Overview
The Siemens LOGO! 8 (6ED1052-xxx08-0BA0 base family) does not support direct PT100/PT1000 connection on its integrated analog inputs. A 2-wire RTD can be wired to the base unit AI1/AI2 only by adding two precision resistors in series and applying a software linearization that the LOGO! 8 firmware does not ship. In practice, every platinum RTD application on a LOGO! 8 requires the LOGO! AM2 RTD expansion module (article number 6ED1055-1MD00-0BA2), which provides two dedicated PT100/PT1000 channels and on-board linearization across the −50 °C to +200 °C measurement range.
One of the most common engineering gaps in a LOGO! 8 RTD application is fault handling. The AM2 RTD firmware does not expose a discrete "open circuit" or "short circuit" flag, and the LOGO! 8 logic engine has no built-in module-health bit for the expansion bus. Wire break, short circuit, corrosion drift, and module hang-up must therefore be reconstructed from limit comparisons on the analog value, watchdog timers on the AI update, and a heartbeat sent from a remote output. This reference documents the field-proven method for implementing PT100 fault detection in a LOGO! 8 + AM2 RTD program, including parameter tables, ladder snippets, and a troubleshooting matrix.
AM2 RTD Module Specifications
| Parameter | Value |
|---|---|
| Article Number | 6ED1055-1MD00-0BA2 |
| Product Family | LOGO! 8 expansion module |
| Power Supply | 12 / 24 V DC |
| Analog Inputs | 2 (PT100 or PT1000, software selectable per channel) |
| Sensor Wiring | 2-wire or 3-wire (jumper on the terminal block selects mode) |
| Measurement Range | −50 °C to +200 °C (PT100/PT1000) |
| Resolution | 0.25 °C typical after linearization |
| Update Time | ~50 ms per channel (firmware 1.81.x and later) |
| Galvanic Isolation | No (sensor common tied to module 0 V) |
| Bus Detection by Base | None (module presence not flagged in logic) |
The full hardware manual is published in the Siemens Industry Online Support portal as the LOGO! 8 System Manual (edition 2020-06). The chapter "Analog Value Processing" lists the scaled AI word for each AM2 RTD channel and confirms that no error bit is exposed when the input leaves the linear range.
Why PT100 Faults Are Not Native
A PT100 element changes resistance from 100 Ω at 0 °C to about 175.84 Ω at 200 °C. The AM2 RTD measures this resistance with a constant excitation current and computes temperature in firmware. When the lead is broken, the measured resistance rises to the excitation-source saturation point, which the firmware clamps to a value corresponding to the upper end of the conversion table. When the leads are shorted, the measured resistance collapses to near 0 Ω, which the firmware clamps to the lower end. In both cases, the AI word presented to the LOGO! 8 logic engine is a normal-looking scaled integer — there is no dedicated error flag, no diagnostic interrupt, and no bit set in the analog input status word (such as AI1.STAT on a S7-1500). Detection must therefore be implemented at the application layer.
Limit-Based Fault Reconstruction
The canonical technique is to compare the AI scaled value against two software thresholds that sit just outside the process operating window. When the AI falls below the lower limit or rises above the upper limit for more than the filter settle time, a fault latch is set and the message scroll on the LOGO! display is updated.
| Fault Type | Observed AI | Recommended Threshold |
|---|---|---|
| Wire break (sensor or lead) | Drifts toward the upper end of the conversion table | AI > ProcessMax + 5 °C (e.g. 195 °C for a 0–190 °C process) |
| Short circuit at terminal or in cable | Drifts toward the lower end of the conversion table | AI < ProcessMin − 5 °C (e.g. −45 °C for a 0–190 °C process) |
| Corrosion / contact resistance drift | Reads within range but is offset from expected | Compare against a redundant sensor; raise drift alarm if |Δ| > 10 °C |
| Sensor unplugged mid-run | AI saturates high within one update cycle | Same as wire break |
Use LOGO!Soft Comfort analog threshold triggers (block B003) or analog comparators (block B008) for the comparison. The hysteresis of the analog comparator should be at least 2 °C to prevent chattering on noisy cables; the on-delay (parameter "On-delay") should be at least 200 ms to ride through the AM2 RTD filter settling transient.
Ladder / FBD Snippet — Two-Level Fault Latch
The following Function Block Diagram snippet implements a sticky fault for a single channel. It uses an analog threshold (B003) feeding a set/reset latch (B007) that drives a digital output, a flag for the message handler, and a one-shot pulse that increments a fault counter.
[AI1 Scaled] ---> [B003 Analog Threshold, On=195 °C, Off=190 °C] ---> [OR] ---+
|--> [B007 RS Latch (Set)] ---> M01 (Sensor Fault Hi)
[AI1 Scaled] ---> [B003 Analog Threshold, On=−45 °C, Off=−40 °C] ---> [OR] ---+
M01 ---> [B004 On-Delay 0.2 s] ---> Q1 (external alarm output)
M01 ---> Flag M02 (Display message "PT100 S1 OPEN")
M01 ---> Counter B002 (CV = number of fault events, retained on BM)
Drop this into LOGO!Soft Comfort, assign the AM2 RTD channel 1 to the AI block, and download to the base module. The latch must be reset manually from the LOGO! display or by a remote SCADA command, which prevents the fault from being auto-cleared while the operator is investigating.
Detecting a LOGO! Internal Fault
The LOGO! 8 does not expose a "module healthy" bit in the user program. Three practical methods are used in the field:
- Watchdog output. Configure a free digital output (e.g. Q3) to toggle every 1 s using a clock generator (B001) and a NOT block. Wire Q3 to a digital input on a remote device (an HMI, a higher-level PLC, or the SCADA). If the remote device stops seeing the toggling, the LOGO! has stopped scanning, the expansion bus is hung, or the power supply has failed.
- AI sanity check. Tie a precision 100 Ω reference resistor (0.1 %, low drift) to one AM2 RTD channel. If that channel ever reads outside 0 °C ± 2 °C, the AM2 RTD itself has drifted, lost its calibration, or the excitation current source has failed.
- Network keepalive. When the LOGO! 8 is a Modbus TCP slave, expose the cycle counter flag (NI1 pulse from B001 → VW area) and have the master raise an alarm if the counter stops incrementing for more than 5 s. This is the standard method when the LOGO! is supervised by a WinCC, TIA Portal HMI, or third-party SCADA.
Sensor Wiring and Field Commissioning
The AM2 RTD accepts 2-wire or 3-wire PT100. For fault detection to be reliable:
- Use 3-wire mode when the cable is longer than 5 m. The AM2 RTD subtracts one lead resistance and partially compensates for the other, reducing the offset error that a wire break would have to overcome.
- Route the RTD cable in a dedicated conduit at least 200 mm from VFD output cables and any 400 V AC power runs. Capacitive coupling into a long PT100 lead can shift the measured value by several degrees and trigger nuisance high-temperature alarms.
- Tighten terminal screws to 0.6 Nm (Siemens recommendation in the LOGO! 8 system manual). A loose terminal presents as an intermittent high-resistance joint, which the fault logic will see as a slow drift rather than a hard wire break — annoying to diagnose.
- Verify polarity at the AM2 RTD terminal block. The two channels share the module 0 V; if a sensor is wired in reverse on a 3-wire connection, the reading will be wildly wrong but inside the range, so the limit-based fault will not fire.
Parameter Mapping in LOGO!Soft Comfort
| Block | Parameter | Typical Value | Function |
|---|---|---|---|
| B001 Clock Generator | Period | 1.0 s | Watchdog toggle |
| B003 Analog Threshold | On / Off | 195 / 190 °C | High-side fault |
| B003 Analog Threshold | On / Off | −45 / −40 °C | Low-side fault |
| B004 On-Delay | Delay | 0.2 s | Filter settling |
| B007 RS Latch | Retentive | Yes | Hold fault message |
| B002 Up/Down Counter | Retentive | Yes | Fault event log |
| B008 Analog Comparator | Threshold + Hyst. | 10 °C, 2 °C | Drift alarm between two sensors |
All blocks support the "Retentive" attribute on the LOGO! 8 BM (base module with display). Use this for any fault or counter that must survive a power cycle.
Display and HMI Messaging
The LOGO! built-in display can show 8 message texts cycled by the message text block (B013). To meet the requirement of a single alarm-trip event that holds the screen until acknowledged, configure a "Message Text" block with:
- En = M01 (sensor fault) + an acknowledgement bit (a digital input wired to a panel pushbutton)
- MsgText = "PT100 S1 OPEN — CHECK WIRING"
- Acknowledge input = I1 (operator button) — required so the message clears only after the operator has seen it
- Tick frequency = 0 (steady display) or 1 (flashing) depending on alarm priority convention
For panel upgrades, a LOGO! TDE (6ED1055-4MH08-0BA0) gives 6 lines of text per message and supports bar graphs, which is useful when trending the temperature during a fault investigation. External HMIs from Weintek, Pro-face, or the Siemens SIMATIC Comfort Panel can subscribe to the same VW flags over Ethernet and present the same alarm text in a richer graphics environment.
Troubleshooting Matrix
| Symptom | Likely Cause | Action |
|---|---|---|
| AI shows ~−50 °C steady, fault latched low | Short circuit at the terminal block or in the cable | Disconnect the sensor and measure loop resistance; should be 100 Ω at 0 °C |
| AI shows ~200 °C steady, fault latched high | Wire break at the terminal or in the conduit | Tug-test the cable; measure continuity end-to-end |
| AI drifts slowly up over weeks | Corrosion at the terminal or moisture ingress in the thermowell | Pull the sensor, clean the joints, replace the terminal block |
| AI reads correctly but alarm fires randomly | VFD-induced noise on a long cable run | Re-route the cable, add a ferrite, switch to 3-wire mode |
| AI is zero, no alarm fires | AM2 RTD module not seated correctly or base firmware too old | Power down, reseat module, verify firmware ≥ 1.81 (see TIA Portal / LOGO!Soft Comfort device info) |
| Watchdog output Q3 stops toggling but base is powered | User program stopped or in STOP mode | Switch base to RUN via the menu or the LOGO!Soft Comfort online view |
| Modbus master loses contact but Q3 still toggles | Ethernet switch or cable fault | Bypass the switch, ping the LOGO! from a laptop, check the link LED |
Verification Procedure
- Power the LOGO! 8 base and AM2 RTD from a clean 24 V supply; confirm the green LED on the AM2 RTD is steady.
- Short the AI1+ and AI1− terminals with a wire link. The scaled AI should drop to its lower limit and the low-side fault latch should set within 0.5 s.
- Disconnect the wire link entirely. The scaled AI should rise to its upper limit and the high-side fault latch should set within 0.5 s.
- Connect a 100 Ω 0.1 % reference resistor across AI1. The scaled AI should read 0.0 °C ± 0.5 °C and no fault should be latched.
- Replace the reference with a precision decade box set to 138.5 Ω. The scaled AI should read 100.0 °C ± 0.5 °C.
- Pull the AM2 RTD module from the bus while the base is powered. The AI word should saturate high within 1 s, latching the high-side fault. This confirms the limit-based method also catches a module-removal event.
- Verify the watchdog output Q3 is toggling once per second by reading it from LOGO!Soft Comfort in online mode or by wiring it to a panel indicator.
Limitations and Caveats
- The AM2 RTD's −50 °C to +200 °C range is hard-bounded. If the process can briefly exceed 200 °C (a sterilization cycle, for example) the AI will saturate and the high-side fault will latch even though the sensor is healthy. Add a software override that inhibits the fault latch for a defined window during known hot cycles.
- Resolution is fixed by the AM2 RTD firmware. There is no averaging block inside the AM2 RTD; any smoothing must be done in the LOGO! program with an analog filter (B006) or a low-pass on a flag.
- Galvanic isolation between the two RTD channels and the LOGO! base is not provided. If the two sensors sit at different ground potentials (e.g. on different phases of a heater), use isolated signal conditioners before the AM2 RTD.
- This reference applies to LOGO! 8 firmware 1.81.x and later. Earlier firmware versions use different block IDs and the analog filter behavior is different; consult the LOGO! 8 System Manual for the version in use.
Can the LOGO! 8 base module read a PT100 directly without an AM2 RTD?
No. The base module's analog inputs accept 0–10 V or 0–20 mA signals only. A PT100 requires the AM2 RTD (6ED1055-1MD00-0BA2) for excitation and linearization.
Does the AM2 RTD provide a built-in wire-break or short-circuit flag?
No. The module reports a scaled temperature only. Wire break and short circuit must be inferred by comparing the scaled value against thresholds placed just outside the operating range, e.g. >195 °C for open and <−45 °C for short.
How do I detect a LOGO! 8 internal fault or stopped program?
Use a free output configured as a 1 s heartbeat via a clock generator (B001). Wire that output to a remote input, SCADA tag, or Modbus register. If the remote side stops seeing the toggling, the LOGO! is stopped, hung, or unpowered.
What is the recommended threshold hysteresis for PT100 fault detection?
Set the analog comparator hysteresis to at least 2 °C and add a 0.2 s on-delay to ride through the AM2 RTD filter settling. This prevents nuisance trips on electrically noisy cables without delaying genuine faults.
Can the AM2 RTD detect a removed or failed module from the LOGO! program?
Not directly. When the AM2 RTD is pulled from the bus the AI word saturates, so the same high-side threshold that catches a wire break will also catch a missing module. Treat module-removal as a high-side sensor fault in the alarm logic.