Wiring an Open-Collector Encoder to a Siemens Digital Input

Overview

This reference describes how to interface a 64 pulse-per-revolution (PPR) incremental rotary encoder with an open-collector (NPN, voltage-sink) output stage to a Siemens SIMATIC S7-300 digital input module. The application runs at a maximum of 40 RPM, producing a 22 ms pulse period and approximately 11.7 ms semiperiod at the output. We cover the required external pull-up resistor, the input-filter timing budget of the SM 321 digital input, A/B quadrature decoding in the PLC program, and the conditions under which a dedicated Siemens counter module such as the FM350-2 becomes mandatory. The wiring principles also apply to ET 200M stations and S7-400 systems using the same SM 321 module family.

Working pulse rate at the rated speed:

Encoder Signal Characteristics

The specified encoder is a 64 PPR incremental rotary encoder with an open-collector output stage. Open-collector outputs do not source current; the output transistor only pulls the line to ground when active and floats (high impedance) when inactive. Without a pull-up resistor, the PLC input reads an undefined voltage and produces erratic counts. A pull-up must be wired between each signal line (A, and optionally B) and the positive supply rail (24 V DC in this case).

The 90° electrical offset between channels A and B enables quadrature decoding and direction discrimination. Most industrial 24 V SM 321 modules source current into the input (Type 1 / Type 3 per IEC 61131-2), which means a PNP (sourcing) encoder could be wired directly without modification; an NPN (sinking) encoder like the one in this application always requires a pull-up.

Prerequisites

• Siemens SIMATIC S7-300 (or ET 200M / S7-400) station with an SM 321 digital input module — for example 6ES7321-1BH02-0AA0 (16 DI, 24 V DC, hardware-selectable input filter) or 6ES7321-1BL00-0AA0 (32 DI, 24 V DC, fixed filter).
• 24 V DC encoder supply derived from the same power supply feeding the rated voltage of the SM 321 module (common 0 V reference required between the encoder and the input module).
• Two 1.47 kΩ resistors (1/4 W minimum, 1% or 5% tolerance) — one per channel used (A and B).
• Free digital inputs on a module with the required scan/filter capability, and a common ground reference.
• Shielded twisted-pair cable (e.g., LiYCY 2×0.34 mm²) with shield grounding hardware.
• STEP 7 V5.x or TIA Portal project with the PLC station hardware configured and online to the CPU.

Pull-Up Resistor Sizing

The 1.47 kΩ pull-up value recommended for 24 V open-collector encoders balances three requirements:

1. Adequate sink current for clean saturation of the open-collector transistor. Most 24 V encoder outputs specify a minimum sink current of 10 mA to guarantee V_OL ≤ 0.4 V. At 1.47 kΩ the sink current is 24 V / 1470 Ω ≈ 16.3 mA, comfortably above the typical 10 mA minimum.
2. Limited steady-state current in the active (output on) state. 16.3 mA is well within the 20-40 mA rating of typical 24 V open-collector encoder outputs.
3. Acceptable RC time constant when combined with cable capacitance. For a 20 m run of shielded twisted pair at roughly 100 pF/m, the cable capacitance is about 2 nF. τ = R × C = 1470 Ω × 2 nF ≈ 2.94 µs, which is negligible compared with the 11.7 ms semiperiod.

Wiring Procedure

1. Confirm the encoder data sheet specifies an open-collector (NPN) output. If the encoder is PNP (sourcing), no pull-up is required; Siemens SM 321 24 V inputs source current and read PNP outputs directly.
2. Connect the encoder's V+ terminal to the 24 V DC supply rail that also feeds the SM 321 input module.
3. Connect the encoder's GND terminal to the same 24 V DC common (M) terminal used by the SM 321 module. Common ground is mandatory — separate supplies require a dedicated common reference.
4. Connect one end of a 1.47 kΩ resistor to the encoder's A output terminal.
5. Connect the other end of the same resistor to +24 V DC.
6. Wire the encoder's A output (the resistor-output junction) to the SM 321 input terminal (e.g., I0.0). The PLC input "sees" the resistor-output node.
7. Repeat steps 4-6 for channel B if quadrature decoding is required.
8. Terminate the cable shield at one end only — typically the cabinet-side shield ground bar near the SM 321 module. Do not terminate the shield at the encoder end unless the encoder housing is itself bonded to cabinet ground.
9. Verify wiring with a multimeter before applying power: confirm continuity from V+ to encoder supply, GND to encoder supply M, and A/B outputs isolated from V+ and GND when the encoder is unpowered.
10. Apply 24 V DC. Measure the voltage on the A output with a DMM — should read approximately 24 V when the output transistor is off and approximately 0.3-0.6 V when the encoder mark is opposite the sensor.

SM 321 Filter Configuration and Pulse Timing Budget

On SM 321 modules with hardware-selectable filters (1BH02 family), the filter is set by a rotary switch on the rear of the module. For S7-300 module specifications and input-delay curves refer to the SIMATIC S7-300 Automation System Manual (ST70), chapter 4.

Switch position mapping on the 1BH02:

For 42.67 Hz operation, switch position B (3 ms filter) is the correct choice. Position A also works but provides no noise benefit. Position C must not be used — it suppresses every legitimate pulse.

Even with the input filter correctly configured, the PLC program cycle can still miss pulses. In an S7-300, OB1 scan time is typically 5-15 ms. If the scan time approaches 11.7 ms, polled counting becomes unreliable. Three reliable alternatives:

1. Hardware counter module (FM350-2 or ET 200S 1Count) — counter runs in hardware, no scan-time dependency.
2. Edge-interrupt inputs (e.g., SM 321-7BH01 with interrupt capability) — the encoder triggers hardware interrupt on each transition, OB40 executes immediately.
3. Fast cyclic interrupt OB (OB35 at ≤ 5 ms) — sample A and B inputs faster than the pulse semiperiod.

Quadrature Decoding with Two Digital Inputs

A single digital input only counts pulse edges; it cannot distinguish direction and cannot reject noise-induced transitions. Wiring both A and B channels to two digital inputs enables quadrature decoding in the PLC program:

• A leads B by 90° (A rising edge while B is low): increment counter (forward direction).
• B leads A by 90° (A rising edge while B is high): decrement counter (reverse direction).

Quadrature decoding also provides 4× resolution: a 64 PPR encoder becomes 256 counts per revolution in 4× mode. Implementation in STEP 7 SCL, called from a fast cyclic OB:

// Quadrature counter — call from OB35 (≤ 5 ms)
IF "DI_A" AND NOT "DI_A_prev" THEN      // rising edge on A
    IF "DI_B" = FALSE THEN              // A leads B → forward
        "Position_count" := "Position_count" + 1;
    ELSE                                 // B leads A → reverse
        "Position_count" := "Position_count" - 1;
    END_IF;
END_IF;
"DI_A_prev" := "DI_A";

Counter Module Alternatives

When the application requires reliable counting, direction discrimination, or noise immunity, a dedicated Siemens counter module is the correct choice. The FM350-2 counter module manual and the FM350-2 configuration package cover parameter assignment, hardware configuration, and example projects.

Use a counter module when any of the following apply:

• Pulse frequency exceeds the ratio of digital-input filter time to OB cycle time.
• Quadrature decoding with direction discrimination is required.
• Vibration may generate spurious pulses that software filtering cannot reliably reject.
• Measurement accuracy and repeatability are critical (the FM350-2 and 1Count modules integrate hardware chatter suppression).

Vibration and Noise Considerations

If the driven machine can come to rest with the encoder positioned near a transition threshold (e.g., adjacent to a ferromagnetic tooth or with the optical disk partially covering the photodetector), vibration can cause the encoder output to oscillate between high and low states while mechanically stationary. A digital input will count each transition, causing the position register to drift while the system is stopped. This is a well-documented field problem for conveyors and indexing tables driven by adjacent vibrating equipment.

Mitigation strategies, in priority order:

1. Use a counter module with built-in chatter suppression. The FM350-2 and ET 200S 1Count include configurable minimum pulse-width parameters that reject transitions shorter than a defined time window (typically 0.1-100 ms).
2. Implement A/B quadrature validation in software. Only count when both A and B transition in the expected sequence within a defined time window; reject sequences that violate the quadrature state machine.
3. Apply a velocity gate. If the time since the last legitimate pulse is shorter than the minimum semiperiod expected for the actual speed range, discard the pulse.
4. Mechanically isolate the encoder from vibration sources with a flexible coupling, vibration-damping mount, or rubber isolator.
5. Use a Schmitt-trigger input with hysteresis to reject noise near the switching threshold.

Verification Procedure and Distance Calculation

1. With power off, verify all wiring against the schematic: V+, GND, A, B (if used), shield.
2. Apply 24 V DC. Measure voltage at the SM 321 input terminal for channel A — should read approximately 24 V when the output transistor is off and approximately 0.5 V when on.
3. Open a STEP 7 / TIA Portal watch table online with the CPU in RUN. Force the input address (e.g., I0.0) to be monitored.
4. Manually rotate the encoder shaft one full revolution. The input should toggle 64 times on channel A. In 4× quadrature (A and B), the counter should advance by 256 counts.
5. Spin the encoder at the rated 40 RPM and verify the count rate: 42.67 Hz on channel A (one transition every 23.4 ms).
6. Stop the encoder. Observe the count for 60 seconds — should remain stable (no vibration-induced false counts).
7. Reverse the rotation direction. In quadrature mode, the counter should decrement; the direction bit should toggle.
8. Run a full operating cycle and verify the calculated distance matches the expected travel.

Distance calculation from the pulse count:

Example: a 100 mm diameter wheel with a 64 PPR encoder gives distance_per_pulse ≈ 4.91 mm. After 1000 counts, total distance ≈ 4.91 m. In 4× quadrature, distance_per_pulse = π × D_wheel / 256 ≈ 1.23 mm per count.

Troubleshooting Matrix

Frequently Asked Questions