Scaling K-Type Thermocouple on S7-1200 SM 1231 in TIA Portal V13

Scaling K-Type Thermocouple on S7-1200 SM 1231 TC Module in TIA Portal V13 SP1

The SM 1231 thermocouple module (order number 6ES7231-5QF30-0XB0) for the SIMATIC S7-1200 returns a digitally scaled integer that already represents the measured temperature in tenths of a degree. Unlike the 0–27648 normalization used by standard analog inputs, thermocouple channels do not require NORM_X / SCALE_X blocks to obtain engineering units. The conversion factor is fixed: Raw Integer = Temperature (°C) × 10 at 0.1 °C resolution. This reference walks through hardware configuration, raw value interpretation, scaling logic for a 0–1370 °C window, negative temperature handling, and verification on the S7-1200 with TIA Portal V13 SP1.

1. Prerequisites

Reference the SIMATIC S7-1200 Programmable Controller System Manual (entry ID 109751326) and the SM 1231 TC module manual for the channel-by-channel diagnostic layout.

2. Hardware Overview

The SM 1231 TC module provides eight isolated differential inputs for thermocouples of types J, K, T, E, N, R, S, and B. Each channel converts the thermoelectric voltage against the internal reference junction (a Pt1000 sensor at the terminal block) and outputs a 16-bit signed integer. Module-specific characteristics:

3. TIA Portal V13 SP1 Hardware Configuration

Insert the SM 1231 TC module to the right of the CPU in the device view. With the module selected, configure each channel in the Properties > Analog inputs inspector pane.

1. Open the project in TIA Portal V13 SP1 and select the S7-1200 station.
2. Drag the SM 1231 TC module from the catalog into the device configuration slot next to the CPU.
3. Click the module, then open Properties > Analog inputs.
4. For each active channel, set:
   • Measurement type: Thermocouple
   • Thermocouple type: K
   • Temperature unit: Celsius
   • Smoothing: Weak (or as required by the process)
   • Reference junction: Internal
   • Diagnostics: Enable wire break, overflow, underflow
5. Compile the hardware configuration and download to the CPU.

4. Analog Value Representation for Type K Thermocouples

For the SM 1231 TC at 0.1 °C resolution, the analog input word (IW) carries the temperature multiplied by 10. The format is a 16-bit signed integer (INT), bipolar over the configured thermocouple range.

This is the same convention used across the ET 200eco PN and ET 200SP TC modules. See the Siemens documentation Representation of Analog Values for Thermocouples for the canonical mapping.

5. Why NORM_X / SCALE_X Are Not Required

The standard NORM_X / SCALE_X ladder blocks assume a 0–27648 (unipolar) or -27648 to +27648 (bipolar) raw range and map it to a user-defined engineering range. TC modules already perform the linearization and scaling in firmware, so applying a second NORM_X / SCALE_X pass double-scales the value and yields nonsense.

Pick one path: either scale in the PLC and send a REAL to the HMI, or send the INT and divide at the HMI tag.

6. Scaling Code for a 0–1370 °C Process Range

If the sensor is bonded to a process that physically spans 0 °C to 1370 °C, the engineering value is simply IW[p] / 10.0. Use SCL or LAD with INT-to-REAL conversion to keep precision.

6.1 SCL Implementation (TIA Portal V13 SP1)

// "Temp_Raw" is an INT mapped to IW96 (first channel of the SM 1231)
// "Temp_Celsius" is a REAL exposed to the HMI
// Channel start address depends on slot position - adjust accordingly

Temp_Celsius := INT_TO_REAL(Temp_Raw) / 10.0;

// Optional: clamp to the 0..1370 process range
IF Temp_Celsius < 0.0 THEN
    Temp_Celsius := 0.0;
ELSIF Temp_Celsius > 1370.0 THEN
    Temp_Celsius := 1370.0;
END_IF;

6.2 LAD / FBD Equivalent

[IW96] --> [INT_TO_REAL] --> [DIV_REAL (10.0)] --> [MD100] "Temp_Celsius"

6.3 Handling Negative Temperatures

The ÷10 conversion is sign-safe because S7-1200 INT is 16-bit two's complement. Division of a negative INT by a positive REAL yields a negative REAL, so 0.1 °C resolution is preserved all the way down to -3276.8 °C (well below the Type K lower limit of -200 °C).

// Read TC channel 0 (raw INT in IW96)
// Convert to engineering units in °C
#Temp_Celsius := INT_TO_REAL(#IW_TC_Ch0) / 10.0;

// Apply process scaling (e.g. 0..1370 °C -> 4..20 mA equivalent at HMI)
#Process_PV := ((
#Temp_Celsius - 0.0
) / (
1370.0 - 0.0
)) * 100.0; // 0..100 %

6.4 Variant: 0.01 °C Resolution (×100)

On modules configured with the higher-resolution mode, replace the divisor:

Temp_Celsius := INT_TO_REAL(Temp_Raw) / 100.0; // 0.01 °C per count

7. Wire-Up and Reference Junction

Thermocouple wire must be of the same calibration type (K-type) all the way to the module terminals. Avoid copper intermediate blocks unless a true isothermal terminal with external compensation is used. Polarity for Type K per IEC 60584-1: positive leg is NiCr (yellow in ANSI, green in IEC), negative leg is NiAl.

• Internal reference: simplest; accuracy ± 1–2 °C near terminal block temperature.
• External reference (channel 0 as Pt100): required when the module sees wide ambient swings or sits away from the process.
• Fixed reference value: use this only when the terminal is held at a known temperature (e.g., oven-mounted terminal block).

8. Diagnostic Bits and Error Codes

The SM 1231 TC raises a diagnostic interrupt (OB82) when configured. The diagnostic address word carries error flags; user program typically polls for wire break before trusting the scaled value.

9. Verification Procedure

1. Online monitor: Go online with the CPU and add %IW96 (or appropriate channel) to a watch table. Observe the integer value.
2. Reference check: Apply a millivolt source equivalent to known Type K temperature (e.g., 4.096 mV ≈ 100 °C for K-type at 0 °C reference). Confirm IW reads 1000 ± a few counts.
3. Ice-bath test: Submerge the TC bead in a 0 °C slush bath; IW should read 0 ± 2 (terminal-strip error band).
4. HMI tag: Confirm the HMI tag is bound to the REAL variable, not the raw INT. If the HMI shows a 10× error, the divisor is wrong or the tag is bound to the wrong symbol.
5. Diagnostic interrupt: Disconnect one TC leg and confirm OB82 fires and the diagnostic bit is set in the module's status word.

10. Troubleshooting Matrix

11. Common Mistakes

• Applying 0–27648 NORM_X. TC modules do not return a 0–27648 count. They return degrees × 10 directly.
• Forgetting the unit suffix. Engineering staff may interpret a value of 213 as 213 °C instead of 21.3 °C. Always label HMI fields with the correct unit and decimal point.
• Using copper wire between TC and module. This introduces a second reference junction. Use K-type extension wire (KX) all the way to the terminals.
• Mismatched smoothing vs. update time. Weak smoothing on a 0.5 Hz filter still updates every 200 ms; verify that the control loop can tolerate the noise floor before disabling smoothing.

12. Migration Notes to TIA Portal V14 / V15

If you migrate the project to V14 or V15, the same scaling logic applies. The only field-level change is the addition of the higher-resolution mode on the 6ES7231-5QF32-0XB0 MLFB. Re-verify the channel properties inspector after migration; the slot address (IW96 in this example) does not change unless the slot position changes.

13. Field Commissioning Checklist

• [ ] Module seated firmly, locking lever down
• [ ] TC wire color code verified against standard
• [ ] Channel configured: Type K, °C, smoothing weak, internal reference
• [ ] Wire-break diagnostic enabled
• [ ] Hardware compiled and downloaded without errors
• [ ] HMI tag bound to REAL, decimal places = 1
• [ ] OB82 attached to diagnostic interrupt
• [ ] Ice-bath or millivolt source calibration recorded