1. Problem Overview
A Siemens LOGO! AM2 RTD expansion module that returns a steady -50 °C on one or both RTD channels is exhibiting the lower rail of its scaled output. The reading is independent of the actual probe temperature and is typically caused by one of three fault classes: (a) wiring topology, (b) sensor-type selection in the LOGO! Soft Comfort program, or (c) analog-input (AI) address binding. This guide isolates each layer with field-proven procedures, official reference values, and an LED-based diagnostic path that resolves the issue without removing the probe from service.
The symptom – both channels at exactly -50 °C – is highly diagnostic. It is the value the AM2 RTD pushes out when it cannot reconcile the input with the configured sensor profile, or when the amplifier function in the program is told to ignore the sensor type entirely.
2. Root Cause Analysis
The LOGO! AM2 RTD is a 2-channel RTD input module (Siemens 6ED1055-1MD00-0BA2 for LOGO! 6/7, 6ED1055-1MD00-0BA6 for LOGO! 8/8.3) that supports only 2-wire or 3-wire RTD probes. A native 4-wire PT100 cannot be terminated on the AM2 RTD in its 4-wire form; the four leads must be combined at the sensor head to form a working 3-wire (or 2-wire) connection. If the lead pairs are left floating, or if the program amplifier is set to "No sensor", the module returns the rail value of -50 °C. Lead-resistance imbalance and broken solder joints at the probe head compound the issue.
Per the official LOGO! 8 System Manual (Siemens Industry Online Support), the AM2 RTD auto-detects PT100 versus PT1000 at every power-on, scales the raw count to -50 °C … +200 °C with 0.25 °C resolution, and reports sensor-break, sensor-short, and out-of-range conditions on the module LED. The -50 °C rail is the lowest legitimate scaled output; values below the rail and any of the four error states will display the same way in the program if the amplifier sensor field is left at "No sensor".
3. Hardware: Converting a 4-Wire PT100 to 3-Wire for the AM2 RTD
A 4-wire PT100 element has two current-excitation leads (the same color – typically both white or both red) and two voltage-sense leads (the other color). Inside the probe head, all four leads connect to the same platinum element; the four-wire configuration exists to cancel lead resistance in high-accuracy measurements. The AM2 RTD cannot perform a 4-wire Kelvin measurement, so the excitation pair must be paralleled at the probe head and the sense pair used as a 3-wire connection.
Conversion procedure:
- Identify the two same-color leads on the probe (typically both white on Honeywell / WIKA / Endress+Hauser 4-wire PT100). These are the excitation pair.
- Solder or crimp the two excitation leads together at the sensor end. Do NOT tie them at the AM2 RTD end – they must be a single common lead that lands on one terminal only.
- Connect the two sense (signal) leads – typically both red – to the AM2 RTD channel's
I+andI-terminals. Use one terminal per sense lead; do NOT parallel them. - Connect the joined excitation lead to the AM2 RTD channel's
M(return/common) terminal. - Tighten all terminals to 0.5–0.6 N·m. Tug each lead after torquing.
Resistance verification at the probe (powered down):
- Sense lead 1 to sense lead 2: ≈ 100 Ω + lead resistance (≈ 1 Ω per 10 m of 26 AWG copper).
- Excitation lead to each sense lead: ≈ 100 Ω + lead resistance, on both leads.
- Any lead to probe body/ground: > 10 MΩ (probe body must be isolated from the element).
- Lead-to-lead isolation between the two excitation leads: < 0.5 Ω (must be continuous, as they are now joined).
4. PT100 Reference Resistance Values (IEC 60751)
Use this table to verify probe health and confirm a -50 °C rail is not a true reading. A 4-wire probe placed in melting ice must read 100.00 Ω ±0.06 Ω (Class A) or ±0.12 Ω (Class B) at 0 °C.
| Temperature (°C) | Resistance (Ω) | Temperature (°C) | Resistance (Ω) |
|---|---|---|---|
| -50 | 80.31 | 25 | 109.73 |
| -25 | 90.19 | 50 | 119.40 |
| 0 | 100.00 | 100 | 138.51 |
| 10 | 103.90 | 150 | 157.33 |
| 20 | 107.79 | 200 | 175.86 |
The Callendar–Van Dusen equation for the PT100 in the -200 °C to 0 °C range is:
R(T) = R₀ · [1 + A·T + B·T² + C·(T-100)·T³] (T < 0)
R(T) = R₀ · [1 + A·T + B·T²] (T ≥ 0)
where R₀ = 100 Ω, A = 3.9083×10⁻³, B = -5.775×10⁻⁷, C = -4.183×10⁻¹². If the measured resistance does not match the table above for a known ambient temperature (use a calibrated reference thermometer), replace the probe.
5. Software: Sensor Type in LOGO! Soft Comfort
The AM2 RTD amplifier block in LOGO! Soft Comfort determines how the raw 0–1000 count from the module is interpreted. Three options are available for an RTD channel:
- No sensor – passes the raw 0–1000 count straight through. Selecting this on an RTD channel is the single most common cause of the -50 °C fault.
- PT100 – scales raw count to -50 °C … +200 °C with 0.25 °C resolution. Auto-detected on power-on when a 100 Ω element is connected.
- PT1000 – scales raw count to -50 °C … +200 °C, auto-detected on power-on when a 1000 Ω element is connected.
Reconfiguration procedure (LOGO! Soft Comfort V8.3 / V8.4):
- Open the project in LOGO! Soft Comfort and go online with the LOGO! BM.
- In the diagram, locate each analog amplifier block. In the source case the blocks were B010, B002, and B051.
- Double-click the amplifier. In the block properties, change the Sensor dropdown from
No sensortoPT100(orPT1000if a PT1000 element is fitted). - Verify the input address binding: the amplifier's Ax field must match the AI address of the AM2 RTD channel (see Section 6).
- Save the program, recompile, and download to the BM (RUN/STOP switch must be in STOP, or use Ethernet online transfer for hot download on LOGO! 8.3).
- Switch the BM to RUN and observe the on-line value. The reading should change from -50 °C to a value within ±2.5 °C of the reference within two scan cycles.
6. Analog Input (AI) Addressing on the LOGO!
LOGO! extension inputs do not always start at AI1. The first analog input on the first AM2 RTD can land at Ai1, Ai3, or Ai5 depending on the LOGO! base module and on the number of digital/analog modules that appear ahead of it in the I/O map. Common mappings for LOGO! 8 (6ED1052-xx08-0BAx) and LOGO! 8.3 (6ED1052-xx08-0BAx with firmware 1.83.x or later) are:
| Base Module | Onboard AI Count | First AM2 RTD Channel 1 | Channel 2 |
|---|---|---|---|
| LOGO! 8 BM (pure digital, e.g. 6ED1052-1MD08) | 0 | Ai1 | Ai2 |
| LOGO! 8 BM with 2 onboard AI (e.g. 6ED1052-2MD08) | 2 | Ai3 | Ai4 |
| LOGO! 8 BM with 4 onboard AI | 4 | Ai5 | Ai6 |
| LOGO! 8.3 BM (Ethernet, 6ED1052-2HB08) | 0 or 4 | Ai1 / Ai5 | Ai2 / Ai6 |
If the program references Ai1/Ai2 but the AM2 RTD is mapped to Ai3/Ai4 (or vice versa), the amplifier block reads the rail value of -50 °C because the bound address is not the address of any physical input. The user reported Ai3/Ai4 as the live address, which is consistent with a LOGO! 8 BM that has 2 onboard analog inputs (e.g. 6ED1052-2MD08-0BA1).
Address-discovery procedure:
- In LOGO! Soft Comfort, click Tools → Determine I/O Status (or press F5) with the PC connected to the BM.
- The status table lists every input and output of every connected module. Note the address of each AM2 RTD channel.
- Open each amplifier block in the diagram and bind the input to the discovered address.
- Recompile and download.
7. AM2 RTD LED Diagnostics
The AM2 RTD has a single two-color status LED on the front face. The LED is the fastest on-site diagnostic available.
| LED State | Meaning | Action |
|---|---|---|
| Off | No 12/24 V DC supply, reverse polarity, or module not detected by the BM. | Check supply voltage and polarity; confirm module is on the right-hand bus. |
| Solid green | Module OK, sensor connected and valid, auto-detect complete. | If the read-out is -50 °C, the fault is in the program (sensor type or AI address). |
| Solid red | Sensor break, sensor short, sensor type mismatch, or wiring fault on at least one channel. | Inspect wiring, then power-cycle to re-run auto-detect. |
| Red briefly at power-on, then green | Normal start-up self-test. | No action required. |
| Flashing red/green | Bus fault to BM. | Check ribbon cable between modules; re-seat the AM2 RTD. |
A solid green LED combined with a -50 °C reading on the LOGO! display is the diagnostic fingerprint of a software-side fault: the module sees a valid probe, but the program amplifier is misconfigured or bound to the wrong AI address. A solid red LED combined with -50 °C on the read-out indicates a hardware-side fault on the wiring or probe.
8. Step-by-Step Diagnostic Procedure (Field-Proven)
- Power down the LOGO! BM and all expansion modules. Wait 10 seconds for bus capacitors to discharge.
- Disconnect every EM except the AM2 RTD. Power up the BM and observe the AM2 RTD LED. Note the LED color and the on-line values of
Ai1/Ai2,Ai3/Ai4, andAi5/Ai6. - Power down. With a calibrated ohmmeter (Fluke 87V or equivalent, 4-wire mode preferred), measure the resistance at the probe head. Verify against Section 3 expectations.
- Verify lead-to-lead isolation: 100 Ω between each excitation lead and each sense lead, > 10 MΩ between any lead and the probe body/ground.
- Reconnect the EMs in their original order. Power up and confirm the LED turns green within 2 seconds.
- In LOGO! Soft Comfort, open the program and change every analog amplifier from "No sensor" to "PT100" (or "PT1000").
- Re-bind the amplifier input to the correct AI address observed in step 2 (use Tools → Determine I/O Status).
- Save, recompile, and download the program to the BM in STOP. Switch the BM to RUN.
- Compare the on-line temperature to a calibrated reference thermometer (Fluke 1523, Hart 1521, or a secondary SPRT in a dry-well). Accept a tolerance of ±0.6 °C (Class AA), ±1.6 °C (Class A), or ±2.5 °C (Class B) at the reference temperature per IEC 60751.
- Power-cycle the BM a second time to confirm the auto-detect re-runs and the reading remains stable.
9. Power-On Auto-Detect of PT100 vs PT1000
Per the LOGO! 8 System Manual, the AM2 RTD applies a 2 mA excitation current at every power-on and measures the resulting voltage across the element. The voltage is interpreted as:
- ≈ 0.2 V → PT100 family (≈ 100 Ω at 0 °C).
- ≈ 2.0 V → PT1000 family (≈ 1000 Ω at 0 °C).
- Outside either band → sensor-break or sensor-short, LED red.
The auto-detect result is latched in the module until the next power cycle. To force a re-detect, cycle the 12/24 V DC supply. The amplifier block in the program must match the auto-detected element; if the program is set to "No sensor", the auto-detect result is ignored and the rail value of -50 °C is reported.
10. AM2 RTD Detailed Specifications
| Parameter | Value |
|---|---|
| Number of inputs | 2 (differential, isolated) |
| Sensor type | PT100 or PT1000, auto-detected at power-on |
| Connection method | 2-wire or 3-wire (NOT 4-wire) |
| Measuring range, PT100 | -50 °C to +200 °C |
| Measuring range, PT1000 | -50 °C to +200 °C (older -0BA2); -50 °C to +850 °C on -0BA6 with firmware 1.83+ |
| Resolution | 0.25 °C |
| Accuracy at 25 °C ambient | ±2.0 °C (PT100), ±2.0 °C (PT1000) |
| Temperature coefficient | ±0.025 °C/°C of ambient |
| Conversion time per channel | ≈ 80 ms |
| Galvanic isolation | Yes (500 V DC to bus) |
| Supply voltage | 12 V DC or 24 V DC (jumper-selectable on older -0BA0; auto-detect on -0BA2/-0BA6) |
| Current consumption from 24 V DC | ≈ 50 mA |
| Power loss | ≈ 1.2 W (both channels active) |
| Terminal torque | 0.5–0.6 N·m |
| Wire cross-section | 0.5–2.5 mm² (solid or stranded, with ferrule) |
| Dimensions (W × H × D) | 35.5 × 90 × 58 mm |
| Weight | ≈ 90 g |
| Approvals | CE, cULus, RCM, KC, EAC |
Source: LOGO! 8 System Manual and the AM2 RTD data sheet section of the LOGO! product documentation.
11. Symptom-to-Cause Matrix
| Symptom | Likely Cause | Field Fix |
|---|---|---|
| -50 °C on both AI channels, LED green | Amplifier set to "No sensor" in program | Change amplifier sensor to PT100/PT1000 and re-download |
| -50 °C on a single channel, LED green | Open lead on that channel, or wrong AI address binding | Inspect wiring; verify AI address via Tools → Determine I/O Status |
| -50 °C on both channels, LED red | Sensor break on at least one channel, or 4-wire probe not converted to 3-wire | Re-wire probe per Section 3; verify lead resistance |
| Value floats ±5 °C with wire movement | Loose terminal, broken solder joint at probe head, oxidized ferrule | Re-crimp or re-solder lead terminations; re-torque to 0.6 N·m |
| Value stuck at +200 °C | Sense lead shorted to excitation lead | Re-wire per 3-wire diagram; verify with ohmmeter |
| Red LED but valid resistance at probe | PT1000 connected but program set to PT100 (or vice versa), out of compensation range | Switch sensor type in amplifier, then power-cycle |
| Green LED, value off by 3–5 °C | Lead resistance uncompensated (2-wire mode) or leads of unequal length | Re-wire to 3-wire with matched lead gauge and length |
| LED off, no -50 °C reading | 12/24 V DC missing or reversed, or module not seated | Verify supply 20.4–28.8 V DC; re-seat module on the bus |
| Value sticks at -50 °C for one power cycle, recovers next | Auto-detect failed (probe resistance out of band) on first boot | Check probe health; power-cycle; if persistent, replace probe |
12. Lead-Resistance and Tolerance Notes
For Class A 4-wire PT100, IEC 60751 guarantees ±0.15 °C at 0 °C. The 3-wire conversion used on the AM2 RTD introduces a lead-resistance error proportional to the difference between the two sense lead resistances. With copper leads of equal length and gauge, the residual error is below 0.1 °C for runs under 10 m. If one lead is shortened (e.g. one lead cut at the junction box while the other is extended), the error can exceed 5 °C at 200 °C. Always run the three leads as a single multi-conductor cable of matched length.
13. Verification
After the fix, the AM2 RTD should report a temperature within one 0.25 °C LSB of the reference, and the LED should remain solid green through normal operation. A repeat power-cycle must reproduce the same reading without a -50 °C excursion. If the LED turns red after a power-cycle, the AM2 RTD has auto-detected a different sensor family; switch the amplifier from PT100 to PT1000 (or vice versa) in LOGO! Soft Comfort, or replace the probe with the type declared in the program. The full I/O loop should be documented: probe serial number, lead length, calibration date, amplifier block number, AI address, and last calibration value. This record enables any future re-commissioning to start from a known-good baseline.
14. Frequently Asked Questions
Why does my LOGO! AM2 RTD show exactly -50 °C on both channels?
-50 °C is the lower rail of the AM2 RTD's scaled output. It is reported whenever the amplifier in the program is set to "No sensor" and the raw count is below the offset, when the AI address is unbound, or when the wiring is open. Check the amplifier sensor field first, then re-verify the AI address with Tools → Determine I/O Status in LOGO! Soft Comfort.
Can I connect a 4-wire PT100 directly to the AM2 RTD?
No. The AM2 RTD supports 2-wire and 3-wire only. Parallel the two same-color leads (typically both whites) at the sensor head and connect the two sense leads (typically both reds) to the I+ and I- terminals, one per terminal. The common excitation lead lands on the M terminal. This converts the 4-wire probe to a functional 3-wire device.
How does the AM2 RTD tell a PT100 from a PT1000?
The module applies a 2 mA excitation current at every power-on and measures the resulting voltage. About 0.2 V indicates a PT100 element (≈ 100 Ω at 0 °C), about 2.0 V indicates a PT1000 element (≈ 1000 Ω at 0 °C). The result is latched until the next power cycle. To force a re-detect, cycle the 12/24 V DC supply.
What supply voltage does the AM2 RTD require?
12 V DC or 24 V DC, with the module accepting the full LOGO! range of 20.4–28.8 V DC on the 24 V tap. Reverse polarity or AC on the supply terminals will produce a red LED and no readings. Always fuse the 24 V feed with a 1 A slow-blow fuse as recommended in the LOGO! 8 System Manual.
Can I use the AM2 RTD inputs as 0–10 V or 4–20 mA analog inputs?
No. The AM2 RTD is an RTD-only module. For 0–10 V use the LOGO! AM2 (6ED1055-1MA00-0BA2) and for 4–20 mA use the AM2 EX (6ED1055-1MM00-0BA2). The AM2 RTD inputs are not voltage-tolerant; applying 24 V across an RTD input will damage the front end and void the warranty.
My reading is off by 3–5 °C even though the LED is green and the wiring is correct. What now?
This is almost always a lead-resistance compensation error from running the AM2 RTD in 2-wire mode. Re-wire the probe in 3-wire mode with all three leads of the same gauge and the same physical length, then re-verify. If the offset persists, the probe is at the end of its calibration life; replace it or apply a single-point offset in the LOGO! Soft Comfort amplifier block.