LOGO! 0BA8 Remanent Memory: Storing Threshold and Analog Values

Overview

The Siemens LOGO! logic module is a compact controller widely deployed in small automation cells, building services, and OEM machine skids. When a project that originally used the LOGO! 0BA6 generation with two analog potentiometers is migrated to the 0BA8 (LOGO! 8) platform, the typical task is to replace the manual potentiometers with an external Text Display (TDE) and to enter the timing or threshold parameters from the keyboard. The migration immediately surfaces two engineering questions that are rarely answered clearly in marketing material:

1. How are operator-entered values actually stored inside the LOGO! 0BA8, and what survives a power cycle?
2. How does an operator change thresholds at run time without entering the engineering programming mode?

This reference addresses both questions with concrete procedures, parameter tables, and field-proven block configurations. It also documents a second, more demanding problem: how to remanently store an analog process value (such as the position of a roller-blind curtain driven by a tube that needs 15 turns for a full cycle) using the LOGO! 8 instruction set.

LOGO! 0BA6 to 0BA8 Hardware Migration

The 0BA8 series (also marketed as LOGO! 8) is a substantial redesign of the LOGO! 6 (0BA6) platform. Memory capacity, program-block count, networking options, and operator-panel integration all changed. The table below summarises the migration impact for projects that store threshold or analog setpoints.

Two part-number families are common in the field:

• LOGO! 8 12/24 V variants (with display): 6ED1052-1MD08-0BA0
• LOGO! 8 230 V variants (with display): 6ED1052-1CC08-0BA0
• External TDE: 6ED1055-4MH08-0BA0

Refer to the Siemens LOGO! product page for the full current catalog and the LOGO! system manual available on the Siemens Industry Online Support portal for rated voltages, I/O counts, and approvals.

LOGO! 0BA8 Memory Architecture

The 0BA8 stores program code and parameter data in three physically distinct regions:

1. Working memory (RAM) — holds the currently executing program and live process values. Lost on power down.
2. Retain memory (EEPROM) — non-volatile storage for the program itself and for any block marked Remanence = ON.
3. MicroSD card (optional) — program copy, data logging, and recipe storage. FAT16/FAT32 formatted cards up to 32 GB are supported.

When an operator adjusts a parameter from the TDE during operation, the new value is written to the block’s parameter slot in RAM. Whether that value also lands in EEPROM depends on the block’s Remanence setting, which is a per-block attribute configured offline in LOGO! Soft Comfort.

Remanence Fundamentals

Remanence in LOGO! is enabled per block, not globally. The relevant blocks for threshold and analog storage are:

The retained counters and timers share a fixed budget of 250 values across the program. Exceeding this budget will be flagged by LOGO! Soft Comfort at compile time, and the LOGO! 0BA8 will refuse to download the program if it overshoots the hardware limit.

Configuration Mode vs Operator Mode

The 0BA8 front panel and the TDE distinguish two user roles. Each role has different edit rights, which is the foundation of how operators change thresholds without reprogramming the device.

The Editable in operator mode flag is set in LOGO! Soft Comfort on each block’s parameter tab. When the flag is set, the parameter appears on the operator menu of the base-unit display and on the TDE without password challenge. When the flag is clear, the parameter is hidden from operator edit and can only be modified in configuration mode.

Configuring Threshold Values from the TDE

The following procedure sets a timer parameter that an operator can later edit from the LOGO! 8 base unit or the external TDE without entering configuration mode.

Prerequisites

• LOGO! Soft Comfort V8.x installed on the engineering PC.
• LOGO! 0BA8 base unit on the same Ethernet subnet, or USB cable connection.
• Block program loaded into the LOGO! and the device running in run mode.

Step-by-Step Procedure

1. In LOGO! Soft Comfort, open the project and select the timer block whose threshold you want to expose.
2. On the block’s parameter dialog, set the timer value (for example T = 00:00:30 for a 30-second on-delay) and tick the box Remanence.
3. Tick the box Editable in operator mode. The block will now appear in the operator menu of the LOGO! display and on the TDE.
4. Compile and download the program to the LOGO! 0BA8.
5. On the LOGO! display or TDE, navigate with the cursor keys to the parameter name. The value will be shown with an editable cursor.
6. Press OK to enter edit, use the ↑/↓ keys to change the value, and press OK again to commit.
7. The new value is stored both in RAM and, because Remanence is ON, in EEPROM. It survives a power cycle.

Storing an Analog Threshold with the Min/Max Block

The Min/Max block is the documented LOGO! mechanism for analog value retention. Although its primary purpose is to record the smallest and largest value seen on its input, it exposes a side-effect that is extremely useful for remanent storage.

Mode Parameter

The Min/Max block has a Mode parameter that takes values 1, 2, or 3. Mode 3 is the key to retention:

When combined with the block’s remanence flag, Mode 3 effectively turns the Min/Max block into a sample-and-hold with non-volatile storage: drive EN HIGH to capture the current analog value, drive EN LOW to freeze it, and the frozen value remains in EEPROM across power cycles.

Wiring Example

• Input AI1 (0-10 V) connected to the analog source.
• Min/Max block set to Mode 3, Remanence = ON.
• EN driven by a digital flag that the operator controls from the TDE.
• Output of the Min/Max block wired to an analog threshold trigger block whose setpoint is the desired window.

Retentive Up/Down Counter Method

For integer-quantised analog storage (for example, a 0-100 % curtain position on a 15-turn tube), the most robust pattern is the retentive up/down counter feeding an Analog MUX:

1. Insert an Up/Down Counter block. Set Remanence = ON. Configure Cnt range 0…1500 (100 % × 15 turns).
2. Wire the count-up input to the rotary encoder channel that pulses on curtain-up rotation.
3. Wire the count-down input to the rotary encoder channel that pulses on curtain-down rotation.
4. Insert an Analog MUX block. Use a constant selector so the MUX passes the counter output directly to its analog output.
5. Divide by 15 in LOGO! Soft Comfort using the Gain and Offset parameters on the next block (for example an Analog Comparator with reference value 100 for fully open).

This pattern guarantees that the absolute position is retained in EEPROM, because the counter value itself is flagged Remanence. Even after a complete power loss the LOGO! 0BA8 powers up with the last stored count, and the curtain resumes from the correct position.

Curtain Position Application (15 Turns, 5 kOhm Multi-Turn Pot)

The original field installation used a 5 kOhm 10-turn potentiometer geared 1:2 onto the curtain tube. The 10-turn pot gives only 5 effective turns before wrapping, and the gearing doubled this to 10; the total of 15 turns for a full open/close cycle exceeds the linear range, so the wiper eventually leaves the resistive track and the reading becomes unreliable. The corrective migration is to replace the potentiometer with an incremental rotary encoder (typically 100 or 360 pulses per turn) and decode the pulses with the LOGO! high-speed counter inputs.

Encoder Wiring Notes

LOGO! 0BA8 base units accept 24 V incremental encoders on the high-speed digital inputs I3 and I4 (on 12/24 V variants). Maximum input frequency is 5 kHz, which corresponds to roughly 800 RPM at 360 ppr — more than adequate for curtain drives. The encoder must be a two-channel quadrature device so the LOGO! firmware can resolve direction internally before applying the count to the Up/Down counter block.

Variable Memory (VM) and Tag Addresses

LOGO! Soft Comfort exposes the analog working memory through VM (Variable Memory) addresses. These are useful when a SCADA, HMI, or third-party device (Modbus TCP, S7 communication) needs to read or write the threshold stored in the LOGO!:

Each function block reserves a fixed offset in this VM area. Once the program is downloaded, the LOGO! Soft Comfort “Tools → Parameter VM Mapping” dialog will print the exact addresses. These addresses are stable for a given compiled program; recompiling only changes them when the block order changes.

Commissioning and Verification Checklist

1. Download the program with Remanence flagged on every block that must survive power-down.
2. Place the LOGO! in STOP and switch off supply power for at least 10 seconds.
3. Restore supply and verify in the operator menu that the timer setpoint and stored counter values are unchanged.
4. From the TDE, edit one threshold (for example timer from 30 s to 45 s) and confirm OK writes to RAM and EEPROM.
5. Cycle power again and confirm the new 45 s value is still present.
6. Connect LOGO! Soft Comfort in Online → Test and read back the VM word containing the parameter; verify it matches the operator display to within one LSB.
7. If the value resets, open the block properties and confirm the Remanence flag is ON. Also confirm the program fits inside the 250-value retain budget.

Troubleshooting Matrix

Notes on LOGO! Soft Comfort

LOGO! Soft Comfort V8.4 is the current engineering environment for the 0BA8 generation. Older V7.x projects can be migrated via File → Migrate. The simulation mode (View → Simulation) supports double-click parameter editing and is the fastest way to validate retention logic without a physical LOGO! on the bench. The integrated parameter VM mapping export provides a CSV that documents every VM address used, which is the recommended cross-reference for any external Modbus TCP or S7 communication that needs to read or write these thresholds.

FAQ