Siemens LOGO! Digital Input Debounce: 4 Hz Scan Rate, Edge Detection, and Field Wiring

Mechanical switches, push buttons, and relay contacts do not produce clean logic-level transitions. When two metal contacts close or open, they physically bounce for a few milliseconds to a few tens of milliseconds, generating a burst of edges that a high-speed counter would see as multiple events. The most common field question about the Siemens LOGO! logic module is therefore: does LOGO! debounce its digital inputs, and what is the maximum usable input change rate? This reference covers the input scan architecture of the LOGO! 8 family (specifically the 6ED1052-1HB08-0BA1 variant), how the 4 Hz / 250 ms limit is implemented, where fast inputs (5 kHz) apply, and which function blocks to use for reliable push-button and counter applications.

1. What "Debounce" Means in a PLC Context

Debouncing is the act of removing unwanted multiple transitions from a signal that is otherwise a single, well-defined event. The MDN Glossary defines debouncing as discarding operations that occur too close together during a specific interval and consolidating them into a single operation. In a PLC, debounce can be implemented in three places:

1. Hardware RC / filter on the input – analog low-pass filter on the input pin.
2. Firmware input filter – a minimum-pulse-width or integrator that rejects pulses shorter than a threshold.
3. Application logic – edge detection, on-delay, off-delay, or pulse generator blocks placed after the input.

The 4 Hz specification on LOGO! is a firmware-level rate limiter on the digital inputs, not a hardware RC filter. It is more accurately described as an input update rate than as a debounce filter, but in practical terms it performs the same function: pulses shorter than 250 ms cannot reach the program as multiple edges.

2. LOGO! 8 Hardware: 6ED1052-1HB08-0BA1 Variant

The part number discussed in field discussions – 6ED1052-1HB08-0BA1 – is a LOGO! 12/24 RCE module of the LOGO! 8 generation. The MLFB decodes as follows:

Key specifications of the 6ED1052-1HB08-0BA1 (and the closely related 6ED1052-1MD08-0BA1 12/24 RCEo):

Because the 6ED1052-1HB08-0BA1 does not have fast inputs, every digital input on it is subject to the 4 Hz / 250 ms rate limit. Variants that do have fast inputs – for example the LOGO! 230 RCE (6ED1052-1FB08-0BA1) on inputs I3 … I6 or the LOGO! 12/24 RCEo on I5/I6 – allow counter operation up to 5 kHz on those specific terminals only.

3. Input Scan Architecture: 4 Hz vs 5 kHz

Siemens documents the input scan behavior in the LOGO! 8 System Manual (entry ID 109751654). The relevant parameters are summarized in the table below.

The 4 Hz figure means that the input image used by the LOGO! program updates at most every 250 ms for normal digital inputs. Mechanically, even a slow push button cannot produce legitimate 4 Hz press events (that is 4 full presses per second), so the rate is more than sufficient for human-operated controls. It is also a comfortable margin over the worst-case bounce envelope of industrial push buttons (typically 5 … 20 ms).

3.1 Why the 250 ms ceiling exists

The limit is a side effect of how LOGO! schedules its input handling. Each scan of the program, the firmware reads the physical input pins through an internal debounce / integrator whose time constant is set so that a pulse shorter than 250 ms cannot reliably toggle the input flag. The benefit is that the user does not need to add any debounce function block in their program for the standard digital inputs – the hardware does it.

3.2 What the program cycle is and is not

The cycle time of a LOGO! program is the time the CPU takes to evaluate all active function blocks sequentially. On a LOGO! 8 the per-block evaluation is roughly 0.1 ms, so a fully populated 400-block program completes in a few tens of milliseconds. That is the rate at which the program can react to the input, not the rate at which the input changes. Because the input is filtered to a 4 Hz update, even an instant program response still sees the filtered value.

4. Practical Debounce Strategies on LOGO!

For most LOGO! applications, the firmware's 250 ms filter is enough and no extra debounce logic is required. However, three situations call for explicit application-level debounce:

1. The signal source is a fast input (5 kHz capable) and the program must respond to a single edge without counting bounces.
2. The wiring carries induced noise (long cable runs near VFDs, solenoids) that produces brief glitches longer than the bounce time but shorter than 250 ms.
3. The application uses fast counting and only wants to count the first edge of a press, not multiple bounces.

4.1 Using the On-Delay block (B004)

The simplest debounce is a short on-delay. Wire the input to an On-Delay (B004) block with a time of 10 … 20 ms. The output of the On-Delay becomes the debounced signal:

I1 ----[ On-Delay B004, T = 15 ms ]---- Q1
        |_ EdgeTriggered = no         |
        |_ Retentivity   = no         |

Any pulse shorter than 15 ms is dropped. A real button press (typically 50 … 200 ms in a human-pressed scenario) is preserved.

4.2 Using the Wiping Relay (Pulse Output) (B010 / B011)

The Wiping Relay (Pulse Output) and Wiping Relay (Edge-Triggered) blocks in the LOGO! Soft Comfort library already implement an edge-triggered single-pulse behaviour. Because the input itself is filtered to 250 ms, the wiping relay will produce exactly one output pulse per button press, regardless of contact bounce:

I1 ----[ Wiping Relay B010 ]---- M1 (internal flag)
        |_ TH (pulse width) = 1 s    |
        |_ Trg = I1                 |

For a manual control that needs "one press = one action", this is the recommended pattern.

4.3 Using the Up/Down Counter (B002 / B003) for hardware debounce verification

If the goal is to measure the bounce of a switch, connect the contact directly to a fast counter input. The counter will record every edge the firmware sees, including bounces. On a 5 kHz fast input, a typical button will register 5 … 50 counts per press. That same signal wired to a normal 4 Hz-limited input will register 1 … 2 counts at most, demonstrating the effect of the firmware filter.

5. Function Block Reference for Input Conditioning

The following table maps the most useful LOGO! blocks to debounce / edge-detect / pulse-train tasks. All blocks are available in LOGO! Soft Comfort V8.x and later, in the Special Functions library.

6. Wiring Considerations for Clean Inputs

Even with the 250 ms firmware filter, the wiring layer matters. The following practices keep the firmware's debounce effective and avoid the situations where the filter is bypassed (e.g. by using a fast input unnecessarily):

1. Use shielded cable for cable runs longer than 10 m, especially near VFD output cables. Ground the shield at one end only (LOGO! end).
2. Keep input commons grouped: LOGO! 8 inputs share common terminals in groups; do not mix AC and DC sources on the same group.
3. Source-type wiring for sinking sensors (NPN) requires the sensor common tied to M (0 V) of the LOGO!; the firmware expects positive logic on the input.
4. Add a 0.1 µF ceramic across the contact if the cable is long and the source is a dry contact – this attenuates RF-induced bounce that is longer than mechanical bounce.
5. Do not use inputs I1 … I4 as analog (0 … 10 V) and digital on the same channel; the analog path is slower and disables the digital threshold filter.

7. Verification Procedure

To verify the 4 Hz / 250 ms filter is in effect on a 6ED1052-1HB08-0BA1:

1. Open LOGO! Soft Comfort and connect to the module via Ethernet.
2. Create a program with one Up Counter (B002) wired to input I1 and a display block showing the count.
3. Toggle I1 by hand with a piece of wire at a rate of roughly 5 Hz (use a function generator or a slow-bounce free source for repeatability).
4. Observe the counter: it should increment by 1 per 250 ms window, i.e. 2 … 4 counts per second, never more.
5. Repeat on a fast input variant (e.g. a 6ED1052-1FB08-0BA1 on I3) – the counter should now follow the source frequency up to 5 kHz.

If the count on a normal input exceeds 4 counts per second, the source signal has a pulse width longer than 250 ms, which means the firmware filter is not the limiting factor – the wiring is delivering a fast or noisy waveform. Add a 10 … 20 ms on-delay in the program to debounce further.

8. Troubleshooting Matrix

9. Related Topics and Cross-References

The same 4 Hz / 250 ms input model applies to every LOGO! 8 base unit that lacks fast inputs. The same debounce-by-time-block technique applies to LOGO! 7 (6ED1 052-x series) and to the LOGO! CMR (communication module with relay outputs) safety extension. The fast-input 5 kHz path is documented separately in the LOGO! 8 manual section on counters and frequency measurement, and is functionally equivalent to the high-speed counter (HSC) inputs on a SIMATIC S7-1200 – though without the S7-1200's configurable input filter (which can be set to 0.1 ms … 20 ms per input group via the PLC properties).

For applications where the 4 Hz limit is not fast enough and a fast input is not available on the LOGO! variant in use, the practical options are:

• Select a different LOGO! variant that includes fast inputs.
• Pre-scale the input with an external hardware debounce (74HC14 Schmitt trigger + RC network, or a 555-timer one-shot).
• Migrate to a SIMATIC S7-1200 where the input filter can be tuned per channel.

10. Summary of Key Parameters