S7-1200 HSC for Hengstler Encoder: Fixing Zero Count and Wiring

1. Problem Statement

An Siemens S7-1214C is wired to a Hengstler RI58-O/5000EK.42KB incremental encoder (5000 pulses per revolution) on HSC1. The configuration in TIA Portal uses:

• Type of counting: Count
• Operating phase: A/B counter (also tried A/B counter fourfold)
• Counting direction: Count Up
• Wiring: channel A → I0.0, channel B → I0.1

The application reads the HSC value through input address %ID1000 and compares it to 500 in OB1 to set Q0.2. When the encoder shaft is rotated, the tag does not increment through the expected 5000/20000 count range. Instead, the value toggles between 0, -1, and -2, occasionally flickering.

2. Hardware Identification

2.1 CPU 1214C High-Speed Counter Resources

The S7-1214C DC/DC/DC (firmware 4.x) provides six onboard HSCs with fixed input assignments. The first four HSCs use the onboard 24 V DC inputs and are usable without any signal board:

* HSC3 and HSC4 share I1.0 on the 1214C; do not enable both with overlapping edges.

For the present application, the encoder is correctly wired to I0.0 (A) and I0.1 (B), which is the canonical HSC1 pinout. The 1214C HSC supports up to 100 kHz single-phase and 80 kHz quadrature, well above the 1.4 kHz theoretical limit of a 5000 PPR, 4x encoder running at 240 RPM.

2.2 Hengstler RI58-O/5000EK.42KB Decoder

The Hengstler RI58 is a 58 mm industrial shaft encoder. The part number breaks down as follows:

3. Wiring and Shielding Best Practices

The encoder must be wired with a twisted-pair shielded cable. The RI58EK cable assigns the following conductors (Hengstler standard colour code):

• Connect the cable shield at the cabinet entry only using a 360° EMC gland; do not ground the encoder end if the housing is not isolated.
• Keep the encoder cable at least 200 mm from VFD motor cables and cross at 90° when unavoidable.
• Verify that the encoder 24 V supply is referenced to the same 0 V as the CPU M terminal; floating supplies cause quadrature jitter.

4. TIA Portal HSC Configuration

4.1 Enable and Mode Selection

Open the device view in TIA Portal, select the CPU 1214C, and navigate to Properties → High-speed counters (HSC). Expand HSC1 and enable the function:

1. Set Function enable = Enable.
2. Set Type of counting = Count (or Frequency if you prefer the firmware to derive speed directly).
3. Set Operating phase = A/B counter fourfold (preferred for 5000 PPR — 4x evaluation quadruples resolution to 20,000 counts/rev).
4. Set Counting direction = Count up if shaft rotation is mechanically fixed; Count up / down (hardware gate via B-direction input) if direction can reverse.
5. Set Initial count value = 0.
6. Set Initial reference value = 0.

4.2 Input Address Assignment

Under Hardware input, confirm the input address. The default for HSC1 is %ID1000 (DINT, 32-bit signed). If you have additional HSCs configured, Siemens increments this address automatically. Use this tag as the source for the count.

5. Root Cause: The Default Input Filter

The single most common reason an S7-1200 HSC fails to count encoder pulses is the digital input filter. By default, every standard digital input on the 1214C is set to a filter time of 6.4 ms (firmware 4.0 and later). At 6.4 ms the filter rejects any pulse shorter than ~78 µs, which means:

With the 6.4 ms filter active, the HSC sees only noise from the input stage and counts random quadrature transitions, producing exactly the 0 / -1 / -2 behaviour reported in the field case.

5.1 Correcting the Filter

1. In TIA Portal, navigate to Device view → CPU → Properties → Digital inputs.
2. Select channels I0.0, I0.1, and I0.2 (the channels assigned to HSC1).
3. In the inspector pane, change Input filter from the default to HSC preset (sometimes labelled high-speed counter or 10 µs depending on firmware).
4. Compile and download the hardware configuration. Re-test by manually rotating the encoder shaft at low speed; the count should advance smoothly.

6. Quadrature Mode and Effective Resolution

Selecting A/B counter fourfold makes the firmware count on every edge of channels A and B (four events per physical pulse period). The effective resolution becomes:

Effective_counts_per_rev = PPR × 4

For 5000 PPR:

• 1x mode: 5,000 counts/rev
• 2x mode: 10,000 counts/rev
• 4x mode: 20,000 counts/rev (recommended)

The relationship between shaft RPM and the HSC pulse rate at 4x evaluation is:

f_pulse = (RPM × PPR × 4) / 60

At a typical conveyor speed of 60 RPM, f_pulse = 60 × 5000 × 4 / 60 = 20,000 Hz, comfortably below the 80 kHz 4x limit.

7. Reading the HSC Value in OB1

Once HSC1 is enabled and the input filter is set to the high-speed preset, the count is exposed at the configured input address. Read it directly in OB1 (or any cyclic OB) and store it in a DINT tag:

// SCL - read HSC1 count
"HSC1_Count_Value" := "HSC_1";   // direct read of configured input address %ID1000

For debugging, drop the value on a watch table or directly into a comparator:

// SCL - debug comparator
IF "HSC1_Count_Value" = 500 THEN
    "Debug_Q0_2" := TRUE;
END_IF;

With the input filter corrected, manually turning the shaft one full revolution should advance HSC1_Count_Value by 20,000 in 4x mode (or 5,000 in 1x mode). If it does not, re-check the wiring and the filter.

8. Calculating RPM from the HSC Count

8.1 Counting-Mode Speed Calculation

To calculate shaft speed in RPM, sample the HSC count periodically and divide the delta by the sampling interval:

RPM = (ΔCounts / ΔT_seconds) × (60 / (PPR × 4))

Where ΔCounts is the difference between the current sample and the previous sample, ΔT is the sampling interval in seconds, and the divisor is the effective counts per revolution (PPR × 4 in 4x mode).

// SCL - periodic RPM calculation in OB1
// Sample every 1 second using a cyclic interrupt OB (OB30..OB38)
IF "Sample_1Hz".Q THEN
    "Sample_1Hz"(IN := FALSE);

    // Capture current HSC value
    "CurrentCount" := "HSC_1";
    "DeltaCount"  := "CurrentCount" - "LastCount";
    "LastCount"   := "CurrentCount";

    // RPM with 5000 PPR, 4x evaluation, 1 s sample
    "Encoder_RPM" := (INT_TO_REAL("DeltaCount") / 20000.0) * 60.0;

    // Linear conveyor speed in m/min
    // Wheel diameter 160 mm -> circumference = π × 0.160 m
    "Conveyor_m_per_min" := "Encoder_RPM" * 3.14159265 * 0.160;
END_IF;

"Sample_1Hz"(IN := TRUE, PT := T#1s);

8.2 Frequency-Mode Speed Calculation

Alternatively, configure HSC1 in Frequency mode and let the firmware return pulses per second directly:

1. Set Type of counting = Frequency.
2. Set Time base = 1.0 s (longer = better resolution at low speed, shorter = faster update).
3. Set Operating phase = A/B counter fourfold.

The HSC returns a DINT in Hz at the configured input address:

// SCL - RPM from frequency mode
"HSC1_Frequency_Hz" := "HSC_1";   // value in pulses per second

// At 4x evaluation: counts per rev = 20000
"Encoder_RPM" := "HSC1_Frequency_Hz" / 20000.0 * 60.0;

Frequency mode removes the need for a cyclic timer and an internal "last count" variable, but the time base dictates the trade-off between resolution and response.

8.3 Conveyor Mechanics

If the encoder is mounted on a 160 mm conveyor pulley driving an axis of 25 mm (e.g., a roller), the mechanical ratio is:

i = Ø_conveyor / Ø_axis = 160 / 25 = 6.4

If the encoder is on the conveyor wheel, axis RPM = encoder RPM × i. If it is on the axis, conveyor RPM = encoder RPM / i. Linear belt speed is then:

v [m/min] = RPM × π × D [m]

9. Alternative: Motion Control Technology Object

For applications where the conveyor speed ultimately feeds into a position-controlled drive or where HMI/web-server integration is desired, the S7-1200 Motion Control technology object (TO_PositioningAxis) can consume the HSC directly. The motion control library exposes the current position and velocity without manual counting logic:

• Add a new Technology object → Positioning axis → TO_PositioningAxis.
• Under Configuration → Encoder → Encoder on high-speed counter (HSC), select the configured HSC and verify the filter times are still set correctly.
• Configure Mechanics with the wheel diameter or leadscrew pitch.
• Read <TO>.ActualVelocity and <TO>.ActualPosition from your user program.

Refer to the TIA Portal V21 documentation on encoder on HSC for the S7-1200 positioning axis for the exact property mapping, and the Siemens support article 109747174 for the configuration procedure.

10. Commissioning Verification

Use this checklist to validate the HSC encoder on the 1214C before commissioning the conveyor logic:

1. Power check: Verify +UB at the encoder terminals is between 10 V DC and 30 V DC and 0 V is bonded to the CPU M terminal.
2. Wiring check: A on I0.0, B on I0.1, Z (if used) on I0.2. No cross-over.
3. Filter check: Inspect CPU properties → Digital inputs → I0.0/I0.1/I0.2 → Input filter; confirm the HSC preset is selected.
4. Online watch: Add HSC_1 (%ID1000) to a watch table. Manually rotate the encoder shaft by exactly one revolution. The value should advance by 20,000 (4x mode) or 5,000 (1x mode).
5. Direction check: Note the sign convention. If rotation in the "production forward" direction decrements the count, swap channels A and B or change the count direction in the HSC configuration.
6. High-speed verification: Drive the conveyor at rated speed and confirm the count rate in the PLC trace. At 80 RPM, 4x, you should see 80 × 5000 × 4 / 60 = 26,667 counts/s.
7. Compare with hand-held tachometer: The calculated RPM should match the tachometer reading within ±2% at steady state.

11. Troubleshooting Matrix