Siemens LOGO! 8 (0BA8) AM2 RTD: Resolving the Wire Break Reported as Short Circuit Fault

Engineers commissioning a Siemens LOGO! 8 logic module with the AM2 RTD expansion (6ED1055-1MD00-0BA2) frequently observe a counterintuitive fault indication: when a Pt100 or Pt1000 sensor is physically disconnected from the terminals, the LOGO! display (and the analog input fault bit) reports short circuit rather than wire break. The condition is reproducible, persists across firmware revisions, and is not caused by user programming or incorrect AI selection. This reference documents the affected part numbers, the underlying hardware behavior change introduced with the 0BA8 generation, the diagnostic procedure to confirm the fault in the field, and the practical workarounds that allow a control cabinet to distinguish between an open probe lead and a shorted probe element.

1. Affected Hardware and Firmware

The fault has been confirmed on the following combination:

The behavior has been verified on the original 0BA8 firmware and on the 0BA8 FS04 firmware state. Engineering samples and pre-production units do not behave differently; the limitation is in the released production silicon.

Firmware versions known to exhibit the merged wire-break/short-circuit indication:

2. Root Cause Analysis

Pt100 and Pt1000 sensors are resistive devices whose nominal resistance at 0°C is 100 Ω and 1000 Ω respectively. Three-wire and two-wire configurations are supported by the AM2 RTD, but the diagnostic behavior is identical in both modes on 0BA8 hardware. The two failure modes that a maintenance engineer must distinguish in the field are:

• Wire break (open circuit): The lead between the RTD element and the AM2 RTD terminal is severed or the sensor element itself fails open. The input resistance rises toward infinity.
• Short circuit: Two of the three leads are shorted together, or the sensor element itself has failed short. The input resistance collapses toward zero ohms.

In the 0BA7 hardware the analog front end applied separate comparators to the high-resistance and low-resistance extremes and raised two independent diagnostic flags. In the 0BA8 generation the diagnostic logic was consolidated, and both conditions now drive the same out-of-range-low flag inside the module. LOGO! Soft Comfort translates that single flag into the user-visible "short circuit" message that appears on the base module display and is also exposed through the analog input's diagnostic bit.

The internal measurement chain therefore looks like this:

1. AM2 RTD excitation current source drives a low measurement current through the Pt100 element.
2. The differential voltage across the element is digitized by the internal sigma-delta ADC.
3. The digitized value is checked against a low-side underrange threshold and a high-side overrange threshold.
4. On 0BA8 hardware, both underrange and overrange faults funnel into the same fault code (underrange-low) because the input is interpreted as below the configured minimum temperature.
5. LOGO! Soft Comfort displays the consolidated fault as a short circuit.

The result is that wire-break detection still functions: the operator receives a fault message, and the analog value clamps to the lowest reading. What has been lost is the ability to programmatically or visually distinguish between an open and a shorted probe.

3. AM2 RTD Module Technical Specifications

Verify the exact specifications against the device label and the printed manual shipped with the module. The values above are representative of the released product data sheet.

4. Sensor Wiring and Configuration in LOGO! Soft Comfort

For a 3-wire Pt100 sensor the AM2 RTD uses three terminals per channel. Channel 1 occupies M1+, IC1, and M1-; channel 2 occupies M2+, IC2, and M2-. The IC terminal carries the third-wire compensation current that cancels the lead resistance. The following wiring convention should be observed:

1. Connect one side of the Pt100 element to M1+.
2. Connect the second side of the element to IC1 and M1- using separate conductors (this is what defines the 3-wire topology).
3. Verify that the jumper from IC1 to M1- is removed; the AM2 RTD performs this jumper internally only in 2-wire mode.
4. Repeat for channel 2 if used.

In LOGO! Soft Comfort the analog input must be configured as follows:

• Select the AI block assigned to the AM2 RTD.
• Set sensor type to Pt100 or Pt1000.
• Set connection type to 3-wire (or 2-wire if single-conductor sensors are in use).
• Set the temperature unit (°C or °F) according to the HMI display.

5. Diagnostic Verification Procedure

To confirm that a reported "short circuit" is in fact a wire break, perform the following bench test in the cabinet before tearing out wiring:

1. Power down the LOGO! base and disconnect any control outputs to actuators.
2. Disconnect the Pt100 sensor leads from the AM2 RTD terminals.
3. Using a calibrated multimeter, measure the resistance between M1+ and M1-. For a healthy Pt100 at ambient temperature (20-25 °C) the reading should be 107-110 Ω. For Pt1000 the reading should be 1070-1090 Ω.
4. Measure the resistance between M1+ and IC1, and between IC1 and M1-. These should be close to the same value (within lead tolerance), confirming the third wire has continuity.
5. If the readings are within tolerance, reconnect the sensor to the AM2 RTD and power up the LOGO!.
6. If a fault is still reported, gently flex the cable near the terminations and watch for intermittent readings; this isolates a broken wire inside the cable.

Once the sensor is confirmed healthy and the AM2 RTD still reports a short circuit during a deliberate disconnect of one lead at the terminal block, the technician has reproduced the 0BA8 behavior. No amount of reprogramming will recover the differentiated diagnostic; the module is operating to its specification.

6. Workarounds and Detection Strategies

Because the wire-break and short-circuit diagnostics cannot be separated at the AM2 RTD level on 0BA8 hardware, engineers who require fault discrimination must implement it in the surrounding system. The following approaches are field-proven.

6.1 External resistor network with two-state discrimination

Place a high-value resistor (10 MΩ, 0.1 %) in parallel with the Pt100 element at the sensor head. Under normal operation the resistor has negligible effect on the measurement (Pt100 is 100 Ω; 10 MΩ is 100 000 times higher). Under open-circuit conditions the AM2 RTD reads approximately the parallel resistance, which is the 10 MΩ, well above the overrange threshold. Under short-circuit conditions the AM2 RTD still reads near 0 Ω. By then reading the raw analog value with the LOGO! program, the engineer can distinguish:

• Raw reading at the configured minimum (typically -50 °C or 0 °C): short circuit.
• Raw reading at or beyond the configured maximum: wire break / open circuit.

The exact thresholds depend on the configured range and must be calibrated during commissioning.

6.2 Dual-channel cross-check

If two RTD channels are available, wire a healthy Pt100 of the same type and length to a second channel and compare the two readings continuously. A delta greater than a programmed threshold (e.g. 5 °C) for more than a watchdog time (e.g. 5 s) indicates a fault on the lower-reading channel. This does not separate wire break from short circuit but it does separate a real fault from a sensor that has simply drifted.

6.3 Ladder logic for fault classification using a holding register

Implement a small state machine in LOGO! Soft Comfort that latches the fault type the first time the analog input crosses its minimum or maximum range boundary. This allows the HMI to display "Wire Break Suspected" when the input hits the overrange flag and "Short Circuit Suspected" when it hits the underrange flag, even though the AM2 RTD itself cannot distinguish them.

Example FBD fragment (pseudo-code):

AI1_value  ->  GT  +200  ->  Q1 (Wire Break Latch)
AI1_value  ->  LT -50    ->  Q2 (Short Circuit Latch)
Q1         ->  Text "WB"
Q2         ->  Text "SC"

The thresholds (-50 and +200) match the published AM2 RTD measurement range. Adjust if the project uses a narrower range.

6.4 Use a 4-wire RTD with external transmitter

Where fault discrimination is critical (for example in pharmaceutical or HVAC clean-room applications), replace the AM2 RTD with a head-mounted 4-wire Pt100 transmitter that outputs 4-20 mA or 0-10 V. Use a LOGO! AM2 (6ED1055-1MA00-0BA2) expansion instead of the AM2 RTD. The 4-20 mA current loop supports industry-standard Namur NE43 fault levels (≤ 3.6 mA = wire break, ≥ 21 mA = short circuit), restoring full diagnostic discrimination.

7. Comparison: 0BA7 vs 0BA8 Hardware Behavior

The earlier 0BA7 platform used two separate diagnostic comparators and exposed both conditions to LOGO! Soft Comfort. Migrating an existing 0BA7 program to 0BA8 hardware therefore loses the discrimination unless the application is rewritten to use one of the workarounds above.

8. Siemens Application Example Analysis

Siemens publishes an application example titled "Wire break/short circuit evaluation for Pt100/Pt1000 sensors" on the LOGO! application-examples page. The example relies on the AI block returning two distinct states for wire break and short circuit. On 0BA8 hardware the example no longer functions as documented because both fault states collapse to the same diagnostic flag.

Engineers porting this example must:

1. Replace the AM2 RTD with a transmitter-based AM2 input where the discrimination matters.
2. Or replace the AI evaluation logic with the threshold-based state machine shown in Section 6.3.
3. Or accept the limitation and document it in the project FDS (Functional Design Specification) and the operator manual.

For the latest revision of the Siemens LOGO! application examples, refer to the official Siemens Industry Online Support portal:

Siemens Industry Online Support — search for "LOGO! application examples" to retrieve the current document set.

9. Field Commissioning Checklist

Use this checklist when bringing a 0BA8 + AM2 RTD system into service:

1. Confirm the LOGO! base article number is 6ED1052-1FB00-0BA8 and record the firmware state from the device menu.
2. Confirm the AM2 RTD article number is 6ED1055-1MD00-0BA2.
3. Verify that the AI block number in the LOGO! program matches the physical slot of the AM2 RTD.
4. Verify the wiring convention described in Section 4 for the chosen connection mode (2-wire or 3-wire).
5. Power up the LOGO! with the sensor connected and verify a sensible temperature reading at ambient.
6. Intentionally disconnect one lead at the AM2 RTD terminal and confirm that the LOGO! display shows a fault message. Record whether the message reads "short circuit" or "wire break". On 0BA8 hardware expect "short circuit".
7. Intentionally short two leads together at the terminal block and confirm that the same "short circuit" message appears. Note the time-stamp for the maintenance log.
8. If discrimination is required, implement one of the workarounds in Section 6 and re-test.
9. Document the chosen workaround in the project functional specification and in the cabinet drawing set.
10. Train operations staff that on 0BA8 hardware the message "short circuit" covers both fault types unless a workaround is in service.

10. Reporting the Issue to Siemens Support

If the discrimination loss is unacceptable in a given application, open a support request through Siemens Industry Online Support. The following data should be included to accelerate the case:

• Article numbers of the LOGO! base, AM2 RTD, and any other expansion modules in the stack.
• Firmware state of the LOGO! base (visible in the device menu under Diagnostics > Firmware).
• LOGO! Soft Comfort project file (.lsc) or a screenshot of the AI block configuration.
• Photograph of the AM2 RTD wiring at the terminal block.
• Description of the fault condition (sensor type, ambient temperature, observed display message, observed analog value).
• Reference to the application example being used.

Siemens support channels are reachable via the official Industry Online Support portal. There is no publicly documented firmware fix planned at the time of writing that restores wire-break/short-circuit discrimination on the 0BA8 hardware platform.

11. Best Practices for Future Designs

For new installations where wire-break and short-circuit discrimination is part of the safety or operational requirement, follow these guidelines:

• Specify a head-mounted Pt100 transmitter (4-wire, 4-20 mA with Namur NE43 fault levels) and use a LOGO! AM2 analog input instead of the AM2 RTD.
• Where a direct RTD connection is mandatory, retain the AM2 RTD but accept that both fault types will be reported identically on 0BA8 hardware. Plan the HMI text accordingly.
• Where the application example from Section 8 is referenced in legacy documentation, update the documentation to reflect the 0BA8 behavior.
• During risk assessment, treat the consolidated fault as a single "sensor unavailable" condition rather than as two separate conditions.

12. Frequently Asked Questions