Configuring 6FX2001-5QN25 for 0-359° Per-Turn Position on S7-1200

Overview

The Siemens 6FX2001-5QN25 is a multiturn absolute encoder from the 1FX2/6FX2 product family. The encoder optically resolves one mechanical revolution into 8,192 increments (13 bits) and uses a geared multiturn stage to count 16,384 revolutions (14 bits) before the turn counter wraps. The 27-bit absolute position is then transmitted as PROFINET IO to the controller. Telegram 81 is the simplest Siemens-specific free telegram for this device and is the correct choice when the application only needs the position value with minimum cyclic bus load.

This reference documents how to wire the encoder, configure a SIMATIC S7-1214C, select Telegram 81 in the TIA Portal, and then extract a 0-359° per-turn position in the PLC program. The key transformation is NORM_X followed by SCALE_X, which converts the raw 0-8191 count value to engineering units in degrees. The same code path also works for the S7-1215C and S7-1217C CPUs; the only difference is the larger process image and the availability of a second PROFINET port on those CPUs.

Encoder Identification and Type Plate

The 6FX2001-5QN25 is identified by its Siemens order number (MLFB). The part number decodes the electrical and mechanical options:

Confirm the actual resolution and turn count on the encoder nameplate. Stock units read ST 13 / MT 14; variants with ST 12 / MT 12 or ST 13 / MT 12 exist in the 6FX2001 family and require different scaling values. The values entered in TIA Portal and in the GSDML device description must match the type plate.

For the steps-per-revolution parameter used in SIMATIC technology objects, the value to enter is 8,192 for a stock 6FX2001-5QN25. If the encoder is parameterized to a different resolution through PROFINET record sets, the steps per revolution must be matched in the TIA Portal encoder parameterization. The Siemens Encoder Parameterization for the SIMATIC S7 Technology Functions document lists the full set of supported values (e.g., 512, 1024, 2048, 4096, 8192) and the GSDML record numbers used to write them.

PROFINET Telegram 81 Data Layout

Telegram 81 is a Siemens-specific free telegram designed for absolute encoders with SSI-compatible position data. The telegram carries 4 input bytes (the 32-bit position) and 4 output bytes (control words and preset). Telegrams 82, 83 and 84 extend the layout with additional status information, additional encoders, and speed / acceleration values respectively. Telegram 860 adds PROFIsafe on top of Telegram 82. For a single 6FX2001-5QN25 used purely for 0-359° position, Telegram 81 is the smallest and the most efficient choice.

The exact input address (n) is shown in the device overview of TIA Portal once the encoder is added and Telegram 81 is selected. With default slot assignment, the first slot (slot 0) is the status word / control word interface, and slot 1 carries the 4 bytes of position. The S7-1214C therefore typically places the position value at IW 8 (low word) and IW 10 (high word) of the input process image, which the discussion thread confirms.

Hardware Configuration and Topology

The S7-1214C has one PROFINET port on the CPU (X1P1). Wire the encoder port 1 to the CPU directly, or through a PROFINET switch if you have multiple devices. Set the encoder's PROFINET device name to something descriptive (e.g., enc-6fx2001-001) using the PRONETA tool or the TIA Portal topology editor. Address the encoder with a fixed IP in the same subnet as the CPU (e.g., CPU 192.168.0.1, encoder 192.168.0.10, subnet mask 255.255.255.0).

Use shielded PROFINET cable (e.g., 6XV1840-2AH10) with shield grounded at both ends through the M12 connector. The encoder's M12 D-coded connector (4-pin) carries both data and 24 V supply on a hybrid cable (e.g., 6XV1860-2H).

TIA Portal Device Configuration

1. Open the TIA Portal project that contains the S7-1214C.
2. In the project tree, right-click Devices & Networks and select Add new device → PROFINET IO Device.
3. Search for the GSDML file of the 6FX2001-5QN25 in the hardware catalog. The file is installed by the Siemens HSP (Hardware Support Package) HSP0180 — Encoders 6FX2 / 1FX2 PROFINET, available on the Siemens Industry Online Support portal. After installation, the device appears under Other field devices → Encoder → SIEMENS AG → 6FX2001.
4. Drag the encoder onto the PROFINET subnet of the S7-1214C.
5. Open Device view on the encoder, then open Properties → Module parameters → Slot 1.
6. Select Telegram 81 as the I/O type. The slot layout updates to show 4 input bytes and 4 output bytes.
7. Set the encoder's PROFINET device name to a unique value. The CPU uses the name to find the device at boot via DCP.
8. Assign the encoder to the IO controller (the S7-1214C) and download the configuration.

After the download, the Online & Diagnostics view of the encoder should show a green PROFINET state, and the input bytes should update at the configured send clock rate.

Process Image Address Mapping

Once Telegram 81 is selected, TIA Portal automatically assigns a starting input and output address to the slot. The starting address can be changed in Properties → IO tags of the encoder. With slot 1 set to 4 input bytes + 4 output bytes, the defaults are typically:

Combine IW 8 and IW 10 into a 32-bit DWORD:

// SCL — assemble the 32-bit position
#Position_DWORD := DWORD#16#0000_0000;
#Position_DWORD.%W0 := "Encoder".IW10;   // high word (turns)
#Position_DWORD.%W2 := "Encoder".IW8;    // low word  (counts in rev)

If the S7-1200 is configured in big-endian byte order, the assignment of high and low words is reversed. In the default TIA Portal byte order (little-endian, aligned with the S7-1200 system), the example above is correct.

Extracting 0-359° with NORM_X and SCALE_X

The per-revolution count is the low word IW 8 and ranges from 0 to 8191. Two IEC instructions convert this raw value into engineering units in degrees:

• NORM_X — Normalizes the input to a floating-point value in 0.0 .. 1.0, given the input minimum and maximum.
• SCALE_X — Scales a normalized 0.0 .. 1.0 floating-point value to an output range, given the output minimum and maximum.

Used together, the pair is the standard Siemens way to convert any linear input to any linear output. For a 0-359° output, the parameters are:

The SCL implementation in OB 1 or a cyclic OB:

// SCL — convert IW 8 to 0.0 .. 359.0 degrees
IF "Encoder_OK" THEN
    // 1) normalize raw count to 0.0 .. 1.0
    #norm_x_out := NORM_X(
        VALUE := "Encoder".IW8,
        MIN   := 0,
        MAX   := 8191);
    // 2) scale 0.0 .. 1.0 to 0.0 .. 359.0 deg
    #Angle_deg := SCALE_X(
        MIN   := 0.0,
        VALUE := #norm_x_out,
        MAX   := 359.0);
ELSE
    #Angle_deg := 0.0;
END_IF;

The result #Angle_deg is a REAL in the range 0.0 .. 359.0. The encoder reports a value of 8191 at almost a full revolution, but the mapping to 359° means the last count never produces a full 360°; if the application needs a true 0-360° scale, set the SCALE_X MAX to 360.0 and accept the linear error of one LSB at the top of the range.

Single-Turn vs Multiturn Configuration

If the application does not need the turn count (i.e., the 14-bit multiturn part of the 32-bit word), Telegram 81 still works but the upper 16 bits are simply ignored. The user has three options:

To switch the encoder to singleturn, write a parameter record via PROFINET to the encoder's parameter interface (record index 0x2001 / subindex 0x01 for resolution, 0x2001 / subindex 0x02 for turns). The exact record indices are documented in the GSDML file. The simplest way to write the record in TIA Portal is through a WR_REC call from the user program, or through the Online & Diagnostics page of the encoder in TIA Portal V17 or higher.

Turn Counting and Wrap-Around Handling

The 6FX2001-5QN25 has 14 bits of multiturn resolution, so the high word wraps from 16383 to 0 every 16,384 revolutions. If the application needs the cumulative turn count (e.g., a winding machine that unspools more than 16,384 turns over its lifetime), the PLC must keep a software counter that increments on the wrap and a software counter that decrements on a reverse wrap. The standard pattern in SCL is:

// SCL — track cumulative turns across the 16,384-turn boundary
#Turn_raw := "Encoder".IW10;          // 0..16383
IF #first_scan THEN
    #Turn_soft := 0;
    #Turn_prev := #Turn_raw;
    #first_scan := FALSE;
END_IF;

IF (#Turn_raw - #Turn_prev) > 8192 THEN
    // wrap from 0 to 16383 (backward rotation)
    #Turn_soft := #Turn_soft - 1;
ELSIF (#Turn_prev - #Turn_raw) > 8192 THEN
    // wrap from 16383 to 0 (forward rotation)
    #Turn_soft := #Turn_soft + 1;
END_IF;
#Turn_prev := #Turn_raw;

// Cumulative revolution count
#Total_rev := DINT_TO_LREAL(#Turn_soft) * 16384.0 + LREAL_TO_DINT(#Turn_raw);

The threshold of 8192 is half the multiturn range; this is the canonical Siemens pattern for handling encoder wrap-around in the PLC. The pattern is robust to the encoder's range being reconfigured (e.g., 12-bit multiturn) by changing the modulus to half the configured range.

Verification and Commissioning Checks

1. Watch table — monitor IW 8 in online mode. Rotate the shaft by hand through one full revolution. The value should sweep 0 → 8191 → 0 with no jumps and no missing codes.
2. Watch table — monitor IW 10. Rotate the shaft slowly through 16,384 revolutions. The value should increment monotonically and wrap to 0 at the 16,384th revolution.
3. Trace — record #Angle_deg over one full revolution in a TRACE recording (TIA Portal V17 or higher). The plot should be a clean sawtooth from 0° to 359°.
4. Step response — set a fixed reference angle in degrees, command the encoder to that angle mechanically, and verify that #Angle_deg matches to ± 1 LSB (≈ 0.044°).
5. Power-cycle test — turn off the encoder supply for 5 seconds, then turn it back on. After re-initialization, the value should be the same as before the power cycle to within 1 LSB.

Troubleshooting Matrix

Integration with SIMATIC Technology Objects

For motion applications, the per-revolution position can also feed a TO_PositioningAxis technology object on the S7-1200. The TO expects a steps-per-revolution parameter; the value to enter is 8,192. The encoder parameterization page of the technology object must reference the same PROFINET telegram and slot. With the axis configured in positioning mode, the technology object automatically applies the same NORM_X / SCALE_X logic to expose the position in user units. The Siemens Encoder Parameterization for the SIMATIC S7 Technology Functions FAQ documents the steps-per-revolution field and lists the supported resolutions.

References to Siemens Documentation

The following Siemens Industry Online Support pages and manuals are the authoritative reference for the topics covered in this article:

• Encoder Parameterization for the SIMATIC S7 Technology Functions — official Siemens FAQ, lists the supported steps-per-revolution values and the GSDML record indices.
• SIMATIC S7-1200 Programmable Controller — System Manual, entry ID 109741175 on Siemens Industry Online Support.
• STEP 7 Basic V17 in the TIA Portal — Programming and Operating Manual, entry ID 109751942 on Siemens Industry Online Support.
• PROFINET Field Bus Installation Guidelines, entry ID 49948838 on Siemens Industry Online Support.