Extending LOGO! 8 OBA8 Up/Down Counter Past 32767 for Flow Meters

1. Problem Statement

A 4-wire Hall-effect flow sensor (e.g., the Seeedstudio G3/4) outputs 330 pulses per litre. On a Siemens LOGO! 8 (OBA8) base module, accumulating that signal into a "total fuel dispensed" register quickly runs into a numerical wall: the on-board Up/Down Counter can hold 0 to 999,999 internally, but the moment you scale pulses into litres with a Gain or analog math block, the legacy 16-bit signed arithmetic ceiling of 32,767 clips the result. With one decimal of resolution (0.1 L) that means 3,276.7 L maximum — less than a single 8-hour shift on a 9 L/min dispenser.

This article documents the field-proven workaround: a two-stage cascaded counter pair. Counter 1 accumulates 0 to 330 pulses; on every 330-pulse carry, Counter 1 resets to zero and Counter 2 (whole litres) increments by one. Counter 2 then tracks 0 to 999,999 L in real time, with no division or scaling math required. The value can be displayed on the LOGO! Web Server, on the LOGO! TDE text panel, or polled by a SCADA tag.

2. Prerequisites

• Siemens LOGO! 8.3 (OBA8) base module: 6ED1052-1MD08-0BA1 (LOGO! 12/24RCE), 6ED1052-1MD08-0BA8 (12/24RCEo), 6ED1052-1CC08-0BA1 (LOGO! 24CE), or 6ED1052-1FB08-0BA1 (24CEo). The high-frequency inputs I3, I4, I5, I6 are rated for 5 kHz on these 24 VDC variants. See the LOGO!Soft Comfort online help: Up/Down Counter for the input-frequency table.
• LOGO!Soft Comfort V8.3 engineering software. Free download from the Siemens Industry Online Support portal.
• 24 VDC power supply, fused at 1 A, with the LOGO! and the flow sensor sharing the same GND reference.
• Hall-effect flow sensor: Seeedstudio SEN0216 (G3/4) or equivalent, 330 pulses/L ±10 %, open-collector NPN, 5 to 24 VDC, 1 to 30 L/min.
• 10 kΩ pull-up resistor if the sensor output is open-collector. The Seeedstudio G3/4 includes one internally; verify on the specific board revision.
• Ethernet cable for the LOGO! Web Server view.

3. Why the 32,767 Limit Exists

The Up/Down Counter block (function-block icon in FBD) in LOGO!Soft Comfort stores its value in a 32-bit register. When you reference the counter's numeric value in a Message Text, an Analog Threshold, or a Gain block, the legacy function libraries up through LOGO! 7 applied 16-bit signed integer math, clamping the result to -32,768 to 32,767. LOGO! 8 OBA7 (firmware 1.16.x and later) and OBA8 (firmware 1.81.x and later) extended several math blocks to 32-bit, but the on-screen text renderer and some analog blocks still exhibit 16-bit clipping depending on context.

The simplest, most portable fix is to never scale. Count whole units in a second register, and let the second register's natural 0 to 999,999 range do the work for you. The technique works on every LOGO! 8 firmware version, which makes it safe to deploy without auditing the controller's firmware build.

4. Hardware Wiring

4.1 Terminal Layout

For a LOGO! 12/24RCE (article number 6ED1052-1MD08-0BA1) the relevant terminals are:

• 1, 2: 24 VDC sensor supply output (fused internally, 200 mA total)
• 3, 4: 0 V (GND)
• 5: I1 (4 Hz, regular digital input)
• 6: I2 (4 Hz, regular digital input)
• 7, 8: AI1, AI2 (0 to 10 V analog, not used in this project)
• 9: I3 (5 kHz, fast counter) — FLOW SENSOR SIGNAL
• 10: I4 (5 kHz, fast counter)
• 11: I5 (5 kHz, fast counter)
• 12: I6 (5 kHz, fast counter)
• 13, 14: Q1, Q2 (relay outputs, optional — used here for a "tank low" or "refill needed" flag)

4.2 Sensor Connection

The Seeedstudio G3/4 has three leads: red (+), black (GND), yellow (signal).

The internal 10 kΩ pull-up on the Seeedstudio board pulls the open-collector transistor to +24 V when no flow is present, registering as a logic "1" on I3. Pulses are active-low; the LOGO! high-speed counter accepts both polarities depending on the counting direction configured in the block. Use a shielded cable, terminate the shield to PE at the LOGO! end only, and keep the run under 3 m to avoid capacitive degradation of the 5 kHz edges.

5. Cascaded Counter Logic

5.1 Block Diagram

5.2 Function Block Layout

The FBD program uses three blocks:

1. B001 — Up/Down Counter (pulse accumulator, 0 to 330)
2. B002 — Up/Down Counter (whole-litre accumulator, 0 to 999,999)
3. B003 — Reset (clears B001 on the carry event)

5.3 B001: Pulse Accumulator

• Counting input: I3
• Direction input: tied to logic "1" (count up)
• On threshold: 330
• Off threshold: 0
• Start value: 0
• Output: "On" flag fires for one scan cycle each time the count crosses 330 going up. This is the carry pulse.

5.4 B002: Whole-Litre Accumulator

• Counting input: B001 On output (a 1-scan pulse at the carry moment)
• Direction input: tied to logic "1"
• On threshold: 999,999
• Off threshold: 0
• Start value: 0
• Output: not used for control; the value is the running total

5.5 B003: Pulse-Counter Reset

• Trigger input: B001 On output
• Target: B001
• Action: resets B001 to 0 immediately after the carry edge

The two events — B002 increment and B001 reset — occur in the same LOGO! scan, which runs in roughly 1 ms on OBA8 firmware. The 1 ms scan is two to three orders of magnitude faster than the carry period at maximum flow (165 Hz pulse rate, carry every 2 s), so no race condition exists. If you observe a one-cycle glitch on B001 reading 331 before the reset fires, update to OBA8 firmware 1.82.x or later, which executes the Reset block in the same scan as the On-flag transition.

6. Program Code (FBD)

+24V ──┐
       ├──[I3]──► [B001 U/D]──(B001.On)──┬──► [B002 U/D]──► Fuel_Total_L
GND ───┘             Th=330  St=0         │         Th=999999  St=0
                                          │
                                          └──► [B003 Reset B001]

7. Parameter Table

8. Real-Time Display on the LOGO! Web Server

1. In LOGO!Soft Comfort, open Tools → Options → Ethernet. Enable the LOGO! Web Server and assign a static IP, for example 192.168.0.10/24.
2. In the Message Text editor, create a display line that references B002, e.g., Total: [B002] L. Rename the variable tag to Fuel_Total_L in the variables table for clarity on the web page.
3. Download the program to the LOGO! and cycle power.
4. From a PC on the same subnet, browse to http://192.168.0.10. The web server shows the live B002 register, updating every 1 s by default.
5. The web server does not reset the counter. It is a non-destructive viewer, satisfying the "I want to see real time" requirement that pure data-log roll-overs cannot meet.

9. Verification Procedure

1. With the program loaded and the sensor dry, observe B001 in Online Test. It should read 0 and remain stable.
2. Manually trigger the sensor with 1 L of fuel poured through it. B001 should briefly hit 330, B002 should increment from 0 to 1, and B001 should drop back to 0.
3. Pour exactly 10 L through the sensor. Confirm B002 reads 10 at the end of the test.
4. Run the dispenser for 60 minutes at 20 L/min. Expected B002 ≈ 1,200 L, accuracy ±10 % of nominal sensor calibration.
5. In Online Test, place a probe on the wire from the sensor to I3. At 20 L/min you should see a 110 Hz square wave with 50 ±5 % duty cycle.
6. Open the LOGO! Web Server from a phone. The page should refresh and show the value updating once per second without gaps.

10. Calibration

The Seeedstudio G3/4 datasheet quotes 330 pulses/L as "typical" with no tighter tolerance. To calibrate for custody-transfer or inventory control:

1. Pump 10 L of fuel through the sensor into a calibrated container.
2. Read B002 in Online Test. If it reads 9.7 L equivalent, the effective pulse density is 330 × (10 / 9.7) = 340 pulses/L.
3. Adjust B001's On threshold to 340. Subsequent readings will match the calibrated volume to ±0.5 % (limited by the integer pulse boundary).
4. Repeat after 90 days or 5,000 L of use, whichever comes first. Mechanical wear in the turbine bearings can shift the calibration by up to 3 % per year.

11. Retentive Behaviour

By default, both B001 and B002 lose their value on a power cycle. Enable retention on each block (Rem = On) to keep the cumulative total across outages. Keep retention off for B001; it is always reset by the carry and retains no useful information. Enable retention on B002 — this is your long-term total.

On the LOGO! 12/24RCEo and 24CEo variants, retention is part of the base module's on-board non-volatile memory. On the LOGO! 12/24RCE (non-o) and 24CE (non-o), retention uses the same memory area but the base module is non-Ethernet. The 12/24RCEo is the typical choice when the Web Server is required.

12. Troubleshooting Matrix

13. Comparison with S7-1200 High-Speed Counter

Engineers familiar with the S7-1200 may wonder why the LOGO! needs this workaround. The S7-1200 HSC (high-speed counter) supports a 32-bit integer range natively and can be configured for a single-shot or cyclic count mode, so a 0 to 999,999 L register with no scaling is the default behaviour. The LOGO! 8 OBA8 is positioned as a low-cost logic relay for sub-200-I/O applications, and the Up/Down Counter is its general-purpose primitive. The cascaded-counter trick is a LOGO!-idiomatic solution that costs three function blocks and zero additional hardware, which is well within the LOGO! 8's block budget (350 blocks on a standard base module, 400 on the -o Ethernet variants).

If you need 32-bit counters, four-quadrant arithmetic, or HSC interrupt routines, the S7-1200 is the right tool. If you need a 1-L-resolution totaliser on a 24 VDC input with 16 function blocks of logic, the LOGO! 8 OBA8 with a cascaded counter is the right tool. The decision pivots on I/O count, required precision, and existing engineering familiarity, not on raw counting performance.

14. Field Notes and Edge Cases

• Reverse flow: if the line can be pumped backwards, wire a direction signal to B001's Dir input. The carry logic still increments B002 only when the count crosses 330 going up, which is the desired behaviour for "net dispensed".
• Water hammer: a 5 kHz input rating gives you 6:1 headroom over the worst-case pulse rate, so water-hammer-induced spikes are filtered by the LOGO! input filter.
• Multiple sensors: parallel two G3/4 sensors by routing one to I3 and the other to I4. Each gets its own B001 / B002 / B003 chain. The two litre totals can be summed in a separate analog math block if you need a system total.
• Long cable runs: keep the sensor cable under 3 m and use shielded twisted pair tied to PE at the LOGO! end only. The 12/24RCE / 24CE inputs are Schmitt-triggered and tolerate up to 1 Vpp of common-mode noise.
• Firmware update: the OBA8 firmware is upgradable in place via LOGO!Soft Comfort. After any firmware update, re-verify the carry behaviour — Siemens has changed block execution order in minor releases (1.81.x → 1.82.x) and a one-scan delay on B003 can occasionally allow B001 to read 331 for one cycle.
• SD card data log as backup: if the LOGO! has an SD card slot, enable Data Log for the B002 variable. The SD card records to a CSV that can be imported into Excel for daily, weekly, and monthly totals. This is a non-destructive backstop — the live web server view remains the primary HMI.

15. Frequently Asked Questions