Siemens LOGO! 8 4-20mA Sensor Fault Detection and OT Reset

1. Application Overview

The Siemens LOGO! 8 logic relay (6ED1052-xxx08-0BA8) is a compact controller used in small pumping, HVAC, and process skids. This reference documents three integration tasks for a pump station with a 4–20 mA pressure transducer (0–10 BAR range) and an integrated operating time counter:

• Analog fault detection — fire a text message on the LOGO! display when the loop current falls below the sensor's valid range.
• Operating time (OT) reset — zero the maintenance counter from the LOGO! built-in display when the pump is replaced, while keeping a separate total-hours counter intact.
• Alarm display — present fault states to the operator with clear, unambiguous visual feedback.

For sensor break and out-of-range detection, the convention adopted across process instrumentation is NAMUR NE43, Standardization of the signal level for the failure information of digital transmitters. NE43 partitions the 4–20 mA span into valid measurement, out-of-range, and failure-information bands. The thresholds chosen in this article respect those bands so that the LOGO! fault logic is interoperable with field instruments from any manufacturer.

2. Sensor Scaling Mathematics

Mapping a 4–20 mA pressure transducer to 0–10 BAR is a linear transform. Use the formulas below as the design baseline before configuring any LOGO! block.

2.1 Linear transfer function

For a current I in mA at the LOGO! analog input, the engineering value P in BAR is:

P [BAR] = (I [mA] − 4) × (P_max − P_min) / (I_max − I_min) + P_min

With P_min = 0 BAR, P_max = 10 BAR, I_min = 4 mA, I_max = 20 mA:

P = (I − 4) × 10 / 16 = (I − 4) × 0.625

2.2 Internal LOGO! AI scaling

The LOGO! 8 analog input block reports the measured current on an internal scale of 0 to 1000 units, where 0 corresponds to the configured zero point (0 mA in 0–20 mA mode, 4 mA in 4–20 mA mode) and 1000 corresponds to the configured span (20 mA). The relationship is:

AI_units = (I − I_zero) × 1000 / (I_full − I_zero)

For 4–20 mA mode (I_zero = 4, I_full = 20):

AI_units = (I − 4) × 62.5

To produce a fault at 3 mA:

AI_units(3 mA) = (3 − 4) × 62.5 = −62.5

Round to −62 to keep the threshold just above the noise floor of the AM2 input stage. The Analog Threshold Trigger block (B002) compares the AI value against this fixed offset and sets its output high when the input is less than or equal to the threshold.

3. Configuring the Analog Input Block

Open LOGO! Soft Comfort V8.x or later and add the Analog Input (AI) block from the analog function group. The parameter dialog contains the following fields:

To display the value directly in BAR without a downstream amplifier, change the Gain so that 1000 AI units = 10 BAR. With Gain = 0.01 the output reads in BAR. With Gain = 1.0 the output reads in AI units and an additional Analog Amplifier (B001) maps it to the engineering range. The choice depends on which downstream block uses the value first.

Reference: see the Siemens LOGO! 8 (0BA8) System Manual, edition 06/2022 for AM2 input ranges and parameter limits. Additional analog block reference data is in the LOGO! 8 application examples KB article.

4. Setting the Fault Threshold

The Analog Threshold Trigger (B002 in this design) implements the under-range detection. Place a comparator block on the AI output and configure the threshold (On) and hysteresis (Off) parameters carefully to avoid chatter when the loop current hovers near the trip point.

4.1 Threshold and hysteresis table

The hysteresis band of 12 AI units (≈0.192 mA) is wide enough to absorb AM2 quantisation noise of ±2 units yet narrow enough to release the fault within one scan cycle after a healthy signal returns. Always test the hysteresis with a 4–20 mA calibrator (Beamex MC6, Druck DPI 620, or equivalent) sweeping the loop slowly across the trip point.

4.2 NAMUR NE43 conformance

NE43 specifies that valid measurements occupy 4 to 20 mA, with out-of-range information above 20 mA and below 4 mA. A LOGO! threshold of −62 units (3.008 mA) is conservative and triggers a fault inside the NE43 sensor-failure band of ≤ 3.5 mA. Adjust the threshold upward to −25 units (3.6 mA) if the application requires NE43 out-of-range detection at 3.8 mA — the user's tolerance for nuisance trips should drive the choice.

5. Operating Time Counter Reset

The Operating Hours Counter block (B016 / B019 in the LOGO! function library) accumulates runtime for maintenance scheduling. The block exposes two inputs (R and Ral) and two outputs (OT and AL) that can be wired to a Message Text (B017) and to maintenance alarms.

5.1 Block terminals

5.2 Resetting OT from the LOGO! display

The LOGO! 8 base module with built-in display (6ED1052-1MD08-0BA1, -1BB08-0BA1, etc.) has a dedicated menu for the operating hours counter. The procedure to zero the OT field while preserving the cumulative total is:

1. Use the ◀ / ▶ cursor keys to navigate to the message text that displays OT (B017 in the project).
2. Press OK to enter edit mode; the cursor jumps to the "R" connector marker on the screen.
3. Press the OK button a second time to fire the reset pulse on the R input. OT returns to 0:00, the alarm (AL) is cleared if the service interval was reached, and the cumulative hours remain on a separate Message Text that the maintenance log reads.

The R pulse is one LOGO! scan long (typically 50 ms depending on program length). It is not latching, so there is no risk of the counter being held at zero after a power cycle.

5.3 Resetting OT via a digital input (alternative)

For a remote reset, wire a momentary pushbutton to a digital input (e.g., I5) and route I5 to the R input of B016 through an OR block. Add a second OR with a discrete flag (M8) that the operator can toggle from the LOGO! menu under "Parameter assignment" to enable or disable remote resets. This prevents accidental resets by a curious operator on a remote HMI.

6. Alarm Message Configuration

The Message Text (B017) block is the LOGO!'s primary operator interface. Place a text block per fault condition. Each block has a "Display" parameter that selects where the message appears: on the built-in LCD, on the LOGO! TDE, or both. A second parameter, "Acknowledge", controls whether the message must be acknowledged with a key press to clear.

6.1 Fault table design

6.2 Red background / "red screen" capability

The LOGO! built-in display on 0BA8 base modules cannot render red text or a red background. The amber/white LCD does not support colour. The "red screen" effect — black text on a red field — is exclusive to the LOGO! Text Display External (TDE) module, order numbers:

• 6ED1055-4MH00-0BA0 — LOGO! TDE (6-line, red-capable on alarm)
• 6ED1055-4MH08-0BA0 — LOGO! TDE successor variant with updated firmware

When the message text is enabled (EN = 1) and the Acknowledge (Ack) parameter is set to "high active", the TDE renders the message with a red background and the bell icon. The same block appears as black text on the standard white/amber LCD of the base module, so the user always sees the fault — only the cosmetic emphasis differs.

7. Wiring and Electrical Considerations

The AM2 analog input module accepts passive 2-wire and active 3/4-wire transmitters. For a 2-wire pressure sensor, the AM2 provides 24 V loop power; for 4-wire sensors, the loop is powered by the sensor's own supply and the AM2 only measures the current. The wiring diagrams in the LOGO! 8 system manual show the terminal assignments for each case.

For cable runs longer than 10 m, use shielded twisted pair (e.g., Belden 8760, Lapp Ölflex 110 CY) and ground the shield at the panel end only. The AM2 input has a common-mode range tied to M; do not float the panel ground or the AI reading will be unstable. The maximum loop resistance is 600 Ω at 24 V supply (per the AM2 data sheet); verify that the sum of cable resistance plus sensor burden does not exceed this limit. Reference: LOGO! 8 System Manual, Section 4.6 (Wiring of the Analog Inputs).

8. LOGO! 8 Hardware Variant Selection

Not all 0BA8 base modules expose onboard analog inputs. Confirm the part number before assuming an AI is available without an AM2 expansion:

The onboard AI channels of the base module are 0–10 V only. A 4–20 mA sensor must be wired through an AM2 (6ED1055-1MA00-0BA2) or AM2 RTD (6ED1055-1MA00-0BA7) module. The 4–20 mA mode is selected per-channel in the AI block configuration. The AM2 supports two independent 4–20 mA loops; the AM2 RTD supports one 4–20 mA plus two PT100/PT1000 inputs.

9. Complete Program Skeleton

The following list summarises the blocks of a working program for this application. Block numbering follows the convention used in the original project; substitute local tags as needed. The complete FBD is small enough to review in a single screenshot — roughly 12 blocks total.

Use OR gates to combine all fault flags (M1, M2) and route the result to a single message text that prioritises by sequence order. The Message Text block priority is set under "Properties → Priority" with 1 being highest. If multiple messages are active, the highest-priority one takes the display.

10. Verification and Commissioning Procedure

Walk through the following checklist before handing the panel over to operations.

1. Continuity check. With the LOGO! powered off, measure resistance between the AM2 input terminals. A healthy 2-wire sensor loop reads 50–500 Ω depending on model. An open loop reads > 1 MΩ and confirms the wiring is intact (but not the polarity).
2. Live current check. Power up, break the loop at the AM2 input, insert a multimeter in mA mode. The reading should match the process pressure within the sensor's stated accuracy class. For a 5.0 BAR measurement: I = 4 + 5.0 × 1.6 = 12.0 mA.
3. Threshold verification. With the calibrator injecting 3.5 mA, the LCD should display the sensor-fault message. At 4.0 mA the message should clear. The hysteresis between On and Off should make the transition clean, not flickering.
4. OT counter test. Force the pump output Q1 on from LOGO! Soft Comfort online mode. The OT should increment every minute. Apply I5 (or navigate the display to R) and verify that OT zeroes but the cumulative counter B020 remains.
5. Power-cycle test. Power down the LOGO! for 30 seconds. On reboot, the OT and the fault latches should retain their last state (LOGO! retains retentive flags and counter values by default).
6. TDE-only test. If a TDE is installed, replicate the fault and confirm that the red background appears. Disconnect the TDE cable and confirm that the message is still shown on the base module display (in black).
7. EMC spot check. Energise a 5 kW VFD on the same panel bus and watch the AI value on the LOGO! Soft Comfort online monitor. The reading should not deviate by more than ±2 AI units; anything larger suggests a grounding or shield-termination issue.

11. Troubleshooting Matrix

12. Frequently Asked Questions