Detecting PT100 Wire Break on Siemens LOGO! AM2 Inputs

Overview

The Siemens LOGO! AM2 analog input module integrates 2-wire Pt100 RTD measurement directly into the LOGO! 8 (0BA8) logic controller family. Unlike the S7-1500 F-AI modules or the S7-1200 SM 1231 RTD, the LOGO! AM2 does not expose a separate wire-break flag in the process image, the LOGO! Soft Comfort diagnostics, or the LOGO! display menu. Instead, an open sensor leg produces a saturated or minimum analog value that the application program must interpret as a fault condition.

This reference consolidates the documented behaviors across LOGO! hardware revisions, the FS04 firmware revision observed in the field, and a min/max threshold strategy that produces deterministic fault outputs even when the AM2 cannot distinguish a wire break from a short circuit. Engineers deploying cooling-water monitoring, HVAC, diesel generator coolant monitoring, or process temperature alarming must use the strategy below unless they escalate to an S7-class controller that provides native per-channel diagnostics.

AM2 PT100 Hardware Reference

The AM2 module family is the RTD input block for the LOGO! 8 system. Key specifications applicable to Pt100 measurement:

The three terminals per channel are not interchangeable. IC is the constant current source that excites the Pt100 element. M+ and M- are the differential sense inputs feeding the internal ADC. For 2-wire operation, the Pt100 element is wired between IC and M+, while M- is bridged to M+ at the terminal block to complete the sense loop. The bridging is essential: leaving M- floating is the most common cause of the ~177 °C "phantom high" reading reported in the field.

PT100 Measurement Principle

The LOGO! AM2 sources a small excitation current from the IC terminal through the Pt100 element. The differential voltage between M+ and M- is digitised by the internal ADC and the firmware linearises the resistance reading into a temperature value using the IEC 60751 curve (α = 0.00385 Ω/Ω/°C).

For a Pt100 element per IEC 60751:

• Nominal resistance at 0 °C: 100.00 Ω
• Nominal resistance at -50 °C: 80.31 Ω
• Nominal resistance at +100 °C: 138.51 Ω
• Nominal resistance at +200 °C: 175.86 Ω

Because the excitation current is constant and the lead resistance appears in series with the sensor element in a 2-wire connection, the AM2 does not compensate for lead resistance. Field installations must therefore keep the lead run short (typically <10 m with 0.5 mm² copper) or accept a positive offset error. Each ohm of lead resistance contributes approximately 2.56 °C of positive offset at 0 °C.

Wire Break and Short-Circuit Behavior Across Revisions

Field testing across multiple hardware and firmware combinations produced the following readings when the sensor circuit was deliberately opened or shorted. All values are scaled to the default LOGO! AM2 range (raw 0 ... 1000 over -50 °C ... +200 °C).

The saturated high reading (~177 °C) on M- open and the gradual climb to the maximum raw value (1000, equivalent to +200 °C) on M+ open both result from the ADC integrator floating to one supply rail when the differential sense path is broken. The exact saturation value is not trimmed or specified in the module datasheet; treat the high reading as a qualitative indicator only.

Why Short and Break Cannot Be Distinguished on LOGO! AM2

The LOGO! AM2 firmware maps any under-range or over-range raw ADC input to the same end value without raising a diagnostic flag in the process image. The status byte that higher-tier Siemens analog modules expose (such as the S7-1200 SM 1231 RTD or the S7-1500 AI 8xRTD/TC) is not implemented in the AM2. Engineers who require unambiguous fault diagnostics must either:

1. Add a low-value current source or hardware fault monitor at the sensor head that produces a distinguishable signal when the loop is open.
2. Use the upper threshold for break detection and accept that a short circuit and a break will trigger the same fault alarm (recommended for most non-safety applications).
3. Replace the LOGO! AM2 with an S7-1200 + SM 1231 RTD or an ET 200SP AI RTD when per-channel fault status is mandatory.

The design tradeoff is intentional: the LOGO! AM2 is a low-cost analog block for simple temperature regulation, not a diagnostics-grade module. The application program is responsible for converting saturated readings into useful fault flags.

Workaround: Min/Max Threshold Monitoring

The most reliable field-proven strategy is to alarm any scaled temperature that exits the process envelope. For a cooling-water monitoring application with a valid range of 5 °C to 90 °C, the logic is:

1. Set a high-side threshold above the maximum expected process value. A wire break on M- typically produces ~177 °C, so a high alarm at 150 °C captures all opens on the sense legs and M-.
2. Set a low-side threshold below the minimum expected process value. A wire break on IC or a short circuit produces -50 °C (raw 0), so a low alarm at -10 °C captures both.
3. Latch either alarm for operator acknowledgement. A sustained high or low reading across two scan cycles (approximately 100 ms at the default AM2 update rate) confirms the fault.
4. Optionally combine both alarms into a single "PT100 fault" output that drives a digital output, a LOGO! display text, or a remote notification via the LOGO! CMR2020 / CMR2040 cellular module.

Sample parameter values for a cooling-water loop:

LOGO! Soft Comfort Implementation

The implementation in LOGO! Soft Comfort V8.x uses the Analog Input (AI) block followed by two Analog Comparator (Threshold Trigger) blocks. The wiring for a single Pt100 channel is:

1. Drag an Analog Input block onto the FBD diagram. Select the AM2 from the block selector and assign it to the correct slot and AI number (AI1 or AI2).
2. Set the sensor type to Pt100 and the measurement range to -50 °C ... +200 °C.
3. Set the gain to 1.0 and the offset to 0.0 initially. Adjust the offset after the field zero check.
4. Insert an Analog Threshold Trigger block (block type "Analog comparator"). Wire the analog input to the Ax pin.
5. Set ON threshold to a value just below the minimum expected process temperature (e.g., -10 °C for a cooling-water loop).
6. Set OFF threshold 1 °C above the ON threshold to provide hysteresis.
7. Wire the comparator output to a digital marker M1 (low-side fault).
8. Insert a second Analog Threshold Trigger for the high-side alarm. Set ON threshold to a value above the maximum expected process temperature (e.g., 150 °C).
9. Wire the second comparator output to marker M2 (high-side fault).
10. OR M1 and M2 into a single fault output using a basic OR block, then route to a digital output, a LOGO! display message, or an SMS/e-mail notification via the LOGO! CMR2020.

The threshold parameters in LOGO! Soft Comfort are stored as floating-point values; LOGO! Soft Comfort V8.4 and later support signed temperature scaling without manual unit conversion. The maximum sampling rate of the AI block is the AM2 module update time (approximately 50 ms per channel); do not expect faster response.

Decision Flowchart

Field-Proven Considerations: Static, Grounding, Lead Dress

Operators have reported erratic AM2 readings (slow climbs to the maximum raw value, sudden spikes to ~177 °C, or rapid oscillation between 0 and 1000) when the module is installed in a cabinet with poor earth bonding or near a switching VFD. The cause is common-mode voltage on the sense pair rather than a true sensor fault. Mitigations validated in the field:

1. Bond the LOGO! ground terminal to the cabinet backplate with a short, low-inductance strap (≤50 mm, 4 mm² minimum).
2. Route the Pt100 cable in a separate conduit or at least 200 mm away from VFD output cables and contactor coil wiring.
3. Use shielded twisted pair for the Pt100 cable. Connect the shield at the cabinet end only; do not ground both ends (avoids ground loops).
4. Avoid mounting the LOGO! directly on a painted, anodised, or plastic surface; use a zinc-plated DIN rail bonded to the cabinet frame.
5. If the cabinet has a known static discharge issue (e.g., conveyor belt drives, dry process areas), apply a 100 kΩ bleeder resistor from the shield to ground at the cabinet end to drain induced charge slowly.
6. Verify earth resistance at the LOGO! chassis is ≤1 Ω before commissioning. Document the reading in the SAT report.

Comparison: Dedicated Wire Break Detection in F-AI Modules

Engineers familiar with S7-1500 F-modules expect a per-channel wire-break flag with a defined threshold. The S7-1500 F-AI module (for example, 6ES7536-1MF00-0AB0, an 8-channel F-AI for 0-20 mA with HART) implements a hardware-based wire-break detection threshold. As documented in the Siemens fail-safe module manual collection, wire break is detected when the loop current falls below 0.4442 mA. The module then raises a diagnostic interrupt that the F-CPU routes to the safety program.

The official Siemens reference for the F-AI wire-break threshold is available at Diagnosis: Wire break - F-AI 8xI 0..20 mA. The LOGO! AM2 does not implement an equivalent diagnostic parameter, so the application must derive the same conclusion from the analog value alone.

Edge Cases and Engineered Solutions

Several edge cases must be considered when designing the wire-break alarm strategy:

Edge case 1: Process temperature genuinely reaches -50 °C

If the application legitimately operates at or below -10 °C (cryogenic storage, refrigerant lines, cold-room monitoring), the low-side fault threshold cannot distinguish a true process reading from a wire break. Solutions: (a) use a separate, redundant temperature sensor on a different module (a second AM2 or an external transmitter), (b) accept the ambiguity and alarm only at -40 °C, or (c) use a 4-20 mA RTD transmitter feeding an AM2 AQ module that re-scales the range to 0-50 °C over 4-20 mA.

Edge case 2: Process temperature legitimately reaches +200 °C

Steam lines, autoclaves, exhaust gas monitoring, and oven safety monitoring can reach the upper end of the AM2 range. The high-side fault threshold cannot distinguish a true +200 °C reading from a wire break on the M+ terminal. Solutions: (a) lower the process range with a Pt100-to-4-20 mA converter feeding an SM 1231 AI module, (b) use a transmitter with a 0-500 °C range, or (c) alarm only the M- open signature at ~177 °C (accepting that an M+ open will produce an even higher false reading).

Edge case 3: Sensor leg intermittent due to vibration

Loose terminal screws, vibration on the sensor head, or broken flex leads can produce short pulses into the saturated range rather than a sustained fault. Debounce the fault output by requiring the threshold to be exceeded for at least 5 scan cycles (approximately 250 ms at the default AM2 update rate) before latching the alarm. Use a LOGO! on-delay block with PT = 250 ms on the comparator output.

Edge case 4: Two AM2 modules sharing one power rail

If two AM2 modules share the same +24 V supply rail and the rail collapses briefly during a heavy load event, both modules will report -50 °C simultaneously. Cross-check the module's operating voltage (LOG input > 17 V DC) before declaring a wire-break fault. The LOGO! base unit provides a system flag M28 (Power OK / Power Failure) that can be used to suppress false alarms during power-up or brownout events.

Edge case 5: Sensor replacement during operation

When the operator replaces the Pt100 element while the LOGO! is running, the AI block may latch a false fault for up to 5 seconds after the new element is connected. Add a maintenance mode flag (driven by a digital input or a LOGO! display button) that suppresses the fault output during scheduled maintenance windows.

Edge case 6: Reverse polarity at the AM2 terminals

If the Pt100 element is connected with reverse polarity (across M+ and IC instead of IC and M+), the AM2 will read approximately -50 °C continuously. This is not a wire break but a wiring error. Document the correct polarity in the cabinet drawing and apply wire markers at both ends of each Pt100 lead to prevent installer error.

Verification Procedure

After implementing the min/max threshold logic, verify the system in three stages:

1. Bench test with intentional opens. Disconnect each terminal (IC, M+, M-) one at a time on a known-good Pt100 sensor at room temperature. Confirm that the high-side or low-side fault flag trips within 250 ms and that the digital output activates. Record the AI raw value at each open condition for the SAT report.
2. Bench test with intentional shorts. Short the Pt100 element at the terminals (place a wire link across IC and M+). Confirm that the low-side fault flag trips identically to an IC open. Document the equivalence in the SAT report.
3. Field test under process conditions. Run the system under normal load for at least 24 hours. Confirm that the fault flag does not trip during expected process transitions. Document the maximum and minimum scaled values observed during the run for future reference.
4. Power-cycle test. Power down the cabinet, then restore power. Confirm that the fault output resets cleanly and does not latch a false fault during the first 2 seconds of operation.
5. Maintenance mode test. Engage the maintenance mode flag, replace the sensor, and confirm that the fault output is suppressed during the maintenance window. Disengage maintenance mode and confirm that the fault output returns to normal operation.

Document the verification results in the project's SAT (Site Acceptance Test) report. Include screenshots from LOGO! Soft Comfort online view showing the analog value, the threshold block parameters, and the fault marker state. Capture one screenshot per test condition (open IC, open M-, open M+, short circuit, normal operation).

Frequently Asked Questions