LOGO! VM Mapping: Reading Counter Values via I/O Status (0BA7+)

Overview: Why VM Mapping Is Required to Read Counter Values

Siemens LOGO! base modules expose a fixed set of operands in the on-screen I/O Status panel of LOGO!Soft Comfort (Tools → Transfer → I/O Status): digital inputs (I1…I24), digital outputs (Q1…Q16), analog inputs (AI1…AI8), analog outputs (AQ1…AQ2), and the marker/flag bytes (M1…M27). The actual value of a function block such as a Counter (Cnt, block number B001…B999) is, however, an internal block parameter — it is not part of the I/O image and is therefore not directly visible in the I/O Status window.

To make the counter's current count appear in the I/O Status (and in any external Modbus/TCP or HMI client that talks to the base module), you must map the counter parameter to a Variable Memory (VM) address. The Parameter VM Mapping dialog in LOGO!Soft Comfort is the engineering tool for this task, and it has been available on every 0BA7 (LOGOI/O 8) and later base module, including the 0BA7, 0BA8, 0BA8.FS4, and 0BA9 families. Each base module supports up to 64 VM mapping entries.

Prerequisites: Hardware, Firmware, and Software

Before mapping, verify that the LOGO! program is downloaded and running (LOGO!Soft Comfort: Tools → Transfer → Start LOGO! in Run Mode) and that the counter is actually counting. An open-circuit I1 will keep the counter at zero and you will not see what you expect in the I/O Status.

Variable Memory (VM) Address Space — The Reference Map

LOGO!'s VM area is a flat 16-bit word array used purely as a publication mechanism. Think of it as a small tag database (max 64 entries) that you pre-populate with the values you want the outside world to see. The base module does not execute the VM area — it is read-only from the perspective of the LOGO! program, and writeable from the HMI / SCADA / LOGO!Soft Comfort side.

Step-by-Step: Mapping a Counter Actual Value to VM

1. Open the program in LOGO!Soft Comfort and identify the counter function block (e.g. B001 — Up-Down Counter) that you want to expose.
2. Open the VM mapping editor: from the menu bar, choose Tools → Parameter VM Mapping (or Edit → Parameter VM Mapping depending on LOGO!Soft version). The dialog shows the 64 slots as a table.
3. Add a new mapping row. Click the first empty row and set the following fields:

   Column	Value to enter	Example
   VM Address	Next free VW word	VW100
   Block Number	The counter's B-number	B001
   Parameter	Select Actual Value (sometimes labelled Cnt or CV)	Actual Value
   Symbolic Name	Free-text, 16 chars max, no spaces	Cnt_Production
   Data Type	Auto-detected; for a counter this is WORD (0…65535) or INT (-32768…+32767)	WORD

4. Save the project and download it to the LOGO! base module. The VM mapping becomes active immediately after a STOP → RUN transition.
5. Verify on the LOGO! built-in display (if the unit has one): navigate to Menu → Diagnostics → Variables — the new symbolic name Cnt_Production should appear with the live counter value.

Reading the Counter via I/O Status (Online)

Once the mapping is downloaded, the counter value is available in the on-screen I/O Status dialog of LOGO!Soft Comfort:

1. With the project open and the base module connected, choose Tools → Transfer → I/O Status (alternatively the toolbar button I/O Status).
2. The I/O Status window opens with three tabs: Inputs / Outputs, Flags, and Variables. Click the Variables tab.
3. The Cnt_Production row now shows the live counter value, updated every scan cycle (typically 50–200 ms on 0BA8).
4. The value column also exposes the raw VM address (e.g. VW100) and the engineering unit if one was assigned in the function block.

Data-Flow Diagram (Counter → VM → I/O Status → CSV)

CSV Export: Five Field-Tested Workflows

LOGO!Soft Comfort does not provide a single "Export I/O Status to CSV" button. The values must be captured by one of the following five methods, ordered from simplest (manual, no extra hardware) to fully automated (industrial-grade data historian):

Method 1 — Manual copy/paste from I/O Status (smallest effort)

1. Open the I/O Status window in the Variables tab.
2. Right-click the value cell of Cnt_Production and select Copy.
3. Paste into Excel / LibreOffice Calc, then add the timestamp manually in column A.
4. Save as .csv. This is the workflow used in the original question and is fine for a few readings per day.

Method 2 — LOGO! 0BA8+ data logging to micro-SD card (automated, no extra hardware)

1. Insert a micro-SD card (max 32 GB, FAT32, ≤ 32 GB) in the LOGO! base module.
2. In LOGO!Soft Comfort, add a Data Log function block (File → New → Data Log) and bind it to the same VM word (VW100).
3. Set the trigger source (e.g. on a rising edge of I3) and the sample interval. The LOGO! writes a CSV line to /LOGO/SD/DATALOG.CSV on the card.
4. Pull the card, open the CSV in Excel. The file already contains YYYY-MM-DD HH:MM:SS, value rows.

Method 3 — Web server polling (0BA8+ with Web Server licence enabled)

1. Activate the Web Server on the LOGO! (Setup → Web Server Access → Enable). Default port 80.
2. In a browser, navigate to http://<LOGO_IP>/ → the Variables page lists every VM-mapped parameter with its current value.
3. Use a small PowerShell or Python script to poll the JSON endpoint (http://<LOGO_IP>/status.xml on 0BA8.FS4+) and append each reading to a CSV:

   import requests, csv, datetime
   url = "http://192.168.0.10/status.xml"
   with open('counter.csv', 'a', newline='') as f:
       w = csv.writer(f)
       while True:
           r = requests.get(url, timeout=2).text
           # parse the <Cnt_Production> element; XML structure varies by FS level
           value = r.split('<Cnt_Production>')[1].split('</Cnt_Production>')[0]
           w.writerow([datetime.datetime.now().isoformat(timespec='seconds'), value])
           time.sleep(1)
   

Method 4 — Modbus/TCP pull from a SCADA tag (Ignition, WinCC, Node-RED, etc.)

1. LOGO! 0BA8/0BA9 acts as a Modbus/TCP server on port 502. VM word 0 maps to holding register 0, VM word 100 maps to register 100, and so on.
2. Add a Modbus TCP device in the SCADA, point it at the LOGO! IP, and create a tag at Holding Register 100.
3. Enable the SCADA's built-in historian (e.g. Tag History in Ignition, Tag Logging in WinCC) to write 1-second samples to a CSV/SQL store automatically.

Method 5 — LOGO! + external HMI with local data logging

Pair the LOGO! with a Siemens SIMATIC HMI Basic Panel (KTP400, KTP700, KTP1200) or any Modbus/TCP master. Configure the HMI tag to poll the LOGO! VM register every 200 ms and enable the HMI's Data Log function. The panel writes a CSV to a USB stick or to a network share, which is the most robust industrial method for shift-long trend recording.

Hardware-Specific Notes: 0BA7 vs 0BA8 vs 0BA8.FS4 vs 0BA9

For a pure read counter and dump to CSV use case, the 0BA8.FS4 (article number 6ED1052-1MD08-0BA2) is the sweet spot: built-in web server, built-in Modbus/TCP, micro-SD data logging, and the lowest cost. The 0BA9 is justified only when you need the S7-style tag import or HTTPS authentication.

Diagnostic and Troubleshooting Matrix

Field-Commissioning Checklist

1. After downloading, power-cycle the LOGO! to clear the volatile VM cache.
2. Force the counter to a known value by pulsing I1 five times; verify that the I/O Status Variables tab shows 5.
3. Toggle the trigger source for the Data Log block and confirm a new row appears in DATALOG.CSV within 1 s.
4. If a SCADA/HMI is in the loop, read holding register 40101 (VW100) with a Modbus/TCP test client (e.g. modpoll) and confirm the value matches the I/O Status display.
5. Document the symbolic name, VM address, block number, and engineering unit in a register sheet — handover without this table causes endless support calls.

Frequently Asked Questions