Configuring a 4-20mA Pyrometer on Siemens LOGO! 12/24RCE

1. Problem Overview

A 4-20 mA two-wire pyrometer (Optris CSmicro, -50 to +1030 °C span) is being connected to a Siemens LOGO! 12/24RCE base module (MLFB 6ED1052-1MD08-0BA1). The expected process window is approximately 25 °C to 800 °C. The user observes three distinct symptoms that indicate multiple, overlapping configuration faults:

1. With the 500 Ω sense resistor in place, the analog input reports a raw value of 200-210 counts and a calculated temperature of -5 °C to -36 °C even though the process is known to be in the 25-200 °C range.
2. Switching to a LOGO! AM2 expansion module (MLFB 6ED1055-1MA00-0BA2 or the newer 6ED1055-1MA08-0BA1) and configuring the AI for "4-20 mA" still produces a -36 °C display at 210 AI units, even after applying gain = 1.35 and offset = -320.
3. LOGO! Soft Comfort does not visually surface the AM2 in the hardware catalog; the module is only detected at run time on the bus.

Each of these symptoms has an independent root cause, but they interact. The base-module AI is voltage-only (0-10 V) and requires an external shunt; the AM2 AI is the only hardware path that supports current natively. The scaling arithmetic must be re-derived for the actual process range, and the offset error from the parallel input impedance must be removed.

2. Hardware Identification and Specifications

Confirm the exact base module and expansion before applying any scaling rule. The MLFB printed on the front of the device resolves unambiguously to a specific firmware behavior and I/O map.

The LOGO! 8 system manual (Siemens LOGO! 8 System Manual, 05/2023 edition, A5E50157592-AD) is the authoritative reference for AI terminal assignment. The base-module analog inputs I1, I2, I7, and I8 are documented as 0-10 V with input impedance of 72 kΩ. The 0-20 mA and 4-20 mA ranges are not supported on the base module terminals; they require the AM2 expansion.

3. Sensor and Signal Type Confirmation

The Optris CSmicro is available in several output variants. Verify the output type on the device nameplate or the order code (look for "LT02" or "LT15" current-output options). This article assumes the 4-20 mA two-wire variant, where the current loop is the same pair of wires that powers the sensing head.

The sensor's published accuracy and the 4-20 mA transfer function are documented in the Optris CSmicro operating instructions (Optris CSmicro product page → Downloads). The transfer function is linear:

I_out (mA) = 4 + (T - T_min) / (T_max - T_min) × 16

where T_min = -50 °C and T_max = +1030 °C for the full-scale unit.

4. Wiring Topologies

4.1 Option A — Base Module AI with External 500 Ω Shunt (no AM2)

The base module cannot read current directly, so the loop current is converted to a 0-10 V signal by inserting a precision 500 Ω resistor between the sensor return and the LOGO! 0 V reference. The full-scale voltage swing becomes 2 V (4 mA) to 10 V (20 mA), which is within the AI range.

Parallel-resistance offset. The 72 kΩ input impedance of the base module AI is in parallel with the 500 Ω shunt, producing an effective resistance of:

R_eff = (500 × 72 000) / (500 + 72 000) = 496.55 Ω

This pulls the 4 mA end to 1.986 V (instead of 2.000 V) and the 20 mA end to 9.931 V. The 4 mA point is off by 0.7 %, which corresponds to ~7.6 °C of apparent error at the bottom of the -50 to 1030 °C span and explains the initial -5 °C reading. This error is systematic and is removed by adding a small positive offset to the math block (see Section 8).

4.2 Option B — AM2 Expansion Module (4-20 mA Native)

The AM2 measures current directly across an internal 250 Ω sense resistor and provides a 0-1000 (or 0-10 000 on newer firmware) count output that already accounts for the 4 mA zero. Wiring is shorter and the parallel-resistance error does not apply because the AM2 input impedance is part of the calibration.

Configure the AM2 channel in LOGO! Soft Comfort: open the AI block, click Sensor, and select 4..20 mA. The AI block now returns 0 at 4 mA and 1000 at 20 mA. The AM2 is recognized automatically at power-up; it does not appear as a selectable item in the project hardware catalog because LOGO! 8 discovers expansion modules dynamically.

5. Analog Input Scaling Fundamentals

The LOGO! AI block returns a raw integer in counts. For 0-10 V and 0-20 mA ranges, 0 counts corresponds to 0 V / 0 mA. For the 4-20 mA range on the AM2 (and on the base module with the 500 Ω shunt, treated as 2-10 V), the 0-count point corresponds to 4 mA / 2 V, not zero signal. Two common transfer errors result:

• Treating 4 mA as 0 % when the sensor is actually at 0 % of the process range (i.e., 0 °C in a 0-800 °C window is not the 4 mA point of a -50 to 1030 °C sensor).
• Forgetting that gain and offset in the LOGO! AI block are applied to the raw count, not to the engineering unit.

Reference the TI 4-20 mA / 0-10 V transmitter primer (PLC Analog Output and Transmitter Introduction, TI 4-20 mA Application Brief) for the relationship between loop current, burden resistor, and ADC count.

6. Gain and Offset Calculation Procedure

The objective is to convert the AI raw count (0-1000) into the process temperature in °C, using the sensor transfer function over the actual process window (0-800 °C).

6.1 Map process range to mA range

6.2 Solve for gain and offset

The LOGO! AI block computes the displayed value as displayed = gain × raw + offset. Two endpoints must satisfy this equation:

0 = gain × 46.3 + offset
800 = gain × 786.9 + offset

Subtracting: 800 = gain × (786.9 - 46.3) = gain × 740.6 → gain = 1.0802.
Then: offset = -1.0802 × 46.3 = -50.0.

The correct values are gain = 1.08 and offset = -50, not 1.35 / -320. The 1.35 / -320 pair the user applied corresponds to mapping the full -50 to 1030 °C sensor span into a 0-800 °C display window, which is mathematically equivalent to a gain of 800/593 = 1.349 over the visible window. It only yields the correct numeric reading at 0 °C and 800 °C, but every other temperature is biased by the same factor because the offset is applied to the wrong reference. This is exactly why 210 counts produces -36 °C instead of the expected value.

6.3 Diagnostic table for the user's reading

At 210 counts, the sensor is reporting 176 °C. The process is therefore working; the only problem is the gain/offset pair. Re-apply 1.08 / -50 and the reading will resolve to ~177 °C. If the user actually wants a 0-800 °C indication over the full sensor span, gain = 1.349 and offset = -258 produces the same result at the two anchor points, with smaller in-band error.

7. LOGO! Soft Comfort Configuration

1. Open the LOGO! Soft Comfort project, place an Analog Input block (F3) on AI1 of the AM2 (or I7 of the base module, with the shunt).
2. In the AI block properties, set:
   • Sensor = 4..20 mA (AM2) or 0..10V (base module with shunt).
   • Smoothing = 4 (default for noise rejection on a temperature signal).
   • Gain = 1.08, Offset = -50 (or 1.349 / -258 if mapping the full sensor span).
3. Wire the AI output to a Math instruction block (F2 → Math) if you need a non-linear curve, or directly to a display block for verification.
4. Set the AM2 type in the project tree: Tools → Select Hardware → AM2 → Type 4 (4-20 mA mode). This is required even though the module is auto-detected; the type tells the AI block which physical input range to expect.
5. Compile and download (Ctrl+D). After download, monitor the AI value in online mode (Alt+F3) and confirm it moves between 0 and 1000 as the process varies.

8. Math Instruction for Engineering Units Conversion

For users who prefer to keep the AI block at its native 0-1000 counts and do the linearization in a Math block, the equation is:

T (°C) = ((AI - 46.3) × 1080) / 740.6

Implementation in a LOGO! Math block (gain/offset form):

• Operator 1: Ax (the AI count) with gain 1.4574 and offset -67.51.
• Operator 2: unused, set to +0.
• Operator 3: unused, set to +0.
• Priority: Low.

For the Option A wiring (500 Ω shunt on a base module AI), the math must also compensate for the 0.7 % low-end error from the parallel resistance. The compensating offset is:

ΔV_low = 4 mA × (500 - 496.55) = 0.0138 V → 1.4 counts

Add +2 counts (round up) to the offset in the AI block or Math block to cancel this systematic low-end error.

9. Verification and Calibration

1. Loop-current check. Disconnect the brown wire at the LOGO! and insert a multimeter in series. Verify 4.00 mA at ambient with the sensor aimed at a known reference (ice bath = 0 °C, or a blackbody at process temperature). Confirm 20.000 mA at 1030 °C reference if available.
2. AI count check. In online mode, the AI should read 46 at 0 °C and 787 at 800 °C, with gain 1.08 / offset -50. The 1.349 / -258 alternative gives 0 at 0 °C and 800 at 800 °C by definition.
3. Step response. Apply a sudden change (cover the lens with a hand) and confirm the AI smooths over 4-8 samples without overshoot. If the reading oscillates, increase smoothing to 6 or 8.
4. Field reference. Compare the LOGO! reading with a handheld pyrometer or a calibrated thermocouple at three points: low, mid, high. Acceptable agreement is ±2 °C or 0.5 % of span, whichever is greater.

10. Troubleshooting Matrix