Siemens LOGO! LWE Softkey Bi-Directional Control via V-Memory

1. Overview: The LOGO! Softkey and the Web Editor Gap

Siemens LOGO! 8 (6ED1052-xxx08-0BA8) and LOGO! 8.3 (6ED1052-xxx08-0BA9) logic modules expose four on-device softkeys (F1-F4) on the integrated display and on the LOGO! TDE (6ED1055-4MH00-0BA0). These softkeys are mapped to four softkey function blocks, B001-B004, which behave as digital contacts in the user program. They are the only operator-input elements on the LOGO! panel that are not bound to a fixed input terminal.

The LOGO! Web Editor (LWE) - invoked through the Web Server menu of LOGO! Soft Comfort (LSC) - generates a custom HTML dashboard hosted by the LOGO! on-board web server. The on-board server is split into two distinct services:

• Standard / extended web server (firmware 8.0 onward, refined in 8.3.x and 8.4): exposes variables, flags, network inputs, and tag values through a configurable dashboard.
• Legacy simple web server (pre-8.0): exposes a small fixed set of operands without user-defined screens.

A documented engineering rule applies across both services: a softkey can only be operated from the LOGO! display or the TDE display, and a softkey cannot be operated from the web server or displayed as a writable value on the web server. The extended web server treats the four softkey blocks purely as digital read-only values - it can display their contact state but cannot accept operator input on them, and the LWE page generator does not expose them as editable widgets.

This creates a recurring engineering problem: a binary parameter (mode switch, enable flag, manual override) must be readable and writable from both the panel and the LWE page, with state synchronized bidirectionally and retained across power cycles. The remainder of this document describes the canonical solution using V-memory bits, digital network inputs/outputs, and a pulse relay.

2. Prerequisites

Confirm firmware version by navigating on the LOGO! display to System > Diagnostics > Versions or by reading the GET request banner on the on-board web server at http://<logo-ip>/. LSC and LOGO! firmware must match within the same major release; mixing 0BA7 and 0BA8 firmware on the same PC project is not supported.

3. Why Softkeys Cannot Be Edited in the Web Editor

Softkey blocks B001-B004 appear in the LWE variable memory configuration table as digital values. The on-board web server can read them, and the LWE page can show their contact state as a colored indicator. However, the HTTP PUT handler that processes operator input on the standard/extended web server has no binding to the B001-B004 internal storage. The handler is wired to:

• Variable memory bits V0.0-V850.7 (851 bytes / 6808 bits)
• Flag bits M1-M64 (read-only from web in firmware 8.0; partial read/write on 8.3+)
• Network inputs NI1-NI64 (read-only on the web; writes from the web are not allowed in 8.0, only display)
• Cursor keys and operator panel inputs (LOGO! 8.4 with display-equipped base module)

Softkey block writes are not exposed because the softkey is an HMI input device, not a writable operand. The internal input is the physical button on the display PCB. This is by design and is not affected by firmware updates or LWE revisions.

4. Architecture: V-Memory and Network I/O as the Bridge

The canonical bridge between the LOGO! panel, the LOGO! program, and the LWE page uses three resources per binary parameter:

1. A V-memory bit (e.g., V0.0) bound to a Digital Network Input (e.g., NI1). The on-board web server can write to V0.0; the LOGO! program reads NI1 as the operator-supplied toggle.
2. The four softkey blocks B001-B004, used strictly as trigger contacts (pulse sources) feeding a logic block.
3. An OR block that merges the softkey trigger and the network-input trigger into a single edge for a pulse relay, and a network output (e.g., NO1) that reflects the resulting state back to the web.

The pulse relay (current-flow sensing, function block group "Other") outputs a stable 0 or 1 that is the actual mode state. This output is the only memory of the mode and is the value displayed in the LWE page. The softkey blocks B001-B004 are not latched; they reset to 0 immediately after release, which is why they cannot serve as the mode storage.

The mapping is summarized below.

5. Mapping the Softkey to a Pulse Trigger

Open the LOGO! program in LSC and confirm that the softkey is configured as a momentary contact, not as a latched switch. The default softkey block latches its output while the button is held, which is not desired for an edge-triggered system. To use the softkey as a momentary push button, configure the block as follows:

1. In the program editor, double-click the softkey block (B001).
2. In the block properties dialog, set the function to Edge-triggered reset (pulse), or wire a rising-edge contact in front of the OR block.
3. Verify that the softkey output goes high only on the rising edge of the F1 button press and resets immediately on release.

If you leave the softkey in latched mode, every press of F1 will hold the contact closed indefinitely, and the OR block output will stay high. The pulse relay will toggle once on the first press but the second press will not register because the input is still high. Always use edge-triggered logic for the softkey path.

6. Building the Web-Side Button with a V-Bit

The on-board web server can set a V-memory bit to 1 through the standard web page, but it does not have a native "momentary" or "push-button" action - each V-bit write is a level assignment. To emulate a button press from the LWE page:

1. In LSC, open the project and select Tools > Web Server Editor.
2. On the left-hand variable tree, drag the desired digital value into the LWE page. Most LWE projects bind a checkbox or a button to a V-memory bit.
3. Click the variable to open its properties. Set the Access attribute to Read/Write and bind it to V0.0 through the parameter VM mapping. The on-board web server will display a checkbox-style control bound to V0.0.
4. Save the LWE project and download it to the LOGO!. The generated configuration is stored on the LOGO! flash and served by the on-board web server.

When the operator clicks the checkbox in the LWE page, the on-board web server sends an HTTP POST to the variable servlet with the value 1. The next click sends 0. The LOGO! program reads the value through NI1 (which is mapped to V0.0 in the network input configuration table) and feeds it to the OR block as a level signal. To make the LWE page behave like a push button - the standard expectation for a "change mode" action - configure the LOGO! program to detect a rising edge on NI1 rather than treat it as a level input. The edge detection block (function group "Other") converts the level to a single-cycle pulse on each press.

For a clean toggle button on the LWE page, wire the rising-edge output of NI1 directly to the pulse relay's TRG input alongside the softkey edge. The pulse relay will then toggle on either input - one from the panel, one from the web.

7. Synchronizing Display, Panel, and Web with OR + Pulse Relay

The OR block merges the two edge-triggered trigger sources (softkey F1 and web V0.0/NI1) into a single signal that drives the TRG input of a pulse relay (current-flow sensing). The pulse relay's output is the actual mode state and is the value the rest of the LOGO! program reads.

Pulse relay configuration in LSC:

• Block type: Pulse relay (current flow sensing), function group "Other".
• TRG input: connected to the OR block output.
• Output: connected to marker M2 and to a network output block (e.g., NO1).
• Retentive: enable retention on the M2 marker so the state survives a power cycle.

Expose the M2 marker (or the NO1 network output) on the LWE page as a read-only indicator. The indicator updates on every cycle, so the operator sees the new mode within 100-200 ms of pressing either the softkey on the panel or the button on the LWE page. The typical LOGO! 8 scan cycle is 5-20 ms, so the visible latency is dominated by browser polling or page reload, not by the LOGO! itself.

A common improvement is to add an XOR block between the mode state and a constant 1, which inverts the logic for display. Wire the XOR output to a Meldetext (message text) block on the LOGO! display that shows "Auto" or "Manuell". The XOR output is purely cosmetic and has no effect on the program logic; it simply maps the bistable state to a human-readable string.

8. Retentive Persistence Across Power Cycles

By default, the standard web server initializes all V-memory bits to 0 on every power-up, and the network inputs read those zeros. The on-board web server also loses its session state. To retain the mode across a power cycle:

1. In LSC, double-click the M2 marker. In the marker properties dialog, enable Retentive.
2. Confirm that the pulse relay output is wired to M2 (not just to a transient internal flag).
3. Configure the marker in the LOGO! display Retentive markers list under System > Retentivity. The number of retentive markers is hardware-limited; LOGO! 8, 8.3, and 8.4 each support up to 250 retentive markers in the standard block range.
4. Verify on the LOGO! display that the marker is listed under the retentive range. If the retentive count is exceeded, the program will compile but the excess markers will revert to 0 on power-up.

The network output NO1 is automatically updated from the M2 marker on the next cycle after power-up, so the LWE page will show the correct mode within one scan cycle of the LOGO! being reachable on the network. There is no need to "resync" the web page manually.

9. Reference Implementation: Auto/Manual Rollup Motor Control

A common real-world application is a pair of rollup shutter motors controlled either automatically (temperature, wind) or manually (operator button). The two modes must be selectable from both the panel (softkey F1) and the LWE page (checkbox), and the current mode must be visible on both. The reference implementation below is the recommended starting point.

9.1 Functional Requirements

• Softkey F1 on the LOGO! display toggles between Auto and Manual.
• A checkbox on the LWE page does the same.
• The current mode is shown on both the LOGO! display and the LWE page.
• Mode survives a power cycle.
• Auto mode enables the temperature/wind sensor path; Manual mode enables the operator push-buttons.

9.2 LSC Program

The Auto path uses M2 to enable the analog sensor comparisons; the Manual path uses M2 inverted (through XOR1, or through an explicit NOT block) to enable the operator push-buttons. This guarantees that exactly one path is active at any time and that the two paths cannot fight each other for control of the motor output. The XOR1 output also drives a Meldetext block that displays the current mode on the LOGO! display.

9.3 LWE Page

• Checkbox 1: bound to V0.0, label "Change mode (Auto/Manual)". The checkbox is read/write.
• Indicator 1: bound to NO1 (which mirrors M2), label "Current mode: Auto / Manual". The indicator is read-only.
• Indicator 2: bound to NI1 or the edge detector output, label "Last trigger source: Panel / Web". Optional but useful for diagnostics.

9.4 Scaling to Multiple Parameters

For each additional bi-directional parameter, allocate one V-memory bit, one network input, one network output, one pulse relay, and one retentive marker. LOGO! 8 supports 64 NI and 64 NO, 851 V-bytes, and up to 250 retentive markers - sufficient for most small-machine dashboards. Label the variables consistently (e.g., V0.0/NI1/NO1 for mode, V0.1/NI2/NO2 for enable, etc.) to keep the LWE page readable. Reserve V0.0-V0.7 for the most frequently toggled parameters to keep the variable table compact.

10. Step-by-Step Commissioning Procedure

1. Open the existing LOGO! program in LSC and confirm the LOGO! base module and firmware match the prerequisites in Section 2.
2. Add a new pulse relay (function group "Other") and wire its TRG input to an OR block whose inputs are the softkey B001 and a network input NI1 (bound to V0.0).
3. Connect the pulse relay output to a retentive marker M2.
4. Connect the same M2 to a network output NO1.
5. In the LWE editor, add a checkbox bound to V0.0 and a read-only indicator bound to NO1. Save the LWE project and download to the LOGO!.
6. On the LOGO! display, navigate to the program and test: press F1; the marker M2 should toggle and the LWE indicator should update within 200 ms after a browser refresh.
7. From a PC browser, open http://<logo-ip>, log in, and click the checkbox; the M2 marker should toggle and the LOGO! display should show the new mode string.
8. Cycle power to the LOGO! and confirm that the mode is retained. The LWE page should show the same mode it had at power-down within 5 seconds of the on-board web server becoming reachable.
9. Document the softkey label ("F1 - Mode"), the LWE checkbox label, and the expected indicator strings in the operator manual.

11. Verification Matrix

12. Troubleshooting and Edge Cases

13. Frequently Asked Questions

For the official LOGO! Web Editor user guide, see the LOGO! Web Editor Online Help (LWE_Help_en-US.pdf). Additional LOGO! system manuals, application examples, and firmware notes are available on the Siemens Industry Online Support portal.