Siemens LOGO! OBA7 Clock Variable Workaround Using VM Counters

Overview: The LOGO! OBA7 Clock Variable Limitation

The Siemens LOGO! OBA7 is positioned as a nano-PLC for OEM and small-machine control. Its real-time clock is fully functional for scheduled switching (astronomical timer, weekly timer, yearly timer), but unlike a S7-1200 or LOGO! 8.3, the OBA7 firmware does not expose the RTC hour, minute, or second as a numeric variable that can be read into a VW (Variable Word) and compared in a ladder diagram. Engineers who try to do something equivalent to VW1 := RTC_HOUR; will find the address list in LOGO! Soft Comfort V7 (LSC V7) does not contain an RTC tag. The complete function-block reference is documented in the LOGO! 8 System Manual on Siemens Industry Online Support.

This article documents the three field-proven workarounds for capturing and comparing time values inside a LOGO! OBA7 program:

1. Data logging with the integrated SD-card slot or internal log buffer.
2. A custom hour counter built from a one-second pulse generator plus up/down counters, stored in the 850-byte variable memory (VM).
3. The built-in operating hour counter (OHC) block.

Why Direct RTC Access Is Restricted in LOGO! OBA7

LOGO! is a logic module, not a soft-PLC. Its firmware only exposes the RTC indirectly through dedicated function blocks: weekly timer, yearly timer, astronomical clock, and stopwatch. The internal RTC registers are not mapped into the variable memory. When the LSC V7 program editor shows the address pool, only the following tag types are selectable:

• Digital inputs I1...I24 (depending on base + expansion count)
• Digital outputs Q1...Q20
• Analog inputs AI1...AI8
• Analog outputs AQ1...AQ2
• Flag bytes M1...M64 (or higher in later firmware)
• Variable memory VW0...VW849 (in 16-bit word increments)
• Network inputs/outputs NI/NQ for LOGO! 8.1+

There is no VW_RTC_HOUR tag. The clock value is consumed only by the scheduling blocks. To compare the hour-of-day against a setpoint such as 17:30, you must either:

• Configure a weekly timer set to 17:30 whose output bit drives a comparison branch, or
• Reconstruct an hour counter yourself and load it into a VW.

This is why the search "LOGO! store current time in VW1" returns a question mark: the operation has to be synthesized, not read. The hardware overview and tag limits are summarized on the Siemens LOGO! product page.

LOGO! OBA7 System Architecture and VM Memory Layout

For a counter-based workaround, only three VM words are needed: VW1 (hour count, 0..23), VW2 (comparison setpoint), and VW3 (minute count, 0..59). The rest of VM remains free for application I/O. Retentivity must be enabled for VW1 and VW3 if the OBA7 is expected to ride through short power dips and resume counting from the saved value.

Workaround A: Data Logging with Internal Memory and SD Card

The simplest path is to stop trying to compute the hour inside the program and instead let the firmware write a timestamped CSV. Configure a Data Log block in LSC V7:

1. Open the LSC V7 project, drag the Data Log block onto the FBD sheet (Tools > Data Log Configuration).
2. Select destination: Internal memory (one file, last ~16,000 records) or SD card (continuous, one file per power cycle).
3. Set the sampling trigger: cyclic (1 s, 1 min, 1 h) or event-driven on a flag edge.
4. Choose columns: time stamp (firmware-injected), your process value (e.g., kWh register), optional state word.
5. Save the project and transfer to the OBA7.

The CSV on the SD card looks like this:

Timestamp, kWh_total, load_state
2024-03-12 14:23:01, 12455, 1
2024-03-12 14:24:01, 12458, 1
2024-03-12 14:25:01, 12461, 1

This is the recommended approach for electrical-consumption logging. You get the hour and minute "for free" because the firmware writes them, and the values can be pulled into a PC for graphing, peak detection, and time-of-use tariff analysis.

Workaround B: Building a Synchronous Hour Counter in VM

If the application needs the hour value inside the LOGO! program - for example to switch loads on/off, to compute time-of-day tariffs, or to write to a display - the recommended approach is to build a counter chain that mirrors the RTC behavior closely enough for control purposes. Three blocks are required:

1. A pulse generator set to 1 s on / 0 s off (or use the on/off delay of a flag).
2. A up/down counter preset to 60, counting each pulse, with its output wired to a counter-reset pulse generator for the next stage.
3. A second counter preset to 24, incrementing on each minute rollover.

The hours counter's current value is then available as its CV (current value) output, which can be routed into a VM word by adding an Analog Multiplexer or, more simply, by using the counter's CV display that the HMI/LOGO! TD reads directly.

Pulse Generation

Use a Pulse Generator (clock generator) block with T = 1 s, duty 50%, output wired to the count input of a Up/Down Counter preset to 60. The counter output is the minute pulse (one high-going edge per minute).

Counting Chain

1 Hz pulse -> [Counter C_min] preset 60, CV -> VW3
                |
                +-- on Q (CV=60) pulse 1 cycle, then reset C_min to 0
                    |
                    v
                    [Counter C_hour] preset 24, CV -> VW1
                |
                +-- on Q (CV=24) pulse 1 cycle, then reset C_hour to 0

The reset of each counter is self-triggered by its own Q output through a one-cycle delay (or by routing Q through an OR with the count enable so that the next clock edge rolls the value from preset back to 0).

Synchronizing to the Real RTC

The chain above will drift relative to wall-clock time because the LOGO! oscillator is not temperature-compensated and the cycle time is added to the 1 s period. To align, two options exist:

• SNTP sync (Ethernet base): enable SNTP client, set poll interval to 1 h. On each poll the firmware nudges the counter by adjusting the next 1 s pulse. This is not a direct injection into your counter; you still need a manual calibration block.
• Calendar tick from a weekly timer: configure a one-minute-once weekly timer on every day, connect its output to reset C_min and increment C_hour. This gives you minute resolution matched to the firmware clock.

Ladder Logic Implementation - Counter Chain Design

The FBD implementation in LSC V7 is straightforward. The equivalent ladder representation, useful for engineers familiar with RSLogix, is shown below. The four rungs map directly to LOGO! function blocks.

The "Analog Threshold Trigger" or "Analog Comparator" block in LSC V7 can be used to do the comparison directly, with VW1 on input AX and VW2 on input AY, output Q1 switching on the configured relation (>, <, =, <>).

Using the Built-in Operating Hour Counter

LOGO! OBA7 includes an Operating Hour Counter block (in the Special Functions library). It counts the cumulative hours that its trigger input has been high and exposes the elapsed time as a 16-bit value. This is not the time-of-day, but it is the right answer if your goal is to log the duration of a specific event (e.g., compressor run-time per shift).

For a factory-load-scheduling application the OHC is rarely the right block; it measures duration, not clock time. Use it in combination with a daily-reset flag if you need shift counters.

Power-Cycle and Synchronization Considerations

The OBA7 RTC is maintained by a super-capacitor for several days and indefinitely if the optional battery card is fitted. After a power cycle:

• The firmware RTC continues at the correct wall-clock time (the super-cap or battery holds it).
• The custom counter chain in VM restarts at 0 unless its VM bytes are configured as retentive. Even with retention, the count is offset by the down time.
• The data log file on the SD card is closed and a new one is opened, named with the new power-on timestamp.

For a tight sync between the firmware RTC and your VM counters, schedule a "snap" event once per day. In LSC V7:

1. Insert a Weekly Timer block.
2. Configure it for Every day, 00:00:00 to 00:00:01.
3. Wire its Q output to a Set/Reset block that forces VW1 := 0 and VW3 := 0.

The chain then resets to midnight once per day and counts up from there. Worst-case drift between two snap events is bounded by the LOGO! oscillator accuracy, typically a few seconds per day.

Verification and Commissioning

To prove the chain is working, run through this checklist on the bench or on-site before integrating with the load:

1. Set the LOGO! internal clock to 23:59:55 via LOGO! menu or LSC V7 online > Set Clock.
2. Force the controller into RUN.
3. Open LOGO! TD or the on-board display and observe VW3 (minutes). It should increment by 1 every 60 s and roll over at 60.
4. Observe VW1 (hours). It should increment by 1 each time VW3 rolls over, and roll over at 24.
5. Set VW2 to a value between 1 and 23 and verify Q1 (or your chosen output) goes high when VW1 >= VW2.
6. Trigger a power cycle for at least 5 s and verify the retentive VM bytes retain their values; non-retentive bytes reset to 0.
7. Verify the daily-snap weekly timer fires by changing the LOGO! clock to 23:59:55 on day 2 and confirming VW1 returns to 0 at the configured snap moment.

For a data-log-only application, verification is simply opening the SD card CSV and confirming timestamps match the LOGO! on-board clock. Drift test: read the timestamp at start of shift, read again after 24 h, subtract. With SNTP, drift should be < 1 s; without, expect +/-2 to +/-5 s/day.

When to Migrate Beyond LOGO! OBA7

The OBA7 is a competent micro-controller, but for applications that need direct RTC register access, structured text programming, or a global namespace of time tags, a step up is justified:

For an electrical-consumption manager on a factory floor, the LOGO! OBA7 + data log combination is genuinely sufficient, and a migration is only justified if the application needs real-time tariff switching at sub-hour resolution or billing-grade audit logs. Power-supply sizing for the OBA7 itself: 12/24 V DC at 1.5 W typical, 3.0 W max with full expansion rack, so the controller does not need a separate UPS if the SD-card log is the audit trail.

FAQ