Siemens LOGO! 8 MOVE Function: VM and Analog MUX Workaround

Overview

Engineers migrating from OMRON, Mitsubishi, or Allen-Bradley PLCs typically expect a generic MOVE instruction that writes a constant or copies a tag into a data register (for example, OMRON's MOV #100 D100). Siemens LOGO! 8 does not expose a MOVE block in the function library because the LOGO! 8 is a logic module rather than a register-based PLC: it has no flag words, no data block (DB) area, and no floating-point engine. All values that exist in a program are bound to the input or output parameter of a function block, and the wiring itself constitutes the data transfer.

Despite the absence of a MOVE primitive, you can emulate a register write using three field-proven techniques: the Analog MUX block (4-value selector), the Analog Amplifier and threshold-switch chain (single-value latch), and the variable memory (VM) area that the LOGO! 8 web server and integrated HMI expose for parameter binding. This reference documents the architecture behind those workarounds, the parameter mapping, the FBD sample logic for a daily-production shift roll-over, and the threshold at which you should migrate the application to a SIMATIC S7-1200.

Why LOGO! 8 Has No Native MOVE Block

LOGO! 8 is positioned in the Siemens catalog as a logic module rather than a programmable logic controller in the strict sense. The internal data model reflects that positioning:

• No data registers in the OMRON D-, W-, or H- sense.
• No flag words (MW) or memory bytes (MB) as in S7-300/S7-400.
• No instance data blocks (DB) for global tags.
• No floating-point arithmetic; only 16-bit signed integer math (range −32 768 to +32 767).
• Only two editor languages: Ladder Diagram (LAD) and Function Block Diagram (FBD).

Inside LOGO! Soft Comfort every value lives as an input pin or an output pin of a block. The compiler does not allocate a named variable that you can target with a "MOVE into tag X" instruction. To transfer a number from one block to another, you wire the source pin to the destination pin, or you use a dedicated routing block (Analog Amplifier, Analog MUX, Analog Flag).

LOGO! 8 Memory Architecture: Variable Memory (VM)

The only addressable storage that LOGO! 8 exposes to the user is the variable memory (VM) area, a table of 16-bit words. The VM is what the integrated Web server and LOGO! TD text display reference when you bind a monitoring tag to the screen. The size of the addressable VM depends on the LOGO! 8 base unit and firmware generation; always confirm the actual count in the device properties of LOGO! Soft Comfort before assuming a specific depth.

To write a constant to a VM word from FBD, you wire a constant block or an arithmetic block output to the connector that represents that VM word. The wiring itself is the MOVE. A separate MOVE block is unnecessary.

Workaround 1: Analog MUX Block as a 4-Value Register

The Analog Multiplexer (Analog MUX) block accepts four integer inputs (V1, V2, V3, V4) and routes one of them to its output AQ, depending on the binary state of two selector inputs (S1, S2):

When the LOGO! executes the block, AQ takes the value of the selected input. If you wire AQ into the input of a downstream block (for example, a counter setpoint, a threshold switch, or a VM-bound output), you have effectively executed a register write without a MOVE instruction.

FBD wiring pattern

1. Place four Constant blocks containing the integer values you want to expose (for example, 100, 250, 500, 1000).
2. Feed them into V1..V4 of a single Analog MUX block.
3. Drive S1 and S2 from two digital inputs or two internal flag bits.
4. Wire the AQ pin to the downstream block (e.g., the Setpoint input of an On-Delay timer, or the SP of a counter).

This pattern is the cleanest substitute for an OMRON MOV when the destination is a parameter pin that LOGO! 8 already supports.

Workaround 2: Counter Latch with Threshold Comparator

For the classic "move today's production count into the previous-day register at end of shift" use case, a latch composed of an Up/Down counter, a threshold switch, and a second counter (or a VM word) is more efficient than a 4-value MUX:

1. Counter Cnt1 accumulates units on the current shift; its CV is bound to a tag the HMI reads as "Today."
2. At end-of-shift, a one-shot pulse (Positive Edge of the shift-change flag) triggers Counter Cnt2 to load Cnt1.CV via the Set Value input. Cnt2 is bound to a tag the HMI reads as "Yesterday."
3. A Threshold Switch on Cnt1.CV resets Cnt1 to 0 at the same moment, starting the new shift count.

This method achieves a MOVE of the live count to a retained register without an explicit MOVE block. Both Cnt1 and Cnt2 must be configured as retentive if you need the values to survive a power cycle; the user-flag-bit and the SD card on supported LOGO! 8 variants provide retention.

Workaround 3: Analog Input → Amplifier → MUX → Analog Output Chain

A widely cited method for routing live process values through the program is the "Analog Amplifier + Analog MUX + Analog Output" chain:

The Analog Amplifier scales the raw analog input (for example, 0–10 V from a sensor) into the integer range supported by the LOGO! (the default gain = 1.0 path). The MUX selects between that live value and an alternative, and the Analog Output block exposes the result to a VM word that the web server or a downstream block reads.

Sample Application: Daily Production Counter Shift Roll-Over

The following FBD pattern implements an end-of-shift "MOVE" of the live count to a previous-shift register using only standard LOGO! 8 blocks. This is the canonical example asked about in the user community and is the closest functional equivalent of a register copy in a non-register architecture.

Block list

Sequential state machine

At the rising edge of the shift-change digital input (for example, a pushbutton wired to I1, or an internal flag driven by the time-of-day scheduler), the Threshold Switch fires for one cycle. During that cycle, Counter_2 latches Counter_1.CV into its CV (the previous-shift value), and the Wipe Relay resets Counter_1 to 0 to start the new shift. Both Counter_1 and Counter_2 are retentive, so the snapshot survives a power-down event.

Web server / TD binding

Bind the HMI element "Today" to the VM word that tracks Counter_1.CV, and "Yesterday" to the VM word that tracks Counter_2.CV. The user can read both values from the LOGO! built-in web server or from a connected LOGO! TD display. If a writable element is bound (for example, an operator-set setpoint), set a strong password on the LOGO! web interface; without authentication, any operator on the network can write to the bound tag.

Programming Method: FBD vs LAD for Data Operations

Engineers with a strong Ladder background often default to LAD in LOGO! Soft Comfort. For data-transfer workarounds, FBD is materially more productive because every block pin is visible as a wire, and you can lay the source block, the routing block (MUX, Amplifier), and the destination block out linearly.

If you have a portfolio of OMRON Ladder code that uses MOV heavily, the conversion effort to FBD is typically lower than the equivalent LAD rewrite, because you can place the four candidate constants on the canvas, wire them into the MUX, and route the output directly to the destination parameter.

Data Type Limitations: Integer-Only Processing

LOGO! 8 evaluates all math in 16-bit signed integer (INT16). Practical consequences for the MOVE substitution:

• Constants above 32 767 (for example, 100 000 from a flow totalizer) must be scaled down before being stored. The LOGO! 8 will clip the value at the integer limit and may produce a runtime warning in LOGO! Soft Comfort's online test.
• Floating-point division (for example, 7 / 3 = 2.333) is not supported; the result is the integer 2 with truncation. If you need fractional precision, use a SIMATIC S7-1200 with REAL (32-bit float) arithmetic.
• Negative numbers are supported, so the Analog MUX can hold negative setpoints (for example, V1 = −50) — useful for setpoint offsets in temperature control.
• The Analog Amplifier gain and offset parameters are themselves integers; precision below 1 unit is not achievable in the standard library.

LOGO! 8 vs S7-1200: When MOVE Becomes Critical

Use the table below as a migration-decision matrix. If two or more rows describe your application, plan a step-up to S7-1200. The IOT2000 sits between the two as a Linux / Arduino-class industrial platform: it is a valid option if you need data logging and a Linux file system, but it is not a drop-in replacement for an OMRON PLC program.

Commissioning and Verification Procedure

Use the following procedure to validate a MOVE-substitution program on the bench before deploying to the field.

1. Open the program in LOGO! Soft Comfort and select Simulation → Online Test. The simulator executes on your PC and exposes the VM area and the web-server binding in real time.
2. Force the S1 and S2 selector inputs of the Analog MUX to each of the four combinations (00, 01, 10, 11) and verify that the AQ pin and the downstream block read the expected constant from the MUX input table.
3. For the shift roll-over sample, drive the shift-change digital input with a one-shot pulse and confirm that Counter_2.CV = Counter_1.CV (pre-pulse) and Counter_1.CV = 0 (post-pulse). Record both values in the LOGO! Soft Comfort watch table.
4. Power-cycle the LOGO! 8 base unit and confirm that both Counter_1.CV and Counter_2.CV are retained. If retention fails, the counter is not configured as Retentive, or the battery / SD card retention option is not enabled in the base-unit properties.
5. Open the LOGO! web server in a browser and navigate to the bound tags (Today / Yesterday). Verify the live values match the LOGO! Soft Comfort watch table.
6. If a TD text display is fitted, navigate to the screen bound to the same VM words and confirm the values render correctly on the LCD.
7. Document the four MUX input constants and the selector truth table in the program comment so that the next maintainer can re-derive the relationship without a ladder walk-through.

Edge Cases and Field-Proven Caveats

• Power-failure safe MOVE: The MUX technique is not retentive; the MUX output reflects whatever the selector pins select on the next scan. To persist a value across power-down, the destination must be a retentive counter, a retentive on-delay timer, or a VM word on a supported base unit.
• PWM and motion control: Do not substitute a MOVE-equivalent for a stepper setpoint — LOGO! 8 has no closed-loop motion controller. Use a SIMATIC S7-1200 with PTO outputs or a SINAMICS drive on PROFINET.
• SD card vs internal retention: Confirm retention in the base-unit properties before relying on it for production data; some LOGO! 8 base units require an SD card for full retentive storage.
• Integer overflow on constant write: Constants above 32 767 written to a MUX input will overflow the INT16 range. Scale the value upstream and never trust the offline simulator to catch the wrap.
• Web server authentication: When the VM words are bound to a writable web-server element, set a strong password on the LOGO! web interface; without authentication, any operator on the network can write to the bound tag.
• Selector pin bounce: If S1 / S2 are driven by mechanical contacts, debounce the input with an On-Delay or use the digital input filter of the LOGO! base unit; otherwise a single shift-change flag may fire the threshold switch multiple times per transition.

Frequently Asked Questions