Siemens LOGO! 8 PT100 Window Comparator with Spare 0-10 V AQ

Application Overview

A Siemens LOGO! 8 logic module supervises a glycol-cooled bath that combines continuous refrigeration with on-demand resistive heating. The compressor runs constantly; an immersion heater is energized only when the bath must be warmed toward setpoint. The operator enters the temperature setpoint through a counter value, and a PT100 RTD on AI3 of an AM2 RTD expansion module returns the measured bath temperature. A spare 0-10 V analog output (AQ1) on an AM2 AQ expansion is repurposed as a "correct temperature" indicator by driving an external 24 V LED panel through a current-limiting resistor.

The required behavior is a one-degree window comparator that fires AQ1 to 100 % (10.0 V) only when the measured value lies inside a ±1.0 °C band centered on the setpoint. Outside the band, AQ1 must fall to 0 % (0.0 V). The full operating range is -20.0 °C to +30.0 °C, so the logic must accept negative setpoints and remain stable across the zero crossing.

Prerequisites

Before implementing the logic, confirm that the following hardware and software are available and that the installed firmware supports signed counter values.

• LOGO! 8 base module with Ethernet interface, catalog 6ED1052-1xx08-0BA1 (LOGO! 8) or 6ED1052-1xx08-0BA2 (LOGO! 8.3/8.4). Firmware ≥ 8.1.0 is recommended so the counter block supports signed actual values.
• LOGO! AM2 RTD expansion, catalog 6ED1055-1MD00-0BA2, for two PT100/PT1000/Ni1000 inputs. AI3 of the AM2 RTD occupies the third analog word in the process image when the module is fitted in slot 1.
• LOGO! AM2 AQ expansion, catalog 6ED1055-1MM00-0BA2, for two configurable 0-10 V or 4-20 mA outputs. AQ1 occupies the first analog output word when fitted in slot 2.
• PT100 Class A or Class B sensor wired in 3-wire configuration to the AM2 RTD (2-wire and 4-wire are also supported; see the LOGO! 8 manual).
• LOGO! Soft Comfort V8.3 SP1 or later, catalog 6ED1058-0BA08-0YA1, for FBD programming, online monitoring, and program download.
• Operator panel or LOGO! TDE feeding the setpoint counter value, or a local Up/Down counter wired to discrete inputs.
• 24 V LED indicator with series current-limiting resistor sized for the AQ pin: R = (10 V - V_F) / I_F. For a typical 20 mA red LED (V_F ≈ 2.0 V), R ≈ 390 Ω ¼ W is standard.

Hardware Configuration and Wiring

Mount the modules on the DIN rail in the order base-AM2 RTD-AM2 AQ. Slot numbering in LOGO! 8 is left-to-right; the analog I/O tags AI1...AI4 and AQ1, AQ2 populate automatically when the modules are dragged into the project tree at the correct position. Verify by selecting Tools > Select Hardware in LOGO! Soft Comfort and confirming the I/O map matches the physical layout.

Figure 1 — Hardware topology. The base LOGO! 8 receives the operator setpoint and the PT100 sensor signal; AQ1 of the AM2 AQ drives the LED panel.

PT100 wiring on the AM2 RTD:

1. Connect one PT100 lead to terminal M1+.
2. Connect the second PT100 lead to terminal M1-.
3. Connect the third (return) lead to terminal IC1 for 3-wire lead-resistance compensation. For a 2-wire sensor, jumper M1- to IC1.
4. Repeat for the second channel (M2+, M2-, IC2) if a redundant sensor is fitted.

Refer to the LOGO! 8 System Manual for the exact terminal diagram and for 4-wire variants.

Signal Path: PT100 Scaling in the AM2 RTD

The AM2 RTD module performs linearization and lead-resistance compensation internally when configured in LOGO! Soft Comfort. The analog value presented on AI3 is already scaled in 0.1 °C units over the configured measurement range. The default PT100 range on the AM2 RTD is -50.0 °C to +200.0 °C, which fully covers the application's -20 °C to +30 °C window. The 12-bit ADC combined with internal scaling delivers a numeric resolution of approximately 0.07 °C per LSB over the standard range; an effective step of 0.1 °C is exposed to the user program.

Setpoint Source: Signed Counter and VM Mapping

The operator enters the setpoint through a counter block whose actual value (CV) is used as an integer reference. In LOGO! 8 the Counter block (C001...C024) maintains an actual value that can be used directly as a signed setpoint. To support the application's negative temperature range, configure the counter as follows:

If signed counter mode is unavailable on the installed firmware, route the setpoint through an analog marker or VM (Variable Memory) address that supports negative values. LOGO! 8 AM and VM addresses are signed 16-bit (-32768 to +32767):

1. Place a Numeric Input block on the function diagram.
2. Set the data range to -200 to +300 (covering -20.0 °C to +30.0 °C in 0.1 °C units).
3. Bind the value to a VM address via VM mapping.
4. Reference the VM address as the setpoint in the comparator arithmetic.

An operator entry of "20.0" must place the value 200 into the counter or VM; "-5.0" must place -50.

Window Comparator Architecture

The required behavior is a two-sided threshold test on the deviation between setpoint and measured value. The condition for LED-ON is:

|AI3 − Setpoint| ≤ 10 (i.e. ±1.0 °C)

Equivalently, two simultaneous inequalities must hold:

AI3 ≥ (Setpoint − 10) AND AI3 ≤ (Setpoint + 10)

This is a classic analog window comparator. LOGO! 8 does not provide a single absolute-value block, so the dual-comparator + AND architecture is the idiomatic implementation. Each comparator can be tuned independently with its own hysteresis parameter, which makes the logic easy to debug and easy to verify against the requirements table above.

Figure 2 — Window comparator signal flow. AI3 and the counter CV feed two arithmetic blocks that compute the band edges; the analog comparators generate Q01 and Q02, which the AND gate combines into the band flag that drives AQ1.

LOGO! Soft Comfort Implementation

Step-by-step procedure to build the comparator in LOGO! Soft Comfort V8.3 SP1:

Step 1 — Insert the analog input

From the toolbar, drag AI3 onto the function diagram. Right-click and confirm that the input is mapped to the AM2 RTD channel. The tag AI3 is sourced at runtime with the integer value 190 for +19.0 °C.

Step 2 — Insert the counter

Drag a Counter block (special functions palette) onto the diagram. Configure as Up/Down, assign Cnt to a discrete input wired to the operator panel's increment pushbutton, Dir to a second input wired to decrement, and Reset to a third input for clearing the setpoint. Enable signed mode if available; otherwise route the value through VM mapping.

Step 3 — Compute the lower and upper bounds

Place two Arithmetic blocks (AR01 and AR02) on the diagram. Configure each as follows:

The constant 10 represents the 1.0 °C band width in 0.1 °C units. To change the band width, replace 10 with the equivalent integer (5 for 0.5 °C, 20 for 2.0 °C).

Step 4 — Insert the analog comparators

Place two Analog Comparator blocks from the special functions palette:

The hysteresis parameter prevents output chatter when AI3 dithers around the band edge. With ±0.3 °C hysteresis, Q01 rises when AI3 crosses Setpoint − 10 + 3 = Setpoint − 7 (i.e. +18.7 °C when setpoint is +20.0 °C) and Q02 falls when AI3 crosses Setpoint + 10 + 3 = Setpoint + 13 (i.e. +21.3 °C when setpoint is +20.0 °C).

Step 5 — AND the comparator outputs

Place an AND block (B003 in the basic functions palette). Connect Q01 to its first input and Q02 to its second input. The output Q03 is the boolean band flag.

Step 6 — Drive AQ1 from the boolean flag

The AM2 AQ expects an analog value in the range 0 to 1000 (corresponding to 0 % to 100 % of the configured output range, i.e. 0-10 V). The boolean Q03 must be converted to either 0 or 1000. Two methods are available.

Method A — Arithmetic block conversion (recommended):

1. Place an Arithmetic block AR03 with Function = Multiplication (A × B).
2. Input A = Q03 (interpreted as 0 or 1).
3. Input B = 1000 (constant).
4. Gain = 1, Offset = 0.
5. Output AM03 = 0 or 1000.
6. Open AQ1 properties in the project tree. Set the reference source to AM03.

Method B — Direct digital flag mode:

1. Open AQ1 in the project tree and select Use digital flag (0 / 100 %).
2. Choose Q03 as the flag source.
3. Verify the firmware release notes if the option does not appear — it is supported in LOGO! Soft Comfort V8.2 and later.

With either method, AQ1 produces 10.0 V when Q03 = 1 (within band) and 0.0 V when Q03 = 0 (outside band).

Boolean-to-Analog Conversion for AQ1

A common stumbling block is that an AQ pin cannot be driven directly by a boolean flag. The AQ channel always interprets its reference as an integer in the range 0-1000 representing 0 %-100 % of the configured physical range. Skipping the boolean-to-analog conversion is the most frequent reason for an unexpected mid-range voltage at the AQ pin (e.g. AQ1 stuck at 5 V).

The recommended conversion path is the Arithmetic block with Function = Multiplication, Gain = 1, Offset = 0, Input A = Boolean flag (0 or 1), Input B = 1000. This produces AM = 0 when the flag is low and AM = 1000 when the flag is high. Assign AQ1 to AM. Method B (the digital flag mode of AQ1) achieves the same result internally and is preferred in LOGO! Soft Comfort V8.2 and later.

Hysteresis and Anti-Chatter Tuning

The window comparator architecture provides two independent opportunities for hysteresis. Each analog comparator block has a configurable hysteresis parameter; a typical setting is ±0.3 °C (3 in 0.1 °C units). Increasing the hysteresis:

• Reduces LED flicker near the band edges by widening the gap between the rising-edge trigger and the falling-edge trigger.
• Widens the effective "ON" region in the temperature space (the LED stays lit across a slightly wider band).

For a glycol bath that drifts at < 0.05 °C/s, hysteresis of 0.2 °C to 0.5 °C is appropriate. Faster-changing systems (e.g. high-mass reactors with rapid heating) may need up to ±1.0 °C, but excessive hysteresis defeats the purpose of the indicator. The 4-sample AI3 smoothing option in the AM2 RTD configuration complements the hysteresis and should be enabled in noisy environments.

When adjusting hysteresis, view the comparator outputs Q01 and Q02 independently in the LOGO! Soft Comfort online watch window. If Q01 or Q02 oscillates faster than 1 Hz when AI3 is near a band edge, increase that comparator's hysteresis until the oscillation stops. A clean transition is mandatory for an operator-visible indicator.

Verification and Commissioning Procedure

Once the program is downloaded to the LOGO! 8, perform the following commissioning sequence. Each step is mandatory; do not sign off the system until every step passes.

1. Online watch setup. Connect LOGO! Soft Comfort to the running LOGO! via Ethernet. Open Online > Watch and add tags AI3, AM01, AM02, Q01, Q02, Q03, and AQ1. Verify the values change in real time as the counter is adjusted.
2. Force setpoint +20.0 °C (counter CV = 200). Manually heat or cool the bath and verify the LED transitions at the expected boundaries: AI3 < +18.7 °C → LED OFF; +18.7 °C < AI3 < +21.3 °C → LED ON; AI3 > +21.3 °C → LED OFF. Allow several minutes for the bath to settle across each boundary.
3. Measure AQ1 with a multimeter: 9.95 V to 10.05 V when the LED is on, < 50 mV when the LED is off. If AQ1 shows an intermediate voltage, the AM03 reference is incorrect or Q03 is floating.
4. Negative range test. Force counter CV = -50 (representing -5.0 °C). Verify AM01 = -60 and AM02 = -40 in the online watch. Adjust the bath temperature to -5.0 °C and confirm the LED is on; adjust to -6.5 °C and confirm the LED is off.
5. Edge chatter test. Bring the bath temperature slowly toward the lower band edge at < 0.02 °C/s. Verify the LED transitions cleanly without flicker. If chatter occurs, increase both comparators' hysteresis to 0.5 °C (5 units) and re-test.
6. Sensor fault test. Disconnect the PT100 from the AM2 RTD terminals. LOGO! AI3 should report an out-of-range value (typically -1000 or +10000). AQ1 must drop to 0 V. If AQ1 stays at 10 V, the comparator logic is not evaluating correctly during the fault; verify that the out-of-range value is outside the AM01-AM02 band.

Document each step's result in the commissioning log. The LOGO! web server (built into firmware ≥ 0BA8) can be enabled to allow remote monitoring of AI3 and AQ1 from any browser at http://<logo-ip>; use this for unattended verification.

Edge Cases: Negative Temperatures and Sensor Faults

When the setpoint is negative, the lower-bound arithmetic Setpoint − 10 is also negative, and AI3 returns signed integers. LOGO! 8 arithmetic blocks treat signed inputs correctly as long as the firmware is ≥ 8.1.0. Verify by entering test setpoints of -50, 0, and +200 and confirming the comparator outputs behave as expected.

For setpoints near zero (e.g. -5 °C and +5 °C are both legitimate), the band crosses the zero point. Test that AR01 and AR02 produce the correct signed results (e.g. setpoint = -50, AM01 = -60, AM02 = -40). Use LOGO! Soft Comfort's Online > Watch view to step through the values.

When the PT100 sensor fails open or short, the AM2 RTD drives AI3 to its rail value (typically -1000 or +10000). Both comparators will report false, Q03 falls, and AQ1 drops to 0 V. The behavior is fail-safe (LED OFF) by construction. To add a dedicated sensor-fault alarm, wire AI3 to a third analog comparator with Ay = -50 °C and Ay = +200 °C band edges; a fault outside the sensor range raises the alarm flag.

Alternative Architectures

Single comparator with absolute deviation

Some engineers prefer to compute the absolute deviation |AI3 − Setpoint| with a single chain and feed it to one comparator with a threshold of 10. This requires two arithmetic blocks, an absolute-value trick, and an analog multiplexer:

1. AR01: A − B = AI3 − Setpoint → AM01 (signed).
2. AR02: B × Gain = AM01 × -1 → AM02 (negated).
3. MUX01: select max(AM01, AM02) → AM03 (the absolute value).
4. AC01: AM03 ≤ 10 → Q01 (band flag).
5. Drive AQ1 from Q01 via Method A or B above.

This uses four blocks instead of two comparators + AND (five blocks), but the logic is easier to read for engineers unfamiliar with LOGO!'s analog comparator blocks. It also loses the independent per-edge hysteresis, which is its main drawback.

PWM-style proportional indication

If a proportional indication is desired (e.g. LED brightness proportional to deviation), use LOGO!'s PWM function block instead of an analog AQ. Connect the comparator output to the PWM enable input and the deviation magnitude (AM03 from the architecture above) to the PWM duty-cycle input. The LED brightens as the bath temperature approaches setpoint and dims as it drifts away.

Networked variant (LOGO! 8.3 / 8.4)

If the LOGO! is networked (firmware 0BA2 with Ethernet), expose AI3, AQ1, and Q03 as network variables. A remote SCADA can then read the actual temperature, the band flag, and the analog output simultaneously. Refer to the LOGO! 8.3 Upgrade Notes for the network variable configuration procedure.

Troubleshooting Matrix

Related Documentation

• LOGO! 8 System Manual — primary reference for base modules, AM2 RTD, AM2 AQ wiring, and parameter ranges.
• LOGO! 8.3 Upgrade Notes — adds the digital-flag mode for AQ and the network variable improvements used in the networked variant.
• LOGO! Soft Comfort V8.x product page — download, release notes, and training materials for the FBD programming environment.
• IEC 61131-3:2013 — Programmable controllers, Part 3: Programming languages. Defines the comparator and arithmetic function blocks standardized across PLC vendors and used by LOGO!.
• IEC 60751:2008 — Industrial platinum resistance thermometers. Defines the PT100 resistance-temperature characteristic implemented internally by the AM2 RTD module.
• Wikipedia, Setpoint (control system) — overview of positive and negative error definitions and the convention used for band detection.