Siemens LOGO! 0BA8 Rotary Speed Measurement and Counting

Overview

Measuring rotary speed on a Siemens LOGO! 0BA8 base module (the eighth-generation LOGO! released in 2014 and still in production) requires distinguishing between a true analog 0-10 V tachometer signal and a digital pulse train from an encoder or proximity switch. The 0BA8 family exposes four 5 kHz high-speed counter inputs (I1-I4) on its DC-powered variants and two 0-10 V analog inputs (AI1 on I7, AI2 on I8) on the LOGO! 12/24 RCE base. This article documents hardware selection for 6ED1052-2MD08-0BA0 (LOGO! 12/24 RCEo) and the three other 0BA8 fast-counter-capable base modules, the analog-versus-digital decision, counter and clock configuration in LOGO! Soft Comfort, RPM math, and a ±5% deviation alarm built from analog comparators. The methodology parallels the gated-count frequency principle used in larger Siemens controllers; for comparison see the Siemens TIA Portal rotational-speed documentation for S7-300/400 and the National Instruments rotational-speed reference.

Identifying the Correct LOGO! 0BA8 Base Module

High-speed digital counting at 5 kHz is only available on the DC-supply 0BA8 base modules. AC-supply variants (e.g., 6ED1052-1FB08-0BA0 at 115/230 V AC) omit the fast-counter hardware and must not be used for encoder or proximity-switch applications. The four DC variants that support high-speed counters are listed below; the user-reported order number 6ED1052-2MD08-0BA0 corresponds to the LOGO! 12/24 RCEo, a 12/24 V DC unit with four relay outputs and no on-board display (the trailing "o" denotes "without display").

All four 0BA8 base modules share the same I/O map: I1-I4 are high-speed digital inputs at 5 kHz, I5-I8 are standard digital inputs (max 4 Hz debounce), and the analog inputs AI1 and AI2 share physical pins I7 and I8 respectively. To use a 0-10 V analog input the corresponding digital input function must be disabled in LOGO! Soft Comfort's input configuration. The complete I/O and specification matrix is published in the LOGO! 8 system manual, available as Siemens Industry Online Support entry 109741041.

Signal Type Decision: 0-10 V Analog vs. Digital Pulse Train

The phrase "0-10 V" in the original specification is ambiguous; the LOGO! 0BA8 input hardware, the choice of function block, and the RPM formula all change depending on which signal class the sensor actually emits. The three relevant cases are summarised in the table below.

Case A: 0-10 V Line-Driver Encoder

Some incremental encoders (e.g., selected IFM, Balluff, or SICK models) provide 0-10 V square-wave outputs as an alternative to the more common 24 V HTL or RS-422 levels. The 0-10 V high level is well above the LOGO! digital input VIH minimum of 8 V (at 24 V supply) and below the 30 V absolute maximum. The 0-10 V output must be capable of sourcing the LOGO! input current; consult the encoder datasheet for the output current rating. Treat each rising or falling edge as a count event. The user requirement to "count the changes from 0V to 10V" maps to this case directly.

Case B: 0-10 V Analog Tachometer

True tachometer outputs (e.g., from eddy-current probes, optical tachometers, or some vibration sensors) emit a DC voltage proportional to rotational speed. Wire to AI1 (pin shared with I7) or AI2 (pin shared with I8). Disable the digital input function on I7/I8 in LOGO! Soft Comfort. Apply the linear conversion formula n = (V_out / 10) × n_max, where n_max is the sensor's full-scale rpm rating (e.g., 3000 rpm). The LOGO! 0BA8 analog inputs have 10-bit resolution (0-1000 raw units for 0-10 V), giving a best-case resolution of 9.8 mV. At 3000 rpm full-scale this is approximately 2.93 rpm per LSB.

Case C: 24 V HTL Digital Pulse

Standard industrial encoders (e.g., SICK DFS60, Kübler 8.5800, IFM RN series) output 24 V push-pull or NPN/PNP open-collector signals. The "0-10 V" in the original specification likely refers to the encoder's 5 V or 24 V supply rail, not the signal level. Wire the A channel to I1, I2, or I3 (any of I1-I4) and the optional index (Z) channel to I4 if direction sensing is required.

System Topology

The recommended topology for a three-channel speed monitor on a single LOGO! 0BA8 base module is shown below. All three sensors share a common 24 VDC regulated supply, and the LOGO! exposes RPM data over Modbus TCP via the on-board RJ45 port for visualisation on a PC browser, HMI, or SCADA. The shield of each sensor cable is bonded to the panel entry plate only to avoid ground loops.

Prerequisites and Wiring

1. LOGO! 0BA8 base module from the table above. Recommended 6ED1052-1MD08-0BA0 if on-device display and on-board parameterisation is required; 6ED1052-2MD08-0BA0 if remote configuration only.
2. LOGO! Soft Comfort V8.x engineering software installed on a Windows 10/11 PC. The current version is available from Siemens Industry Online Support; the LOGO! 8 system manual is published as entry 109741041.
3. Ethernet patch cable (Cat 5e or higher) for PC-to-LOGO! connection. The LOGO! 0BA8 supports both DHCP and static IPv4 addressing on the RJ45 port.
4. Two incremental rotary encoders matched to the 6 rpm operating point. PPR selection rule: aim for 1 Hz to 1 kHz output frequency for best LOGO! resolution. With 6 rpm, a 100 PPR encoder yields 10 Hz; a 1000 PPR encoder yields 100 Hz; both are inside the 5 kHz counter ceiling.
5. One proximity switch or inductive sensor with 24 V PNP NO output (or 0-10 V analog output, depending on the signal case selected above). The 20 rpm target yields 0.33 Hz at 1 pulse per revolution; a multiple-pulse target wheel is recommended to keep the measurement within the LOGO!'s cycle-time budget.
6. 24 VDC regulated power supply sized for LOGO! consumption (90-200 mA base) plus sensor load. A 2 A supply provides headroom for cable runs up to 30 m with 0.5 mm² conductors.
7. Optional LOGO! AM2 analog input module (6ED1055-1MA00-0BA0) if the application requires more than two analog channels.

Use shielded twisted-pair cable (e.g., LiYCY 4×0.34 mm²) and ground the shield at the panel entry plate only. Route sensor cables at least 200 mm from VFD power cables to avoid common-mode coupling. The 0BA8 base module accepts 0.5-1.5 mm² conductors in its spring-loaded or screw terminals; torque the screw terminals to 0.6 Nm.

Speed Calculation and Scaling

Rotational speed in revolutions per minute is derived from the pulse frequency f of the encoder or sensor:

where PPR is the pulses-per-revolution rating of the encoder. If the application uses a single-pulse-per-revolution reference (typical of inductive proximity switches aimed at a single key, bolt, or tooth), the formula simplifies to n = f × 60. For multi-pulse target wheels (e.g., 60-tooth gear on a 20 rpm shaft giving 20 Hz output), use the full (f × 60) / PPR form with PPR = 60.

The pulse frequency f is obtained in one of two ways:

1. Count pulses over a fixed time window: f = N / t_window (gated-count method).
2. Measure the time between consecutive rising edges: f = 1 / T_period (period method). The period method gives faster convergence at low speed but is not directly supported by the LOGO! 0BA8 high-speed counter block.

For a 1-second measurement window the LOGO! clock marker M8 (1 Hz) can be used as the gate. The resulting RPM after one update is:

For the user's example with 6 rpm and 100 PPR: n = (10 × 60) / 100 = 6 rpm. For 20 rpm at 1 PPR (single pulse per rev): n = (0.33 × 60) / 1 = 20 rpm. The scaling factor used inside the LOGO! Analog Amplifier block is therefore Gain = 60 / PPR (0.6 for PPR = 100, 60 for PPR = 1).

For a 0-10 V analog tachometer (Case B) the linear conversion is n = (V_out / 10) × n_max. If the sensor full-scale is 3000 rpm, then 5 V = 1500 rpm and 8 V = 2400 rpm. The LOGO! 0BA8 analog input resolution is 10 bits (0-1000 counts), so the LSB at 3000 rpm full-scale is 3 rpm. Apply a moving-average filter of 4-8 samples to reduce display jitter on the LOGO! TDE or web server. The scaled value can be exposed directly on the LOGO! display (in units of 0.1 rpm) by setting the block's "Number of decimal places" property.

LOGO! Soft Comfort Program Example

Build the program in LOGO! Soft Comfort V8.x using Function Block Diagram (FBD) mode. The following block list implements gated-count RPM measurement on three channels plus a ±5% alarm. All blocks fit in the LOGO! 0BA8 program memory (max 400 function blocks per program, 8500 bytes total memory on LOGO! 8.3 firmware).

The block-naming convention in LOGO! Soft Comfort V8.3 is "Up/Down Counter" with the high-speed input assigned to I1, I2, I3, or I4 (the only inputs with 5 kHz hardware counters). The user opens the block properties, selects the input terminal from the dropdown, and sets the "On threshold" to a value above the expected pulses per cycle (e.g., 999999 to disable the threshold-based output). Up to three high-speed counters can be active simultaneously on a single 0BA8 base module; the fourth input I4 is reserved for the acknowledge pushbutton in this example.

For the analog-comparator blocks, set the on-threshold to the upper bound (+5%) and the off-threshold to the lower bound (-5%) of the running mean RPM. The 5% window is computed as a fixed value, e.g., for a 6 rpm target the window is 5.70 to 6.30 rpm. Adjust these bounds once during commissioning using the LOGO! TDE on-device display or the LOGO! Web Editor (LWE) interface.

±5% Threshold and Alarm Logic

Implement the ±5% window as a hysteretic on/off comparator rather than a single point-equality check. The hysteresis prevents relay chattering when the speed oscillates around the 5% boundary. The window is centred on the mean of the three measured speeds, recomputed every 1 s:

The OR block (B13) asserts if any pairwise deviation exceeds the window. The 2-second on-delay (B14) suppresses nuisance trips during acceleration ramps. The RS flip-flop (B15) latches the alarm until the operator presses the acknowledge pushbutton wired to I4.

For applications requiring a different tolerance per channel (e.g., allow ±10% on the proximity switch but ±2% on the encoders), replace B10-B12 with three separate Analog Comparators, each with its own gain and offset parameters. The LOGO! 0BA8 supports up to 64 analog comparator blocks and 32 math instruction blocks per program.

Verification, Commissioning, and Modbus TCP

1. Open LOGO! Soft Comfort V8.x and connect to the LOGO! base module via Ethernet. The default IP address of a factory-fresh 0BA8 is 192.168.0.10; the PC must be on the same subnet (e.g., 192.168.0.1/24).
2. Open the project and verify that the program is in PC → LOGO! direction. Click "Transfer → PC → LOGO!" to download the configuration.
3. Power-cycle the LOGO! and watch the boot LED sequence: green RUN should illuminate within 5 seconds, indicating successful program load.
4. Force a known frequency on each input by rotating the encoder by hand at a steady rate and counting the pulses with an external oscilloscope on I1, I2, I3. Compare the LOGO!'s internal counter value (visible via LOGO! TDE or web server) with the oscilloscope count.
5. Verify the RPM reading using a handheld tachometer aimed at the rotating shaft. The LOGO! display should agree within ±0.5% at 6 rpm and within ±1% at 20 rpm (subject to the 1 s sample window quantisation).
6. Force a deviation by stalling one encoder and confirm the alarm relay Q1 picks up after 2 s. Reset using the acknowledge pushbutton on I4.
7. Monitor the LOGO! web server at http://192.168.0.10 from any browser on the network. Enable the "LOGO! Web Editor" (LWE) in the project settings for live variable display without needing the LOGO! Soft Comfort IDE.
8. Record the Modbus TCP register map (function code 0x03 read holding registers, base address 0x0000) for integration with an HMI or SCADA. The LOGO! 0BA8 Modbus server exposes all digital and analog block values starting at register 0x0000 for inputs and 0x0800 for outputs; consult the LOGO! 8 system manual entry 109741041 for the complete mapping.

Troubleshooting Matrix

Edge Cases and Field Notes

Very low speed. At 6 rpm with 1 PPR the measured frequency is 0.1 Hz; a 1-second sample window yields one pulse per 10 s, and the LOGO!'s 1 s update rate will be unstable. Increase PPR (use a 60-tooth gear on the shaft) or extend the sample window to 10 s using a 0.1 Hz clock marker or a custom counter on M8 (divide-by-10).

Analog noise pickup. If the analog tachometer is on the same cable tray as a VFD output, add an RC filter (10 kΩ + 100 nF) at the LOGO! analog input to attenuate PWM common-mode. The LOGO! 0BA8 input impedance is 72 kΩ for the 0-10 V range, so the filter time constant is approximately 7.2 ms (cutoff ≈ 22 Hz), which is adequate for the 6-20 rpm speed range.

Modbus TCP throughput. The LOGO! 0BA8 Modbus server refreshes its register map at the LOGO! cycle time (typically 10-50 ms). For a 1 Hz update, this is non-blocking. For SCADA polling above 10 Hz, place a Modbus gateway buffer in front of the LOGO! or reduce the SCADA poll rate. The LOGO! supports up to 8 simultaneous Modbus TCP connections per the LOGO! 8 communication manual.

Web server security. The LWE web server on the LOGO! 0BA8 has no built-in authentication on legacy firmware. From firmware version FS:04 (released 2016) onward, the LOGO! supports password protection on the LWE and the LOGO! Soft Comfort remote access. Verify the firmware version in the LOGO! menu under "Diagnostics → Product Info" or via Modbus register 0x0000 (article number). Upgrade to the latest firmware from Siemens Industry Online Support entry 109741041.

Direction sensing. If the encoder is mounted on a reversible drive and the application must distinguish forward from reverse, wire the encoder's B channel to a second high-speed input and select "Quadrature" mode in the Up/Down Counter block. Quadrature decoding multiplies the apparent count by 4 per PPR, so divide the resulting rpm by 4 in the scaling block. Direction is available as the "Dir" output of the Up/Down Counter block and can be displayed on the LOGO! TDE.

Cross-platform validation. For higher-frequency or higher-resolution applications (above 1 kHz pulse rate, or 16-bit+ RPM resolution), the Siemens FM 350-1 / FM 350-2 counter modules on the S7-300 / S7-400 platform implement a native rotational-speed measurement mode that the LOGO! 0BA8 cannot match. The FM 350 modules support gate-time-based frequency measurement with hardware integration, as described in the Siemens TIA Portal counting and measuring documentation for S7-300/400. The same counting principles (gated count, fixed integration time, threshold-based alarm) apply to both platforms.

Sensor diagnostics. The National Instruments rotational-speed reference notes that the most common tachometer technologies are proximity probes and optical transducers. For each, the sensor datasheet must list the PPR (or pulses-per-revolution) rating and the maximum output frequency. Verify that the maximum expected frequency stays below 5 kHz for the LOGO! 0BA8 high-speed inputs; for a 6 rpm shaft with 1000 PPR encoder the maximum is 100 Hz, well within budget.

Performance, Cycle Time, and Platform Notes

The LOGO! 0BA8 executes its program in a fixed scan cycle that scales with the number of active function blocks. A typical 30-block program runs at an 8-15 ms cycle time, and the 1 s sample window therefore introduces a quantisation of roughly ±1% on the measured rpm. For the user's 6 rpm application this is 0.06 rpm, well inside the ±5% alarm window. For the 20 rpm application the quantisation is 0.2 rpm, also inside the window.

If the program grows beyond approximately 50 active blocks, the cycle time can exceed 20 ms and the analog-comparator response begins to lag behind the 1 Hz M8 clock. In that case, the LOGO! 0BA8 DM16 digital/analog expansion modules (e.g., 6ED1055-1CB00-0BA0 for 8 DI / 8 DO, or 6ED1055-1MA00-0BA0 for 4 AI) add I/O without adding scan burden to the main program. For applications beyond 8 high-speed counters or 4 analog inputs, migrate to the S7-1200 platform with the SB1221 high-speed counter signal board, which supports 30 kHz / 200 kHz counting and provides native frequency and period measurement function blocks.

Persistent RPM values (e.g., for an hours-counter or odometer) should be stored in Retain-tagged variables. The LOGO! 0BA8 supports 250 bytes of retain memory; mark the hours-counter block as "Retain = On" in the block properties. Non-retain values are reset to their default (0) on every power cycle, which is the desired behaviour for a running RPM but not for a totalised hours counter.

FAQ