Siemens S7 Analog INT to REAL: FC105, I_DI, DI_R Conversion

1. Overview

Converting an analog input from a Siemens SIMATIC S7 PLC into a usable REAL engineering value is a routine task, but the terminology in the STEP 7 help, the TI-S7 library, and the STL instruction set often confuses new programmers. The original question asked which SFC (System Function) or SFB (System Function Block) performs the conversion from analog INT to REAL. The short answer is: none. Type conversion in S7 is handled by elementary bit/word instructions (ITD/I_DI, DTR/DI_R) or by the standard TI-S7 converting block FC105 "SCALE" and its inverse FC106 "UNSCALE". SFCs and SFBs are reserved for operating-system services (time, diagnostics, communication) and are not the right tool for a numeric cast.

This reference covers the conversion path on the SIMATIC S7-300 using the analog module SM 334; AI4/AO2 x 8/8Bit (order number 6ES7 334-0CE00-0AA0) described in the Siemens Industry Online Support manuals, but the same instructions apply to S7-400, S7-1200 (TIA) and ET 200 distributed I/O. The full conversion chain, scaling mathematics, bipolar handling, overrange behaviour, wire-break detection, the special PT100/PT1000 case, and a commissioning checklist are presented below.

2. Module Identification: SM 334 AI4/AO2 x 8/8Bit (6ES7 334-0CE00-0AA0)

The module 6ES7 334-0CE00-0AA0 is a compact, low-cost S7-300 analog I/O module intended for non-precision applications. Its relevant specifications from the official SIMATIC S7-300 module data manual are:

3. Analog Value Representation in SIMATIC S7

All SIMATIC analog modules return a 16-bit signed integer count that is mapped onto a normalised range. The mapping is documented in the SIMATIC S7-300 Module Data reference manual and in the online help of STEP 7. The relevant tables are reproduced below.

The overrange headroom (27648 to 32511) is reserved for diagnostics: any count above 27648 indicates that the real physical signal has exceeded the nominal range but is still within the diagnostic range. Counts of 32767 (0x7FFF) or 0x8000 (-32768) are reserved as fault indicators and are explained in Section 9.

4. SFC vs SFB vs FC vs FB: Clarifying the Misconception

STEP 7 distinguishes four function-block categories. Mixing them up is a common source of confusion.

For a pure type conversion (INT → DINT → REAL) there is no SFC/SFB; the conversion is performed by elementary bit/word instructions found in every S7 CPU's instruction set. When a scaled engineering value (for example 0.0 to 100.0 % or 4.0 to 20.0 mA) is required, the standard library TI-S7 Converting Blocks provides FC105 SCALE and FC106 UNSCALE. The library is shipped with STEP 7 V5.x and with the TIA Portal, and the source is documented in the Siemens Standard PID Control and FM 350-1/-2 Counter Module reference manuals.

5. Type Conversion Path: I_DI and DI_R

The elementary path from a 16-bit process input word (PIW) to a 32-bit floating-point value uses two instructions, executed either in STL, LAD or FBD.

1. ITD / I_DI – convert INT (16-bit, -32768 to +32767) to DINT (32-bit, -2147483648 to +2147483647). The sign is extended into the upper word.
2. DTR / DI_R – convert DINT (32-bit signed integer) to REAL (IEEE-754 32-bit float). The conversion is exact because the DINT range is a subset of REAL.

STL implementation, reading channel 0 of the SM 334 on slot 4:

// Read the raw 16-bit analog value
      L     PIW 256                // INT in accumulator-1

// Step 1: INT -> DINT (I_DI / ITD)
      ITD                           // Accu1 = DINT, sign-extended

// Step 2: DINT -> REAL (DI_R / DTR)
      DTR                           // Accu1 = REAL

// Store the unscaled REAL value
      T     MD     100             // RealVal_RAW at MD100 (REAL)

LAD/FBD equivalent using the converter palette:

 |---[ PIW256 ]---[I_DI]--[DI_R]--( MD100 )---|

The value stored in MD100 is now the exact numerical representation of the raw count, for example 2.345000e+003 if the count was 2345. No scaling to engineering units has been performed. This value can be displayed, logged, or used as the input to further arithmetic, but it is rarely the final engineering result.

6. Engineering-Unit Scaling: FC105 "SCALE"

To obtain a value in engineering units (psi, °C, %, RPM, etc.) the raw count must be linearised. The TI-S7 function block FC105 SCALE applies the standard Siemens conversion formula:

OUT = [ ((FLOAT (IN) - K1) / (K2 - K1)) * (HI_LIM - LO_LIM) ] + LO_LIM

For a unipolar range (BIPOLAR = FALSE) the constants are:

• K1 = 0
• K2 = 27648

For a bipolar range (BIPOLAR = TRUE) the constants are:

• K1 = -27648
• K2 = +27648

FC105 is a pure FC (no instance DB). The call interface is:

STL call example, reading the 0 to 10 V input on channel 0 of the SM 334 and scaling to 0.0 to 100.0 %:

      CALL  FC   105
         IN       :=    PIW256            // raw count
         HI_LIM   :=    1.000000e+002     // 100.0
         LO_LIM   :=    0.000000e+000     // 0.0
         BIPOLAR  :=    FALSE
         RET_VAL  :=    MW20              // 0000 = OK, 8000 = clip
         OUT      :=    MD30              // scaled REAL (0.0 to 100.0)

A 4-20 mA input scaled to 0.0 to 250.0 °C (typical RTD transmitter output) is scaled by setting LO_LIM = 0.0, HI_LIM = 250.0, BIPOLAR = FALSE; the formula compensates internally for the 4 mA live-zero offset because K1 = 0 corresponds to 0 % of the input range (which is 4 mA on a 4-20 mA transmitter).

7. Reverse Scaling: FC106 "UNSCALE"

To drive an analog output (e.g. the two AO channels of the same 6ES7 334-0CE00-0AA0) a scaled REAL value must be converted back into the raw count that the module expects. FC106 UNSCALE applies the inverse formula:

OUT = [ ((IN - LO_LIM) / (HI_LIM - LO_LIM)) * (K2 - K1) ] + K1

With LO_LIM = 0.0, HI_LIM = 100.0, BIPOLAR = FALSE the block returns the integer count that the SM 334 expects for a 0 to 10 V output, which the program then writes to PQW 256.

      CALL  FC   106
         IN       :=    MD50              // setpoint in %
         HI_LIM   :=    1.000000e+002
         LO_LIM   :=    0.000000e+000
         BIPOLAR  :=    FALSE
         RET_VAL  :=    MW22
         OUT      :=    PQW256            // 0 to 27648 -> 0 to 10 V

8. Bipolar vs Unipolar Configuration

The BIPOLAR parameter of FC105 must match the hardware configuration of the channel in HW Config (STEP 7) or in the device configuration of TIA Portal. The SM 334 (6ES7 334-0CE00-0AA0) is unipolar only (0 to 10 V or 0 to 20 mA). For bipolar signals a different module from the SM 334 family or the SM 331/332 family must be used; in that case BIPOLAR = TRUE selects K1 = -27648 and K2 = +27648. Mixing the parameter with the hardware range is the single most common cause of a factor-of-two or sign-inverted output.

9. Error Codes and Diagnostics

Analog modules signal abnormal conditions by returning specific diagnostic counts. These counts must be filtered before scaling, because feeding 32767 into FC105 will produce nonsense engineering values.

The fault indicator is the same for all SIMATIC analog modules in the S7-300 family, as documented in the SIMATIC S7-300 Module Data manual. A typical guard block:

      L     PIW 256                  // raw count
      L     32767                    // compare to 0x7FFF
      >I                              // ACCU2 > ACCU1?
      JCN   OK1
      S     M    10.0                 // set "Overflow / Wire Break" flag
      JU    DONE
OK1:  L     -32768
      <I
      JCN   OK2
      S     M    10.1                 // set "Underflow / Open Wire" flag
      JU    DONE
OK2:  CALL  FC   105
         IN       :=    MW10
         HI_LIM   :=    1.000000e+002
         LO_LIM   :=    0.000000e+000
         BIPOLAR  :=    FALSE
         RET_VAL  :=    MW12
         OUT      :=    MD30
DONE:  NOP   0

10. PT100 and Temperature-Scaling Special Case

When a temperature input channel is configured as a thermocouple type (PT100, PT1000, Ni1000, type J/K, etc.) in HW Config, the module no longer returns a normalised count. Instead, it returns the temperature multiplied by ten (PT100, type K, type J) or by one (PT1000 with 0.1 ° resolution on newer modules). This behaviour is documented in the chapter "Representation of the Analog Values of Resistance Thermometers and Thermocouples" of the S7-300 module data manual.

Do not pass a PT100 raw count to FC105 SCALE with LO_LIM = 0, HI_LIM = 100. The block will treat the count as a percentage and produce wrong temperatures. Use the explicit division path instead:

      L     PIW 256                  // e.g. 2345 (= 234.5 °C)
      ITD                            // I_DI
      DTR                            // DI_R  -> 2345.0
      L     1.000000e+001
      /R                             // divide by 10.0
      T     MD 200                   // REAL: 234.5 °C

11. SFC51, SFB52 and SFB54 – Reading Module Diagnostics

When the analog value is 32767, it is often useful to read the diagnostic record of the module to identify the root cause. Siemens provides three system calls for this:

A diagnostic interrupt for the SM 334 will deposit information about the failing channel (e.g. "Channel 0 wire break", "Channel 2 overrange") in the diagnostic data record DS0/DS1. The S7-300/400 standard and system functions reference describes the byte layout. If a recurring OB82 is observed and the count is 32767, the diagnostic record distinguishes wire break from overrange from short circuit, allowing the HMI message to be specific.

12. Custom FC/FB with Overflow and Wire-Break Alarms

For applications that require more than the standard FC105 provides, a wrapper FB is recommended. The wrapper:

1. Traps 32767 and -32768 before scaling.
2. Traps overrange (counts between 27649 and 32766).
3. Traps underrange (counts between 1 and 0; only for ranges that do not include zero, e.g. 4-20 mA).
4. Applies FC105 to the validated range.
5. Exposes the scaled value, the limit flags, the raw count, and a quality byte to the HMI.

The data structure for the instance DB of such an FB on a S7-300 (S7-300 supports 8-bit quality field and BOOL flags packed in byte):

DATA_BLOCK DB200 // AI scaling FB instance
STRUCT
   Quality       : BYTE;     // 0=Good, 1=Overflow, 2=Underflow, 3=WireBreak, 4=Bad
   Overflow      : BOOL;     // M100.0 alias in DB
   Underflow     : BOOL;
   WireBreak     : BOOL;
   Bad           : BOOL;
   RawCount      : INT;      // Mirror of PIW
   ScaledValue   : REAL;     // Result of FC105
   Hi_Lim        : REAL;     // Pass-through
   Lo_Lim        : REAL;
END_STRUCT
END_DATA_BLOCK

Calling the FB once per channel inside OB1 (or in OB35 for cyclic execution) consolidates the alarm logic in one place and avoids scattering limit checks across the program.

13. Step-by-Step Implementation Guide

1. Configure the module in HW Config. Drag the SM 334 (6ES7 334-0CE00-0AA0) to its slot, set the input range (0 to 10 V or 0 to 20 mA), enable diagnostic interrupt if the CPU supports OB82, and download the hardware configuration.
2. Open the symbol table and assign symbolic names to the PIW and PQW addresses (e.g. AI_Ch0_Raw for PIW 256).
3. Install the TI-S7 Converting Blocks library. In STEP 7 V5.x: File → Open → Libraries → TI-S7 Converting Blocks; copy FC105 and FC106 into your S7 program container.
4. Create the instance DB for your custom scaling FB (Section 12) or use global flags/MWs.
5. Insert the call in OB1 (or in a cyclic OB such as OB35 for 100 ms update). Wire the input from PIW 256 and the limits from constants or operator-adjustable data blocks.
6. Add the fault guard from Section 9 around the FC105 call.
7. Compile and download the program to the PLC.
8. Verify online using Monitor/Modify in STEP 7: force a known input (e.g. 5 V from a calibrator) and confirm the scaled REAL matches expectations.
9. Document the channel in the HMI tag list with engineering unit, range, and quality mapping.

14. Verification Checklist

15. Troubleshooting Matrix

16. Performance, Cycle Time and Ordering Notes

The elementary ITD + DTR path consumes roughly 2 µs on a CPU 315-2 PN/DP and 1 µs on a CPU 319. FC105 consumes approximately 8 µs on a CPU 315 and 4 µs on a CPU 319, depending on firmware version (FW ≥ V2.6 recommended; see Siemens support entry 109751634 for the documented performance). For a 4-channel module processed in OB1 the additional cycle-time burden is therefore negligible (less than 50 µs even on a CPU 312 IFM).

When ordering spare parts or replacements, be aware that the 6ES7 334-0CE00-0AA0 is a discontinued product. The current equivalent with 8-bit resolution and 4 AI/2 AO is the 6ES7 334-0CE01-0AA0 (ES7 334 module data), and for higher precision the 6ES7 334-0KE00-0AB0 (12-bit) is the recommended replacement. The conversion procedures above apply unchanged to the successors; only the input wiring and the LSB step size change.

17. Frequently Asked Questions