Siemens LOGO! 8: Reading Counter and Block Values via VM Mapping, Web Server, and CSV Logging

Engineers deploying Siemens LOGO! 8 basic modules (BM) such as the 6ED1052-1MD08-0BA2 (LOGO! 8.3) or 6ED1052-1CC08-0BA2 need to expose runtime values of function blocks—counter, up/down counter, hour counter, timer, weekly timer, yearly timer, analog threshold trigger, PI controller, and arithmetic blocks—to a supervisory layer. The default Transfer > I/O Status dialog in LOGO! Soft Comfort (V8.x) only mirrors digital inputs (I1–I24), digital markers (M1–M27), shift register bits (S1–S8), analog inputs (AI1–AI8), analog markers (AM1–AM24), and a subset of network I/O. It does not display the live numeric value of a counter or timer block.

This reference documents four field-proven paths for retrieving block values: (1) VM mapping into analog network inputs, (2) the internal web server of LOGO! 8, (3) the on-board Data Log block writing CSV to a micro SD card, and (4) a third-party HTTP/JSON middleware for remote polling. Each method carries constraints on value width, polling rate, and hardware generation. Where counter values are 32-bit but analog markers are only 16-bit (signed), the workarounds are preserved with concrete implementations.

1. Why I/O Status Cannot Show Counter Values

The I/O Status screen in LOGO!Soft Comfort is a snapshot of the process image. Its symbol set is fixed and limited to:

Counter (CNT), Up/Down Counter, Hour Counter, On/Off Delay timers, and PI Controller blocks expose their current value only through the Variable Memory (VM) area, which is a separate address space starting at byte offset 0 in the LOGO! internal memory map. The I/O Status dialog does not decode VM addresses, hence the apparent invisibility. The same restriction applies to LOGO! TD (text display) function block (FB) parameters unless they are explicitly mapped.

2. Prerequisites

• LOGO!Soft Comfort V8.0 or later installed on a Windows PC. Download via the Siemens LOGO!Soft Comfort V8.x support entry.
• LOGO! 8 BM (6ED1052-1…08-0BA2 or newer) with Ethernet port. Earlier LOGO! 0BA6 (6ED1052-1MD00-0BA6) units have no TCP/IP stack.
• Ethernet crossover or switch cable, IP address assigned (default 192.168.0.1, mask 255.255.255.0).
• Optional: micro SD card (max 32 GB, FAT32) for Data Log CSV output.
• Optional: a remote HMI/SCADA or PC running Python, Node-RED, or any HTTP client for the middleware approach (§7).
• Firmware of the LOGO! BM at FS04 or higher for full web server variable support; the DWORD VM display was introduced in FS04 firmware (released 2015).

3. Method 1 — VM Parameter Mapping (Offline + Online Read)

The Parameter VM Mapping is the canonical engineering tool inside LOGO!Soft Comfort for reading non-I/O block values. It works by binding a block's parameter or actual value register into the VM byte stream, which can then be picked up by an analog network input and finally surface in I/O Status.

3.1 Workflow

1. Open the project in LOGO!Soft Comfort.
2. Select Tools > Parameter VM Mapping (or Extras > Parameter VM Mapping in older localization).
3. The dialog lists every block in the program with its computed VM offset. A typical counter block displays its current value at an offset such as VW x, where x depends on block order.
4. Assign an analog network input (NI) to that VM offset. Drag the NI symbol onto the schematic and configure its address to point at the counter's VM word.
5. Wire the NI to an analog marker (AM).
6. Open Transfer > I/O Status. The AM now displays the live counter value, refreshed once per cycle.

3.2 The 32-bit / 16-bit Width Mismatch

A counter that has counted above 32 767 cannot fit into an AM, because AMs are 16-bit signed integers in the range -32 768 … +32 767. A counter value of, say, 50 000 would wrap to a negative number on the AM. Hour counters and up/down counters face the same issue once their value exceeds the 16-bit signed range.

Workarounds, in order of preference:

3.3 Verification

In I/O Status, the AM cell should update within one LOGO! scan cycle (~10 ms typical, configurable down to 0.5 ms with Cycle Time parameter in the BM properties). Set a known pulse source into the counter and confirm the AM increments by exactly the pulse count. If the value jumps backward after passing 32 767, you have a width overflow and need to apply one of the workarounds above.

4. Method 2 — Mapping Counter Values to a Text Display (LOGO! TD)

The LOGO! TD (Text Display, e.g., 6ED1055-4MH00-0BA2) and the integrated LOGO! TDE on the BM itself can render any value through the Message Text function block. The Message Text block accepts up to four variables which can be wired from AMs that have been populated via VM mapping. This is the field-proven way to display a running counter without a PC.

1. Add a Message Text block to the schematic.
2. Wire the counter's VM-bound AM (or the manually mapped 32-bit reconstruction) into the message's value parameter.
3. Build the project and download to the BM.
4. On the TDE, navigate to the message; the counter value appears as part of the message string.

5. Method 3 — Internal Web Server (LOGO! 8 FS04 and later)

From firmware FS04 onward, every LOGO! 8 BM ships a built-in web server. Activating it removes the 16-bit limitation entirely because the server can expose up to eight variables selected from a list that explicitly includes VM with type DWORD—the 32-bit unsigned container used internally by counter blocks.

5.1 Enable the Web Server

1. Connect LOGO!Soft Comfort online to the BM via Ethernet.
2. Open Tools > Transfer > Options > Ethernet and tick Access Web Server.
3. Assign a username and password. LOGO!Soft Comfort writes the credentials into the BM; without them the web UI is read-only.
4. Restart the BM (power cycle) to commit the change.
5. From any browser, navigate to http://<logo-ip>. The default page shows BM identity and the eight variable slots.

5.2 Configuring the Eight Variable Slots

The web UI lists all available LOGO! variables, grouped by type. The relevant types for counter readout are:

Map the counter's DWORD VM offset to slot 1, the up/down counter to slot 2, and so on. Each slot has a configurable update interval between 1 s and 60 s.

5.3 Message Configuration Without VM Mapping

LOGO!Soft Comfort V8.2 introduced the ability to embed a counter's value directly into a web-served Message Text—without first mapping it to an AM. The procedure:

1. Edit the Message Text block.
2. In the message body, insert a placeholder by pressing the value-insertion key.
3. From the dropdown, select the counter block whose value you want to display.
4. Download. The variable appears in the message on the BM/TDE and on the web UI automatically.

This bypasses the entire 16-bit AM staging chain and is the recommended approach for new deployments where the engineer controls the program from scratch.

5.4 Pulling Variables Programmatically

The web server does not expose a documented REST or JSON API. The only public read path is the HTML UI. For machine-to-machine access, the engineering community commonly reverse-engineers the page's hidden AJAX refresh and parses the rendered table. The official Siemens approach for SCADA integration is to use S7 communication instead (LOGO! 8 supports the S7 protocol with up to 64 bytes of process data), which is documented in the LOGO! 8 system manual.

6. Method 4 — Data Log Block to micro SD Card (CSV)

The Data Log function block, introduced in LOGO!Soft Comfort V8.0, writes a configurable CSV record to a micro SD card inserted in the BM. The card is formatted FAT32 and the file lives in /logo/ with names like DAT00001.csv. Each line is timestamped by the BM's RTC and contains up to eight process values, including counter values.

6.1 Procedure

1. Place a Data Log block on the schematic.
2. Configure the Sampling Interval parameter (10 ms minimum, but for counter logging 1 s is typical).
3. In the value list, choose Counter value as the source for any column. Other columns may hold AI, AM, NI, or block parameters.
4. Set the Stop when SD full behavior; LOGO! overwrites the oldest file by default if Stop when SD full is unchecked.
5. Download the program and insert a micro SD card (max 32 GB) into the BM slot.
6. The BM writes a new CSV file whenever the existing one crosses a size threshold (~512 KB). Files can be retrieved by removing the SD card and reading on a PC, or by accessing the SD card remotely via FTP if Access via FTP is enabled.

6.2 CSV Format

Timestamp;Counter1;AI1;AM1
2024-05-12 08:15:02;0000004352;00537;+00018
2024-05-12 08:15:03;0000004352;00538;+00019
2024-05-12 08:15:04;0000004352;00538;+00019

The counter column is rendered as a zero-padded decimal integer; no overflow is applied because the BM writes the raw DWORD value. A 32-bit counter therefore correctly logs up to 4 294 967 295.

6.3 Verification

After a few cycles, remove the SD card and confirm the CSV opens cleanly in Excel or Notepad++. Confirm the timestamp column advances at the configured interval and the counter column changes by exactly the pulse count fed in.

7. Method 5 — Third-Party HTTP/JSON Middleware

Where the LOGO! is reachable over an intranet or VPN, an HTTP/JSON middleware can act as a polling aggregator, allowing a remote SCADA, Python script, or web dashboard to read variables by simple GET requests. The middleware typically sits between the LOGO! (S7-200-compatible peer protocol on TCP port 102) and an HTTP client.

One commercial middleware in this space is offered by AmberSys. It exposes:

• Read of I1–I4, M1–M4, AI1–AI4, AM1–AM4 over the public internet using a static IP on the LOGO! side.
• Trigger of function keys F1–F4 by HTTP call, enabling remote start/stop of program sequences.
• Authentication via a project-unique logokey passed as a URL parameter, e.g., https://www.ambersys.net/api?logokey=<your-key>.

Open-source alternatives include node-red-contrib-s7 for Node-RED, python-snap7 for Python, and the libnodave library for C. Each speaks the S7 ISO-on-TCP protocol that LOGO! 8 implements, allowing direct read of VM area by absolute address.

8. Multi-LOGO! Polling Architecture Over VPN

When several LOGO! 8 controllers are deployed at remote sites and connected back to a central engineering workstation via site-to-site VPN with static IP addressing, three practical architectures emerge.

8.1 Multiple LOGO!Soft Comfort Instances

LOGO!Soft Comfort allows several instances to run concurrently on the same Windows host, each maintaining its own online session. This is the simplest pattern when the engineer only needs occasional visibility per site.

1. Configure the VPN tunnel to the remote site.
2. Ping the remote LOGO! IP to confirm reachability.
3. Launch one LOGO!Soft Comfort instance per site; in Tools > Options > Ethernet, set the target IP.
4. Open Transfer > I/O Status on each instance. The values shown reflect the current process image of that remote BM.

8.2 Master LOGO! Aggregator

A LOGO! 8 BM can act as an S7 client to up to eight peer LOGO! devices, using the Network Input (Analog) and Network Output blocks. Counter values exposed by peers can be mapped into the master's VM area via VM mapping (per §3) and then written to a single consolidated Data Log CSV. The master effectively becomes a low-cost gateway.

8.3 Central SCADA

For higher polling rates and historical archiving, deploy a SCADA or historian at the central site. Configure the S7 driver with one connection per remote BM, set the read area to VM starting at offset 0, and define eight DWORD tags per BM covering the counters and hour counters of interest. Polling at 1 s is comfortable for any LOGO! 8 generation.

9. Sample Python Poller Using snap7

The following Python snippet reads the first 32 bytes of VM from a LOGO! 8 reachable at 192.168.0.1, then prints the counter at VM offset 0 as a 32-bit unsigned integer. Requires python-snap7.

import snap7
from snap7.util import get_dword

client = snap7.client.Client()
client.connect('192.168.0.1', 0, 1, 102)  # rack=0, slot=1, tcp=102

# LOGO! exposes VM as DB 1 in the S7 address space.
# Adjust DB number per your LOGO! project configuration.
vm_raw = client.db_read(1, 0, 32)
counter = get_dword(vm_raw, 0)  # first 4 bytes
print(f'Counter actual value: {counter}')

client.disconnect()

10. Troubleshooting Matrix

11. Commissioning Checklist

1. Confirm BM firmware version supports the chosen method (FS04+ for web server DWORD).
2. Validate that every counter to be exposed has its VM offset noted in the Parameter VM Mapping table.
3. For 32-bit counters, decide between (a) web server with DWORD slot or (b) two-AM reconstruction. Document the choice in the project functional specification.
4. If the BM is exposed remotely, place it behind a VPN or firewall; disable plaintext HTTP if compliance requires.
5. Establish polling cadence: not faster than 100 ms for S7, not faster than 1 s for the web UI.
6. Generate at least 1 hour of CSV via Data Log; open in Excel and verify column ordering matches the schematic.
7. Document the SD card swap procedure (rotation, archival, replacement) in the maintenance plan.
8. For multi-site deployments, label each remote BM with its static IP and store the .lsc project under version control keyed to that IP.

12. References Within the Siemens Documentation Set

The following official Siemens documents cover the topics in this reference and should be kept on hand during commissioning:

• LOGO! 8 System Manual (6ED1050-1…08) — full description of VM area, S7 connectivity, web server.
• LOGO!Soft Comfort V8.x Online Help / Installation — Parameter VM Mapping menu path, Data Log block editor.
• LOGO! 8 Product Information (FS04 release notes) — DWORD web variable support and message-text variable insertion.