Siemens LOGO! 8.4 AM2 RTD -50°C: PT100 AI Address Fix

1. Problem Overview: -50°C Reading on LOGO! 8.4 with PT100 3-Wire RTD

A field-engineering case occurs when a Siemens LOGO! 8.4 base module (BM) is expanded with an AM2 RTD (6ED1055-1MD00-0BA2) and a PT100 3-wire (3-core) RTD probe, but the LOGO! Text Display (TD) and the LOGO!Soft Comfort online view both report a steady -50°C regardless of actual process temperature. The wiring continuity is correct, the sensor element measures ~108 Ω at room temperature (≈ 20 °C), and the program code appears to be syntactically valid. Yet the amplifier block output remains pegged at the negative end of scale.

This symptom is not a defective sensor, not a faulty AM2 RTD, and not a broken program. It is an input-address offset error caused by the LOGO! 8 base module's default analog-input reservation scheme. The program is reading the wrong physical input pair on the AM2 RTD expansion module, and the analog input that the program is actually sampling is either floating (open circuit) or driven by a base-module analog channel that has no PT100 connected. The amplifier block interprets the out-of-range input as the lower display limit and produces a constant -50°C.

2. Affected Hardware, Firmware, and Software Versions

The fault pattern is tied to the LOGO! 8 generation and is reproducible across every firmware that reserves two analog inputs on the base module. The following components and versions are confirmed in field use:

The behavior also applies to the LOGO! 8.3 (6ED1052-1xxx8-0BA1) and LOGO! 8.2 (6ED1052-1xx08-0BA0) families because the base-module AI reservation is part of the LOGO! 8 architecture rather than a single firmware revision. The fix is identical for every variant.

3. Root Cause: LOGO! 8 Base Module AI Address Reservation

The LOGO! 8 base module integrates two 0–10 V analog inputs on terminals I7 (AI1) and I8 (AI2). These terminals are physically present on every base unit regardless of whether the user needs them. From the point of view of the LOGO! 8 I/O map, the two on-board analog channels are always addressable, and by default they consume the address space AI1 and AI2.

When an AM2 RTD is the first expansion module to the right of the base module, the expansion's two RTD channels are not mapped to AI1 and AI2. They are mapped to AI3 (AM2 RTD channel 1) and AI4 (AM2 RTD channel 2). The full address layout for a one-AM2-RTD configuration is therefore:

The original program in the field case referenced AI1 and AI2 in the analog-amplifier blocks. The LOGO! runtime obediently sampled the base-module 0-10 V inputs I7 and I8, where no PT100 was wired. With nothing connected, the inputs float near 0 V. The amplifier block, scaled for a PT100 RTD curve, interprets 0 V (i.e. the lowest possible signal) as the bottom of the configured temperature range, in this case -50 °C. The output is therefore a constant -50 °C regardless of what the AM2 RTD is actually measuring on AI3/AI4.

4. PT100 3-Wire Sensor Wiring and Verification

Before changing any program or module configuration, confirm the PT100 3-wire probe is correctly connected to the AM2 RTD. The AM2 RTD has six screw terminals arranged in two groups of three. Each group corresponds to one RTD channel.

4.1 Resistance verification with a multimeter

1. Power off the LOGO! system. Disconnect the PT100 leads from the AM2 RTD.
2. Measure resistance between the two same-color lead wires (e.g. red ↔ red) at the PT100 end of the cable. The reading should be 0 Ω — they are joined at the sensor element.
3. Measure between the third (typically white) lead and one of the red leads at the sensor end. The reading should be the PT100 element resistance at ambient: ~100 Ω at 0 °C, ~107.79 Ω at 20 °C, ~119.40 Ω at 50 °C.
4. If the red–red reading is not 0 Ω, or the white–red reading is open (>10 MΩ), the probe or the lead run is broken.
5. Reconnect lead-by-lead to U1+/I1+/I1- (or U2+/I2+/I2- for channel 2). The two same-color leads must land on the I+ and U+ terminals of the same channel; the third lead goes to I-.

4.2 Lead resistance check (long cable runs)

For cable runs above ~10 m, measure each lead's loop resistance from the AM2 RTD terminal back to the PT100 head. The two I-side leads should be within ~0.5 Ω of each other; the U-sense lead should match one of them. Mismatched lead resistance on a 3-wire RTD introduces a direct temperature error: every 0.385 Ω of imbalance equals ~1 °C. If the imbalance is significant, either re-pull the cable with three identical conductors, switch to a 4-wire probe and the AM2 RTD in 4-wire mode, or move the sensor closer.

5. Solution A: Disable Unused Base Module Analog Inputs (File > Properties)

If the program is conceptually correct and the engineer prefers to keep the AI numbering identical to the original code, the LOGO! 8 base-module AI count can be reduced from 2 to 0. This frees AI1 and AI2 for the AM2 RTD, which then occupies AI1–AI4 in the project (with AI3/AI4 still inside the AM2 RTD's internal channel pair mapping). This path is the lowest-risk option when several programs already exist on the same machine and a global remap would force a wider retest.

1. Open the affected program in LOGO!Soft Comfort V8.4 (or later).
2. Select File > Properties (or press Alt+Enter on the project root).
3. In the Hardware tab, locate the Number of analog inputs on the base module field. The default is 2 (for AI1/AI2 on I7/I8).
4. Change the value to 0 to disable the base-module analog inputs.
5. Click OK. The I/O map re-compiles. The AM2 RTD now occupies AI1 (channel 1) and AI2 (channel 2). Existing program blocks that reference AI1 and AI2 will read the RTD correctly without any code change.
6. Recompile, transfer the program to the LOGO! 8.4, and verify the displayed value tracks the actual process temperature.

Set the count to 0 only if no analog source is actually wired to I7/I8. If a 0–10 V transducer is wired to I7/I8 and is also needed, leave the base AI count at 2 and use Solution B instead.

6. Solution B: Remap Program References from AI1/AI2 to AI3/AI4

This is the canonical, least-invasive fix for the field symptom described in the original report. It preserves the base-module AI count (still 2) and instead re-points every analog-amplifier (or analog-comparator, threshold-trigger, math, PI-controller) input from AI1/AI2 to AI3/AI4.

1. In LOGO!Soft Comfort V8.4, open the project.
2. Use the menu Tools > I/O Names to list every analog input in the program. Note the count of references to AI1, AI2, AI3, and AI4.
3. Open the Analog Amplifier block (B006 in the original program) and change the sensor input from AI1 to AI3. Repeat for the second amplifier block (B007), changing AI2 to AI4.
4. If the program uses analog comparators (B008, B009, etc.), threshold triggers, math blocks, or PI controllers reading AI1/AI2, change them to AI3/AI4 as well.
5. For LOGO! TDE message text blocks that reference the temperature value, no change is required — the variable connected to the display tag is the same; only its source is changed.
6. Save the program, then choose PC > LOGO! to transfer. Use the online test view to confirm the value tracks the real probe temperature.

7. LOGO! 8.4 AI Address Allocation Reference

Use the following table as a single-source reference when commissioning a LOGO! 8.4 with mixed digital and analog expansion modules. The address layout is additive: each module's contribution stacks on top of the previous module's last address. The base module always contributes AI1 and AI2 first (unless disabled per Solution A).

Maximum analog-input count in a single LOGO! 8 stack is 8 (AI1–AI8), which permits up to four AM2 / AM2 RTD modules. Pure digital modules (DM8, DM16, DO8) consume digital addresses only and do not advance the AI counter.

8. AM2 RTD Module (6ED1055-1MD00-0BA2) Specifications

Full specifications are in the Siemens LOGO! 8 System Manual (entry ID 109751049) and the AM2 RTD product page (MLFB 6ED1055-1MD00-0BA2).

9. PT100 Resistance vs. Temperature Reference

Use this table to verify a PT100 probe against the AM2 RTD's expected input. IEC 60751 nominal values; tolerance Class A is ±(0.15 + 0.002·|t|) °C, Class B is ±(0.30 + 0.005·|t|) °C.

If a hand-held calibrator is available, inject the equivalent resistance on the AM2 RTD terminals and read the LOGO! display. A 100.00 Ω injection must read 0.0 °C ±0.5 °C on the LOGO! TD; if it reads -50 °C, the AM2 RTD channel is not being addressed (confirm AI numbering per Section 7) or the channel is open.

10. Verification and Commissioning Procedure

1. Power up the LOGO! 8.4. The LOGO!Soft Comfort Online > I/O Status view should show the AI values from every configured analog channel.
2. For each AM2 RTD channel, confirm the raw AI value moves with temperature. A 0 °C reference (ice-bath or 100 Ω decade box) should produce a value of 0 °C after the amplifier block. A 100 °C reference (boiling-point calibrator or 138.51 Ω) should produce 100 °C.
3. Trigger a sensor-break condition: disconnect one lead of the PT100. The amplifier output should snap to the configured under-range behavior. With the default scaling, the LOGO! TD will display -50 °C. This is the correct failure-mode behavior, not a fault.
4. Trigger a short-circuit condition: short the U+ and I- terminals at the AM2 RTD. The amplifier output should show the configured over-range behavior (typically +400 °C or 3276 °C depending on amplifier configuration).
5. Verify on the LOGO! TDE: the message text block should refresh and the parameter should track the real process temperature. The TD's F1/F2/F3/F4 keys can be used to navigate or toggle, but the value display is automatic once the parameter is wired.
6. Document the final AI mapping, the AM2 RTD serial number, the PT100 serial number, and the calibration reference for the maintenance record.

11. Troubleshooting Matrix for Common LOGO! 8.4 + AM2 RTD Faults

12. Field-Proven Lessons from the LOGO! 8.4 + AM2 RTD -50 °C Case

The original field report was resolved by remapping the analog amplifier blocks from AI1/AI2 to AI3/AI4. Three engineering takeaways are worth carrying into future commissioning work:

1. Always verify the AI map on the very first power-up. A 30-second check of File > Properties > Hardware before writing a single line of logic prevents this entire class of fault. The base module's default 2-AI count is rarely what a small retrofit actually needs.
2. Treat a pegged -50 °C reading as a wiring or addressing alarm, not a process alarm. The AM2 RTD cannot physically output -50 °C from a valid PT100 connected at room temperature; it can only display that value when the input is open (sensor break) or when the program is reading the wrong AI.
3. Use 4-wire PT100 probes on long cable runs. The AM2 RTD supports 4-wire mode and on long lead runs the 3-wire lead-resistance imbalance becomes the dominant error source. A 4-wire probe eliminates the imbalance contribution and improves absolute accuracy.

For the LOGO! 8 generation specifically, the AI reservation on the base module is documented in the LOGO! 8 System Manual (entry ID 109751049) and the AM2 RTD chapter of the same manual. The LOGO!Soft Comfort V8.4 online help also documents the Number of analog inputs on the base module setting under File > Properties > Hardware.