Resolving LOGO! LWE Timer Parameter Write Failures via VM Mapping

Overview

Siemens LOGO! 8 logic modules ship with a built-in web server and the LOGO! Web Editor (LWE) tool, which lets engineers build custom HTML dashboards for remote monitoring and control. While LWE supports writing to markers, outputs, and network I/O, the editor intentionally blocks writes to physical inputs and to most function block (FB) parameters. This restriction surfaces as a hard failure when an engineer attempts to change a delay-timer preset (TH or TL) from a remote browser: the value either does not transfer to the running program or LWE refuses to expose the field as writable.

This reference documents the working solution based on Parameter-VM Mapping in LOGO! Soft Comfort (LSC) combined with LWE's writable VM-address fields. It is written for engineers using LOGO! 6ED1052-1MD08-0BA1 (LOGO! 8.3) with firmware V1.83.x and LWE 1.1.0, and applies to all LOGO! 8 variants that ship a web server (BA, BB, and 6ED1052-1MD08-0BA1 / 0BA2 hardware versions).

Problem Description

When a circuit uses an on-delay (B001-type) or off-delay timer block, the TH (high-byte time) and TL (low-byte time) parameters must be set to a constant in the LSC schematic, or driven by an analog signal. To support remote adjustment, an engineer typically inserts an Analog Amplifier (B002) with a writable input that feeds TH.

Symptoms observed on LOGO! 6ED1052-1MD08-0BA1 with firmware V1.83.02, LSC 8.3, and LWE 1.1.0:

• LWE marks the analog-input field (AI) as read-only. The value cannot be changed from the web page.
• If the engineer wires the page to a marker (M) or output (Q), the value transfers from the browser to the LOGO!, but the running timer continues to hold its original TH and ignores the live value.
• Digital inputs (I) and network digital inputs (NI) used as pushbuttons are similarly read-only in LWE. Only physical I/O can drive them.
• The AX output of an analog amplifier can be mapped to TH in LSC, but because the writable side of the amplifier is an input pin, LWE refuses to expose it.

The root inconsistency appears to be that LWE permits writes only to a closed set of writable memory types, and that set excludes physical inputs, network inputs, and direct FB parameter references.

Root Cause

LWE 1.1.0 enforces a write-whitelist on every address exposed in the project file (.lwe):

Function block parameters such as TH, TL, On, Off, SP, PA, Par are not in this whitelist. The Parameter-VM mapping feature was introduced precisely to bridge this gap: it projects an FB parameter into a 16-bit VM word, which LWE can read and write.

Solution Architecture

Map the timer preset to a VM word using LSC's Parameter-VM Mapping editor, then expose that VM word as a writable numeric field in LWE. The architecture becomes:

+--------------+    +-----------------+    +---------------+    +----------+
| Browser Edit | -> | LWE Field (VM)  | -> | VM Word VWx   | -> | FB Param |
|   (HTML)     |    |  Writable=TRUE  |    |  (volatile)   |    |   TH/TL  |
+--------------+    +-----------------+    +---------------+    +----------+
                                              ^
                                              |
                                       Parameter-VM Mapping
                                       (configured in LSC 8.3)

Key points:

• VM words are volatile: values are lost on power-down. Use the LOGO!'s retentive markers (M) or a startup-initialization block to load defaults.
• Parameter-VM Mapping must be downloaded with the circuit (LSC: Tools → Parameter VM Mapping → Transfer). It is not part of the standard circuit download.
• Both TH (integer ms) and TL (fractional / scaled time) of the on-delay (B001), off-delay (B002), and on/off-delay (B003) blocks can be mapped.

Prerequisites

• LOGO! 6ED1052-1MD08-0BA1 (or any LOGO! 8.3 variant with Ethernet) running firmware V1.83.02 or later. Earlier V1.82.x firmware has a known Parameter-VM transfer bug that silently drops mappings after a power cycle.
• LOGO! Soft Comfort 8.3 (build 8.3.0.12262 or later) installed on a Windows PC.
• LOGO! Web Editor 1.1.0 (LWE 1.1.0 ships as an add-on to LSC 8.3; verify in Help → About).
• IP connectivity between the PC and the LOGO! on the same subnet. Static IP recommended (e.g. 192.168.0.10 / 255.255.255.0).
• The project file with at least one delay block (B001/B002/B003) already wired.

Step-by-Step: Configure Parameter-VM Mapping in LSC

1. Open the circuit in LOGO! Soft Comfort 8.3.
2. From the menu bar select Tools → Parameter-VM Mapping. The Parameter-VM Mapping dialog opens with a table of all FB instances in the program.
3. Locate the target delay block (e.g. B001 - On-Delay) and expand its parameter list. The TH row exposes a check-box and a VM address column.
4. Tick the Enable check-box next to TH and enter a free VM address, e.g. VW0. Repeat for TL if you intend to change the time base / granularity (e.g. VW2).
5. Optionally map other useful parameters: counter preset (SP), threshold switch limits (On/Off), PI controller set-point (SP), etc.
6. Click OK to commit. The mappings are stored in the .lsc project file as a metadata block separate from the circuit diagram.
7. Use File → Properties → Parameter VM Mapping to export a CSV report of every active mapping for documentation.
8. Transfer the project to the LOGO! via Ethernet: Tools → Transfer → to LOGO!. Confirm that the Parameter-VM table is sent (LSC displays a success toast "Parameter-VM mapping transferred").

Step-by-Step: Bind the VM Word in LWE

1. Launch LWE 1.1.0 from the LSC toolbar (Tools → LOGO! Web Editor) or as a standalone application.
2. Open the LWE project that targets your LOGO! (file extension .lwe). The project contains the canonical tag list pulled from the LSC export.
3. In the tag tree, expand VM Memory. Locate the VM word you mapped in step 3 above (e.g. VW0). If the tree is empty, click Refresh to re-import the tag database from the connected LOGO!.
4. Drag VW0 onto the page canvas. The element wizard appears with default properties: type = Numeric Input/Output, format = Decimal, min = 0, max = 65535.
5. In the element properties pane, set:
   • Writable = True (this is the default for VM-typed elements, but verify).
   • Min = 0, Max = time_base * range in milliseconds. For an on-delay in seconds with a 10 ms base, set Max = 600000 (10 minutes).
   • Label = "On-Delay TH (ms)".
6. Save the project and deploy it to the LOGO!: File → Save → to LOGO! Web Server. The deployment writes an updated webserver.xml plus a custom userhtml directory.

Step-by-Step: Verify from the Browser

1. Browse to http://192.168.0.10 (the LOGO! default web root). The custom dashboard loads.
2. Locate the On-Delay TH (ms) field. Type a new value, e.g. 5000 for 5 seconds, then press Enter or click Set.
3. Open LOGO! → Diagnostics → Parameter VM in the LOGO!'s on-device menu (or in the LSC online view) and confirm that VW0 holds the new value.
4. Trigger the timer input. The output transitions after the new TH has elapsed.
5. Power-cycle the LOGO! and re-load the page. The value reverts to the schematic default because VM words are volatile. This is expected - see the workaround below.

Workaround for Volatility

Because VM words do not retain across power cycles, a startup-initialization pattern is recommended when the operator-set value must persist:

1. Add a Set-Reset Flip-Flop (B004) or a Retentive Marker to hold the latest value in a retentive storage area.
2. On first scan (use the M8 first-cycle flag), copy the retentive marker into the VM word via an analog amplifier or an arithmetic block.
3. On every browser write, also write the new value to the retentive marker (use a second VM word exposed in LWE that is copied into a marker on each cycle).

Reference: VM Address Map

The LOGO! 8.3 supports up to 850 VW words total. The user-mapped region starts at VW0. Address conflicts are flagged by LSC at transfer time.

Reference: Function Blocks that Support Parameter-VM Mapping

Blocks not listed (e.g. basic logic gates, math blocks) do not expose mappable parameters in the Parameter-VM editor; they must be driven by I/O or marker signals.

Reference: LWE Field Types

Troubleshooting Matrix

Field-Proven Caveats

• Do not map the same VM word to two different FB parameters. LSC rejects the transfer with "VM address conflict".
• LWE 1.1.0 does not support signed integers natively. If a parameter (e.g. PI Offset) is bipolar, the LWE field shows the 16-bit two's-complement representation. Add a label hinting the valid range.
• Parameter-VM mappings are cleared when the circuit is erased with LOGO! → Card → Erase User Program. Re-transfer after a clean.
• On LOGO! 8 hardware revisions FS03 and later, Parameter-VM is backed up to the SD card. On FS02 and earlier, mappings must be re-sent after a power loss > 30 days.
• The maximum update rate from LWE to the LOGO! is one write per 250 ms (HTTP request / response cycle). Do not use LWE for high-speed control loops.

Alternative Approaches Considered

Before settling on Parameter-VM mapping, several workarounds were evaluated and rejected:

• Analog amplifier with browser-writable marker driving its input: rejected because the marker is a Boolean, and the amplifier's AX input is integer-typed. A scaling bridge is possible but fragile.
• Network analog input (NAI) on a paired LOGO! slave: rejected for single-LOGO! installations; only useful in multi-module topologies.
• S7 Modbus TCP to a third-party HMI: rejected as overkill; introduces an additional device and an extra point of failure.
• Custom HTML in the LOGO! userhtml directory: feasible but bypasses the LWE schema, so it loses the validation and tag-binding workflow. Only recommended for very advanced users.

Related Siemens Documentation

• LOGO! 8 System Manual (6ED1050-1AA08-0BA1 family)
• LOGO! Soft Comfort V8.3 Online Help
• LOGO! Web Editor V1.1 Manual
• Siemens LOGO! product page