Implementing Frequency-Based Speed Watchdog on LOGO! 8 PLCs

Problem Statement: Overspeed Detection on LOGO! 8

You are using a Siemens LOGO! 8 (6ED1052-1xxx08-0BA2 series) to command a safe-stop function on an off-highway vehicle. The vehicle is loaded and descending a slope. When the engine is forced into neutral, gravity accelerates the drivetrain rather than decelerating it. The drivetrain must be cut to a safe state the moment the actual speed exceeds the maximum permitted speed, otherwise the machine will run away down the slope.

The natural LOGO! function block to reach for in this situation is the analog watchdog. The standard analog watchdog block has two thresholds (a low limit and a high limit) and one analog input. When the analog value moves outside the window, the watchdog output goes high. The block also has a hysteresis parameter to prevent chatter on the threshold crossing.

The speed signal in this application, however, is a digital pulse train. A magnetic pickup, an inductive proximity sensor, a Hall-effect sensor, or an encoder outputs a square wave whose frequency is proportional to the rotational speed of the wheel, the motor shaft, or the transmission output. The LOGO! base module has no digital-to-analog converter, so the pulse train cannot be read as a 0 to 10 V or 4 to 20 mA value directly. The standard analog watchdog block cannot accept a pulse train input.

This article describes how to build a frequency-based speed watchdog on a LOGO! 8 using only the on-board high-speed counter inputs, the standard HSC block, the asynchronous pulse generator, the off-delay timer, and the latching relay. No DAC is required and no external frequency transducer is required for the basic implementation.

Why the Standard Analog Watchdog Won't Accept a Pulse Train

The LOGO! analog watchdog is internally built around the 12-bit analog-to-digital converter (ADC) of the LOGO! base module. The ADC is fed by the four on-board analog inputs AI1 to AI4 (I7 and I8 are analog on the LOGO! 12/24 RCE and RCEo variants) and by the optional LOGO! AM2 / AM2 RTD / AM2 PT100 expansion modules. The ADC resolution is 12 bits over a 0 to 10 V range (0 to 1000 in LOGO! scaled units) or 0 to 20 mA (0 to 1000 in LOGO! scaled units). The LOGO! base module has no analog output stage; outputs are relay or 24 V transistor only.

A pulse train is a digital signal. Each pulse is a transition from 0 V to Vcc (typically 24 V) and back to 0 V. The mean voltage of a pulse train depends on the duty cycle of the signal, not on the rotational speed. If you wire the pulse train to one of the analog inputs, the ADC will read a value somewhere between 0 V and Vcc, but that value is a function of the duty cycle and not a function of the frequency. The analog watchdog will therefore not detect an overspeed condition correctly. A 50% duty cycle at 100 Hz and a 50% duty cycle at 1000 Hz will both read approximately 5 V on the analog input.

The standard counter block and the high-speed counter (HSC) block are the only blocks that can interpret a digital pulse train as a frequency-related value. They count edges on a digital input and compare the count to a configurable setpoint. When the count exceeds the setpoint, the block output goes high. This is the equivalent of an analog watchdog, but it operates on counts per second rather than on volts. The HSC block on LOGO! 8 is rated to 5 kHz on inputs I3 to I6, which covers almost all vehicle speed sensor applications.

The general concept of a process watchdog that monitors a value and intervenes when the value moves outside a permitted range is documented in industry design notes. The Analog Devices application note on the DS80C320 high-speed microcontroller watchdog timer, available at Using the High-Speed Micro's Watchdog Timer, describes the same principle: a hardware or software monitor that resets or intervenes when the process drifts outside the expected range. The same principle applies to a vehicle speed watchdog.

Prerequisites and Required Components

Before starting, confirm that the following items are available:

• One LOGO! 8 base module with high-speed counter inputs. The relevant models are:
  • LOGO! 12/24 RCE (Siemens order number 6ED1052-1MD08-0BA2) — 8 digital inputs, of which I3, I4, I5, I6 are high-speed (5 kHz) and I1, I2, I7, I8 are 24 V inputs.
  • LOGO! 24 RCE (Siemens order number 6ED1052-1HB08-0BA2) — 8 digital inputs, of which I3, I4, I5, I6 are high-speed.
  • LOGO! 230 RCE (Siemens order number 6ED1052-1FB08-0BA2) — inputs are not high-speed; not suitable for pulse rates above 30 Hz.
• LOGO! Soft Comfort V8.x installation. Soft Comfort V8.4 or later is recommended for the HSC block enhancements.
• A speed sensor that outputs a clean 24 V push-pull or PNP square wave. Sensors with an open-collector output need a 2.2 kΩ pull-up resistor to 24 V.
• A free digital output on the LOGO! to drive the safe-stop relay, the engine kill, or the hydraulic dump valve. The LOGO! 12/24 RCE has four 24 V / 0.3 A transistor outputs (Q1 to Q4).
• The LOGO! 8 system manual, available from Siemens Industry Online Support at LOGO! 8 system manual, and the LOGO! Soft Comfort online help.

Frequency-to-Count Conversion Principle

The relationship between rotational speed, sensor pulses per revolution, and pulse train frequency is:

f = n × p / 60

where:

• f is the pulse frequency in Hz
• n is the rotational speed in rpm
• p is the number of pulses per revolution (PPR) of the sensor

If the sensor is mounted on a wheel of diameter D (meters) and the vehicle speed is v (m/s), the relationship is:

f = v × p / (π × D)

For an inductive pickup with one tooth per gear tooth, p is the number of gear teeth. For an encoder, p is the PPR marked on the encoder housing (commonly 60, 100, 360, 500, 1000, 1024, 2048, or 4096 PPR).

The strategy for the LOGO! is to count the pulses for a fixed time window of one second, then read the count at the end of the window. The count at that instant is the frequency in Hz, provided that the pulse generator period is exactly 1.000 s. A count of 3600 at the end of the window corresponds to 3600 Hz, which corresponds to a known vehicle speed given the sensor PPR and the wheel diameter.

The classic implementation uses:

• A high-speed counter block fed by the high-speed input
• An asynchronous pulse generator that produces a 1-second window
• The pulse generator output resets the counter at the end of each window
• The HSC On threshold parameter defines the high threshold of the watchdog
• The HSC Off threshold parameter defines the low threshold (hysteresis)

Alternatively, a fixed count of 1 can be set as the reference: the period of one pulse is measured by a high-speed counter, and the inverse of the period is the instantaneous frequency. LOGO! 8 does not have a dedicated period-measurement block, so the count-over-fixed-window method is the simplest. The trade-off is that the measurement resolution is 1 Hz (one count per second), and the response time is 1 second plus the off-delay time.

To improve resolution, reduce the pulse generator period. A 100 ms period with the same threshold divided by 10 gives 0.1 s measurement windows and 10 Hz resolution. A 10 ms period with the threshold divided by 100 gives 100 Hz resolution. The trade-off is a higher CPU load on the LOGO! base module and a more demanding pulse generator timing requirement.

Step-by-Step Implementation in LOGO! Soft Comfort

1. Start a new project in LOGO! Soft Comfort V8.4. Select the target module (for example, LOGO! 12/24 RCE, order number 6ED1052-1MD08-0BA2). Place an empty FBD page.
2. Place an Asynchronous Pulse Generator block. Set:
   • Parameter TH (high time) = 50 ms. The pulse width must be long enough for the HSC to recognize the reset edge, but short enough to avoid losing significant pulses within the window.
   • Parameter TL (low time) = 950 ms. The total period TH + TL = 1.00 s.
   • Output Q of the pulse generator becomes the reset signal for the HSC.
3. Place a High-Speed Counter block. The HSC is required if the input frequency exceeds 30 Hz, because the standard counter block samples its input at 5 Hz and will alias a 60 Hz signal. Set:
   • Count input = I3 (high-speed input wired to the speed sensor)
   • Direction input = constant 0 (count up only)
   • Reset input = Q of the pulse generator
   • On threshold (setpoint) = 360 (or the count that corresponds to the maximum safe speed)
   • Off threshold = 350 (defines the hysteresis; the output returns to low only when the count drops below 350, preventing chatter on the threshold crossing)
   • Output Q of the HSC goes high when the count reaches or exceeds 360 and returns low when the count drops below 350
4. Place an Off-Delay block to latch the overspeed output. Set:
   • Parameter T = 5 s (the minimum duration of the safe-stop signal after the overspeed is detected). This ensures that the safe-stop output remains active for at least 5 s, even if the speed briefly drops below the threshold.
   • The trigger input of the off-delay is fed by Q of the HSC.
   • The output of the off-delay drives the safe-stop output relay on the LOGO!.
5. Place a 2-input OR block. The second input of the OR block is fed by a manual reset pushbutton on I1. The OR block output drives the Reset input of the off-delay, providing operator reset.
6. Place an Output block Q1 and assign it the output of the off-delay. Q1 is the actual output that energizes the safe-stop relay on the machine.
7. Save the project as a LOGO! Soft Comfort file. Transfer it to the LOGO! 8 base module via Ethernet, microSD card, or the LOGO! Soft Comfort cable.

The corresponding FBD network is:

I3 -----> [HSC] Count input
[HSC] Direction <- constant 0
[HSC] Reset <- Q [Pulse Gen] (TH=50 ms, TL=950 ms)
[HSC] Q -----> [Off-Delay] Trigger (T=5 s)
             [Off-Delay] Q -----> Q1 (output)
             [Off-Delay] Reset <- [OR] <- I1 (manual reset pushbutton, NO)

Block Parameters and I/O Assignment

The threshold value of 360 counts per second is a worked example. To convert a target vehicle speed to a threshold count:

Threshold (counts/s) = v_target × p / (π × D)

For a 0.8 m diameter wheel and a 60-tooth sensor and a target speed of 15 m/s (54 km/h):

Threshold = 15 × 60 / (π × 0.8) = 358.1 Hz ≈ 360 counts/s

Adjust the threshold to match the actual maximum safe speed for the descent application. Common vehicle speed sensor configurations are listed in the table below:

Wiring the Speed Sensor to a High-Speed Input

Wire the speed sensor as follows:

• Sensor supply +24 V → LOGO! 24 V terminal (terminal 1 or 2 on the LOGO! 12/24 RCE)
• Sensor output (PNP) → LOGO! terminal I3
• Sensor ground → LOGO! 0 V terminal (terminal 3 or 4)

If the sensor is open-collector NPN, add a 2.2 kΩ pull-up resistor between the sensor output and the LOGO! 24 V supply. Without the pull-up resistor, the LOGO! input will not see a defined high level and the counter will not increment. The 2.2 kΩ value is appropriate for a 24 V supply; for a 12 V supply on the LOGO! 12/24 RCE, use 1.0 kΩ.

If the sensor cable is longer than 5 m, use a shielded cable and ground the shield at the LOGO! end only. The high-speed inputs are sensitive to capacitance on the cable, and unshielded cables longer than 10 m will round off the pulse edges and cause missed counts. For cable runs longer than 50 m, install a line receiver such as a MAX3095 or a 26LS32 between the sensor and the LOGO! input to restore the edge rate.

If the sensor is a 2-wire proximity switch, install a 24 V series resistor and verify the residual current does not exceed the LOGO! input off-state leakage. The LOGO! 12/24 RCE has a typical off-state current of 0.5 mA, so the proximity switch must have a residual current below 0.5 mA to guarantee a low input state. A typical 2-wire proximity switch has a residual current of 1.5 mA, which will keep the LOGO! input in a high state even when the switch is open. In that case, use a 3-wire PNP sensor instead.

If the LOGO! base module is installed in a separate control cabinet from the sensor, route the sensor cable through a dedicated cable gland and keep it at least 100 mm away from any 400 V AC motor cable, VFD output cable, or welding cable. Cross high-voltage and low-voltage cables at 90 degrees if they must share a cable tray.

Threshold Trigger and Safe-Stop Latching

The standard analog threshold trigger cannot be used here because it requires an analog input. The equivalent of a threshold trigger on a counter is the On threshold parameter of the HSC block itself. When the count reaches the On threshold, the counter output Q goes high. This is the high threshold of the analog watchdog. The Off threshold implements the hysteresis: the output returns to low only when the count drops below the Off threshold at the next reset, which prevents chatter on the threshold crossing.

For the safe-stop latching, the off-delay timer holds the output high for the configured T time after the trigger condition clears. This is functionally equivalent to a latching relay with a manual reset, except that the latch releases automatically after T seconds. To force a manual reset, the OR block gates the Reset input of the off-delay with a manual pushbutton on I1. The off-delay timer is the LOGO! function block in the Timers category, not the Wiping Relay (which is a one-shot pulse output).

To implement a permanent latching that only releases on manual reset, replace the off-delay with a Latching Relay block. Wire the HSC output Q to the S (set) input of the latching relay. Wire the manual reset pushbutton on I1 to the R (reset) input. The relay will then stay latched until the operator presses the reset button, regardless of whether the speed drops below the threshold. This is the recommended configuration for a safe-stop function on a descent, because the operator must deliberately reset the system after an overspeed event.

To increase the safety integrity, add a hardware latching relay downstream of Q1. The hardware relay is energized by Q1 and is mechanically latched. If the LOGO! loses power or resets, the hardware relay remains in the latched state, and the safe-stop function is maintained. The hardware relay can only be released by a deliberate operator action (pressing the reset pushbutton on I1, which the LOGO! maps to the hardware relay reset coil).

Alternative: External Frequency-to-Voltage Transducer

If you prefer to keep the standard analog watchdog function block, install an external frequency-to-voltage converter between the speed sensor and the LOGO! analog input. Common industrial products are the Red Lion IFMA, the Phoenix Contact MINI MCR-2-F-UI, the WAGO 857 Series frequency transducer, and the Siemens SITRANS F frequency input module. These accept a pulse train and output a 0 to 10 V or 4 to 20 mA signal that is proportional to the frequency.

The wiring becomes:

• Speed sensor → transducer input (terminals + and -)
• Transducer 24 V supply → LOGO! 24 V
• Transducer 0 to 10 V output → LOGO! AI1 (I7 on the 12/24 RCE)
• Transducer ground → LOGO! 0 V

The LOGO! analog watchdog then operates on the 0 to 1000 LOGO! unit value (0 to 10 V scaled). Configure the watchdog with a low threshold of 0 (or 100 if a non-zero minimum is required) and a high threshold corresponding to the maximum safe speed. This approach is functionally identical to the HSC method but adds hardware cost and a single point of failure in the transducer.

For SIL-rated safety functions, the transducer must itself be SIL-rated or the LOGO! analog watchdog must be backed up by a second independent path. Most general-purpose transducers are not SIL-rated. Confirm the SIL rating of the transducer against the manufacturer's datasheet before relying on the analog watchdog as part of a safety function.

Commissioning and Verification

1. Connect the LOGO! to the programming PC and open the project in LOGO! Soft Comfort V8.4. Switch to online mode. The online mode shows the live state of every input, every block output, and every HSC count.
2. Manually rotate the wheel or shaft at a known low speed (for example, 60 rpm with a 60-tooth sensor = 60 Hz = 60 counts per second). Observe the HSC block in online mode. The count should increment from 0 to 60 in 1 second and then reset to 0 at the start of the next window. The HSC output Q should remain low.
3. Increase the speed to the target safe speed (for example, 360 rpm = 360 Hz = 360 counts per second). The count should equal the setpoint, and the HSC output Q should remain low. The off-delay output should remain low. Q1 should be de-energized.
4. Increase the speed beyond the target (for example, 380 rpm = 380 Hz = 380 counts per second). The count should exceed the setpoint on the next measurement window, and the HSC output Q should go high. The off-delay should latch the output for at least 5 seconds. Q1 on the LOGO! should energize the safe-stop relay.
5. Reduce the speed below the threshold (for example, 340 rpm = 340 Hz = 340 counts per second, which is below the Off threshold of 350). The HSC output should go low at the next reset. The off-delay should release after 5 seconds. Q1 should de-energize the safe-stop relay.
6. Press the manual reset pushbutton on I1. The off-delay should release immediately. Q1 should de-energize the safe-stop relay.
7. Power-cycle the LOGO! by removing and reapplying the 24 V supply. The HSC count should reset to 0. The off-delay output should remain low. Q1 should remain de-energized until the next overspeed event. Confirm that any hardware latching relay downstream of Q1 remains in the latched state through the power cycle.
8. Document the threshold value, the sensor PPR, the wheel diameter, the maximum safe speed, the HSC count at the threshold, the off-delay time, and the LOGO! firmware version in the machine file. The firmware version is visible on the LOGO! display (LOGO! > Diagnostics > Software) or on the LOGO! web server home page.
9. Use the LOGO! web server to verify the live state from a remote PC. The web server URL is the IP address of the LOGO!, for example, http://192.168.0.100. The default password is "LOGO" and should be changed on first commissioning to comply with the machinery IT security policy.

Edge Cases, Diagnostics, and Field Considerations

1. Sensor failure (stuck-at-zero): if the sensor wire breaks, the HSC will read 0 and the watchdog will not trigger. Add a stuck-at-zero detector. One method is to use a second HSC that triggers if the count is below a minimum value (for example, 5 counts per second) for more than 5 s. This detects a stalled vehicle with a broken sensor. Wire the second HSC output to a different output (Q2) and use Q2 to drive a dashboard warning lamp. The driver sees the lamp and knows that the overspeed protection is in a fault state.
2. Pulse generator drift: the LOGO! pulse generator is based on the internal oscillator of the LOGO! base module. The accuracy is typically ±2% over the full temperature range of 0 to 55 °C. For a target threshold of 360 counts per second, the actual threshold may be 353 to 367 counts per second. If higher accuracy is required, use an external pulse source or measure the oscillator with the LOGO! web server diagnostics page. The LOGO! web server shows the live HSC count and the live pulse generator output, so you can read the actual pulse generator period and adjust the HSC threshold accordingly.
3. High-frequency interference: in the off-highway environment, the 24 V supply is often contaminated with spikes from the engine cranking, the alternator load dump, and the hydraulic solenoid switching. Install a 24 V suppressor on each inductive load and a power line filter on the LOGO! 24 V supply. The Siemens LOGO! Power 24 V product family includes filtered supplies suitable for the cab environment. Use a 24 V TVS diode rated for the alternator load dump (such as a Littelfuse 5.0SMDJ24CA) on the LOGO! power input.
4. Internal CPU watchdog: the LOGO! base module has its own internal watchdog that resets the CPU if the scan time exceeds the configured limit (typically 400 ms). This is a separate watchdog from the application-level speed watchdog. The internal watchdog cannot be configured by the user. If your scan time approaches 400 ms, the LOGO! will reset and the safe-stop output will momentarily de-energize, which may not be acceptable for a latched safe-stop function. Use a hardware latching relay downstream of Q1 to maintain the safe-stop state through any LOGO! reset.
5. Frequency above 5 kHz: the high-speed inputs on I3 to I6 are rated to 5 kHz. If the sensor PPR is high (for example, 1000 PPR at high rpm), the frequency may exceed 5 kHz and the counter will miss pulses. In that case, fit a frequency divider (such as a CD4018 or a 74HC40103) between the sensor and the LOGO! input. A divide-by-4 divider brings a 20 kHz signal down to 5 kHz, which is within the LOGO! HSC range.
6. Networked reset: if the LOGO! is connected to a master PLC or to a cloud gateway via the on-board Ethernet port, the reset signal can be sent over the network using the LOGO! 8 Modbus TCP server. The reset register is in the variable memory area; refer to the LOGO! 8 communication manual for the exact register map and the supported function codes. Confirm the register map against the manual in your specific Soft Comfort version. The default Modbus TCP port is 502, and the default unit ID is 1.
7. Diagnostic flags: in LOGO! Soft Comfort V8.4, enable the status output of the HSC block. The status bit is high whenever the counter is active. Route this bit to a free output (for example, Q2) and connect Q2 to the dashboard indicator. The driver can then see at a glance that the overspeed protection is armed.
8. Two-sensor redundancy: if the safety integrity target requires dual-channel monitoring, use two high-speed inputs (for example, I3 and I4) and two HSC blocks with the same setpoint. The two HSC outputs go through an AND block. The AND output drives the safe-stop relay. If either sensor fails or disagrees with the other, the AND output goes low and the system is in a safe state. To bring the system back online, the operator must acknowledge the fault and reset both channels manually.
9. Scan time impact: each HSC block adds approximately 0.1 ms to the LOGO! scan time. Two HSC blocks add approximately 0.2 ms. The LOGO! 12/24 RCE has a maximum scan time of 400 ms. Even with eight HSC blocks, the scan time stays well below 1 ms. The scan time is visible on the LOGO! display under Diagnostics > Scan Time. If the scan time exceeds 100 ms, reduce the program size or move to a LOGO! 8 with more memory.
10. LOGO! web server: LOGO! 8 has a built-in web server that is accessible from any browser on the Ethernet network. The web server shows the live state of every input, every output, and every function block. Use the web server for remote commissioning and for operator diagnostics. The web server URL is the IP address of the LOGO!, for example, http://192.168.0.100. The default password is "LOGO" and should be changed on first commissioning to comply with the machinery IT security policy. Enable HTTPS in LOGO! Soft Comfort V8.4 or later to encrypt the web server traffic.
11. Firmware update: before updating the LOGO! 8 firmware, back up the program to a microSD card and to the LOGO! Soft Comfort project file. The firmware update may change the behavior of the HSC block or the pulse generator. After the update, re-run the commissioning procedure in full and re-document the threshold and the response time. The current LOGO! 8 firmware is V8.4 as of 2024; check the Siemens Industry Online Support portal for the latest firmware release notes.

Frequently Asked Questions