal board (SB) installed, the analog I/O defaults are fixed. The two outputs are addressed in the process image output (PIQ):

Scaling rule (current and voltage):

• 0 → 0 % (0 mA or 0 V)
• 13824 → 50 % (10 mA or 5 V)
• 27648 → 100 % (20 mA or 10 V)
• 32767 → full over-range (current clamps to 22.9 mA)

The tag value is unsigned integer. Always scale the engineering value through the NORM_X / SCALE_X instructions or a manual conversion INT_TO_REAL → multiply by 20 / 27648 → REAL_TO_INT. Writing a real-world value such as 0.5 directly into QW64 will be interpreted as 0 because the PLC truncates non-integer writes to the output word.

4. Output Type Selection: Voltage vs. Current

The 1215C outputs are software-configurable per channel. The selection is made in TIA Portal at Device Configuration → CPU 1215C → Properties → Analog outputs:

1. Open the device view of the CPU.
2. Select the Analog outputs node in the device tree.
3. Click on channel 0 (and channel 1 if needed).
4. In the Properties inspector, set Output type to one of:
   • Voltage (±10 V)
   • Current (0-20 mA)
   • Current (4-20 mA)
5. Compile and download to the CPU. A stop-to-run transition is required to apply the new output type.

Reconfigure both channels to Current (0-20 mA), download, and re-test before continuing.

5. Wiring Diagram and Terminal Assignment

The analog I/O terminals are on the bottom edge of the CPU, behind a removable orange connector. Pin assignment for the 1215C DC/DC/DC:

The 2M terminal is internally bonded. Both AQ0 and AQ1 share the same analog common. If the field device is isolated, the device's signal common must still be tied to 2M. If the field device is non-isolated, 2M must be tied to the field device's signal ground at exactly one point to avoid ground loops.

For a 4-wire current loop driven by the 1215C, the correct topology is:

The test load can be a 250 Ω, 500 Ω, or any precision resistor up to the 600 Ω maximum load for current mode. For a bench verification, a 500 Ω resistor across AQ0 to 2M gives a 0-10 V drop at 0-20 mA, which is easy to read on a standard DMM.

6. Common Wiring Errors on the 1215C

The most frequently reported field failures cluster into four categories. Each has a distinct signature.

6.1 Front/Back Terminal Block Reversal

The 1215C has two rows of screw terminals on the bottom — digital inputs on the upper row, analog I/O on the lower row. Engineers with the CPU mounted in a cabinet where the wiring enters from above occasionally land the analog wires on the digital terminal block and vice versa. The 2M terminal in particular looks like a normal digital M terminal and is easy to mistake for a digital common. Verify continuity with an ohmmeter between the terminal and the 2M pin on the device label.

6.2 Missing Analog Common

If 2M is left floating, the output stage cannot complete a loop. The PLC tag value changes correctly, but no current flows because there is no return path. The field device may show a small leakage-induced offset of 1-3 mA — consistent with the 2.09 mA reading some engineers report at 0 %.

6.3 Voltage Output Misused as Current

If the channel is left at the factory default of Voltage, writing 27648 yields 10 V. Connected to a 250 Ω burden, the source attempts to deliver 40 mA but clamps to 22.9 mA. The reading 0.38 mA reported in some threads implies the load is not even completing a path — the field device is reading essentially zero current. The fix is a configuration change, not a wiring change.

6.4 Polarity Reversal on Externally Powered Loops

If a 24 V loop supply is wired in series with the AQ output, the output cannot source against an external supply that opposes it. The 1215C analog outputs are passive 0-20 mA / ±10 V outputs with onboard drive only; they do not include a loop power supply. If the field device requires a loop-powered input, install a separate 24 V supply and a dropping resistor.

7. Diagnostic Procedure with Test Resistor

Follow this bench procedure to isolate a wiring fault from a configuration fault. It is the same procedure Siemens support uses for first-line triage.

1. Place the PLC in STOP.
2. Disconnect all field wiring from AQ0, AQ1, and 2M.
3. Land a precision 500 Ω, 0.1 % or better, 1 W resistor between AQ0 (pin 3) and 2M (pin 2). Repeat with a second resistor on AQ1 (pin 4) to 2M.
4. Connect a DMM across the resistor in voltage mode (4-wire or a separate voltmeter).
5. In TIA Portal, go online and add a watch table. Force QW64 = 0 and QW66 = 0. Verify the voltmeter reads 0.000 V (within ±2 mV noise floor).
6. Force QW64 = 13824. Expected voltage = 5.000 V (current = 10 mA × 500 Ω).
7. Force QW64 = 27648. Expected voltage = 10.000 V (current = 20 mA × 500 Ω).
8. If the readings match within 0.3 % (30 mV at 10 V), the output stage is healthy. The fault is in the field wiring or the field device.
9. If the readings are off by more than 5 %, the channel configuration is wrong (likely voltage mode), or the output is damaged.

8. Interpreting Field Readings: The 18.54 V DC Anomaly

The reported reading of 18.54 V DC at the analog output terminals is, in practical terms, impossible for a 1215C analog output to produce on its own. The output stage is bounded at ±10.5 V. A reading of 18.54 V therefore implies one of the following conditions:

1. An external voltage source is back-feeding the AQ terminal. A typical cause is a mis-wired 24 V supply from the field side (the field device is sourcing the loop instead of the PLC).
2. The voltmeter is reading the digital-output 24 V rail that is physically adjacent to the analog terminal block. The orange removable connector can be inserted one row off, so pin 2 of the analog block is actually pin 2 of the digital block.
3. The CPU has a damaged output stage and the protection zener is conducting to the 24 V rail. In this case the output is destroyed and the channel cannot be recovered without CPU replacement.

To isolate, remove the orange terminal connector entirely and re-measure between pin 3 and pin 2 of the connector (not the CPU). If the voltage is still 18.54 V, the source is external. If the voltage is 0 V on the connector and 18.54 V when seated, the CPU output stage is damaged.

9. TIA Portal Configuration Walkthrough

For a fresh project where the analog outputs do not appear to function, confirm the configuration in the following sequence.

9.1 Device Configuration

Open the project, expand the device tree, and select the CPU 1215C DC/DC/DC. The catalog part numbers are:

Right-click the Analog outputs node and confirm both channels are present. If they are not, the CPU in the project is a 1212C or 1214C — both have only two analog inputs and zero analog outputs.

9.2 PLC Tag Table

Open the default tag table. Confirm the system-default tags:

• QW64 (WORD) — output word for AQ0
• QW66 (WORD) — output word for AQ1

If a signal module (SM) is installed to the right of the CPU, the analog output addresses shift accordingly. The first SM analog output word starts at PQW72, and so on. The on-board analog outputs are always PQW64 and PQW66 regardless of the configuration further to the right.

9.3 Output Type and Range

In the Properties pane of the analog output channel, set:

• Output type: Current
• Current range: 0-20 mA (or 4-20 mA if the field device is loop-powered)
• Diagnostics — short circuit: Enabled (only valid in voltage mode, but does not cause nuisance in current mode)
• Reaction to CPU STOP: Hold last value or Substitute value (default = Hold last value; 0 mA in many projects)

9.4 Programmatic Scaling

The simplest scaling block in Structured Text is:

// Input: aiPercent (REAL, 0.0 to 100.0)
// Output: QW64 (INT, 0 to 27648)

IF aiPercent < 0.0 THEN
    aiPercent := 0.0;
END_IF;

IF aiPercent > 100.0 THEN
    aiPercent := 100.0;
END_IF;

QW64 := REAL_TO_INT(aiPercent * 276.48);

For ladder logic, use the NORM_X and SCALE_X instructions from the Convert operations folder in TIA Portal. The NORM_X instruction normalizes the engineering value to a 0.0-1.0 REAL, and SCALE_X scales it to the 0-27648 output range. Both must be unipolar for a 0-20 mA output.

10. Verification Checklist

Run through the items below in order before declaring the analog output channel functional.

11. Common Field-Reporting Patterns

The following patterns come from the symptoms reported in engineering threads and consistently resolve to a small set of root causes. They are useful as a quick triage matrix when triaging a call from a field technician.

12. Reference Documents

The diagnostics and specifications in this article are taken from the official SIMATIC S7-1200 manual collection. Use these as the source of truth when escalating a case to Siemens support.

• Measurement ranges of the analog outputs for current (CPU 1215C and CPU 1217C) — scaling table, 0-27648 mapping, over-range values
• Digital inputs and outputs (CPU 1215C) — terminal pinouts, isolation, hardware ratings

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