Siemens Logo 8.4 → MQTT

Prerequisites

Hardware

• ☐ Siemens Logo 8.4 PLC
• ☐ 24V power supply
• ☐ Ethernet cable
• ☐ USB to Ethernet adapter (if needed)
• ☐ Jumper wires for testing inputs

Software

• ☐ Siemens Logo Soft Comfort (programming software)
• ☐ Windows PC with administrator rights
• ☐ Eclipse Mosquitto MQTT broker
• ☐ Visual Studio Code

Network Configuration

Set a Static IP on Your PC

Open Network and Sharing Center, click Change adapter settings, then right-click your Ethernet adapter → Properties.

Select Internet Protocol Version 4 (TCP/IPv4) → Properties, and configure:

IP Address:       10.1.10.1
Subnet Mask:     255.255.255.0
Default Gateway: 10.1.10.1

Click OK to save.

Set the Logo's IP

On the Logo device screen, navigate to:

Setup → Network → IP Address
010.001.010.002

This translates to 10.1.10.2. Confirm the settings.

Ping the Logo

Open Command Prompt and test the connection:

ping 10.1.10.2

Logo Configuration

Go Online

Launch Logo Soft Comfort, go to View → Network Project, and click Go Online.

Register the PLC

Click Add New Device, select Logo 8.4, and configure:

IP Address:   10.1.10.2
Subnet Mask:  255.255.255.0
Gateway:      10.1.10.1

Click OK.

Establish Connection

Right-click on the Logo device → Properties, go to the Online Settings tab, and click Connect.

Turn On MQTT Connectivity

In Properties, go to Cloud Connection Settings. Click Enable Cloud Access → Apply.

When prompted, choose Disable all unsecure ports → Continue. Click Yes when asked if the device should stop.

Configure the MQTT Connection

Click Register Thing, select MQTT from the dropdown → Next. Configure:

Broker URL:  10.1.10.1
Port:        1883
Client ID:   logo  # or any unique identifier
Username:    (leave empty)
Password:    (leave empty)

Click Next.

Select TCP (Unencrypted)

For this tutorial, select TCP (not TLS). Click Next.

Define Publish Topic

Published Topic:   /logo/test
QoS:               0
Subscribed Topic:  (leave empty — we're only publishing)

Click Next → Finish.

Data Mapping

Map Input to MQTT

Right-click Logo → Properties, go to Cloud Data Transfer Settings, and click + to add a new data point:

Name:     Input1  # or descriptive: "DoorSensor"
Range:    I       # Input
Type:     Bit
Address:  I1
Length:   1

Set transmission conditions:

• ✓ On Change — Sends when value changes
• ✓ Periodically — Send interval: 60 seconds (1 minute)

Leave Writable unchecked. Click Write to Logo → OK.

Creating the PLC Program

Wire Input to Output

In Logo Soft Comfort, switch to Diagram view. From the Digital section on the left:

1. Drag an Input block to the workspace
2. Drag an Output block to the workspace
3. Connect Input to Output (click and drag from output pin to input pin)

What this does: When Input I1 receives 24V, it activates Output Q1. This creates a simple pass-through program — the Logo will now process and transmit input states.

Save and Transfer

File → Save As → Name: Logo_MQTT_Test

Click the Download button (white down arrow) or right-click Logo → Download. Select your saved program and click OK.

When prompted:

• ✓ Overwrite IP and Device Name
• ✓ Change to RUN mode

Click Yes.

Switch from Admin to OP

On the Logo device, press ESC repeatedly, navigate to Setup, scroll down to Switch to OP (Operating Mode), and confirm.

MQTT Broker Installation

With the Logo configured and running, we now set up the MQTT broker on the PC so the Logo has somewhere to publish its data.

Get the Installer

Go to mosquitto.org, navigate to the Download section, and download the Windows 64-bit installer (e.g., mosquitto-2.x.x-install-windows-x64.exe).

If your browser warns about the download, click the dots → Keep → Keep Anyway.

Run the Installer

Use default/standard installation options. Install to the default location:

C:\Program Files\mosquitto

Click Finish.

Edit the Config File

Navigate to the installation folder and open mosquitto.conf with Notepad or VS Code:

C:\Program Files\mosquitto\mosquitto.conf

Add these lines at the top of the file:

bind_address 0.0.0.0
allow_anonymous true
listener 1883

Save the file (may require administrator rights).

• bind_address 0.0.0.0 — Allows external connections (not just localhost)
• allow_anonymous true — Permits connections without username/password
• listener 1883 — Standard MQTT port (non-encrypted)

Allow MQTT Traffic

Open Windows Defender Firewall with Advanced Security.

Inbound Rule:

1. Click Inbound Rules → New Rule
2. Select Port → Next
3. Select TCP → Specific local ports: 1883 → Next
4. Select Allow the connection → Next
5. Check all network types → Next
6. Name: MQTT Broker → Finish

Outbound Rule: Repeat the same steps under Outbound Rules.

Apply the Configuration

Open Services (Windows key + R → services.msc), find Mosquitto Broker, right-click → Restart.

VS Code MQTT Client Setup

Now we set up a client to subscribe to the Logo's MQTT messages and verify everything is working.

Download VS Code

Download from code.visualstudio.com and install with default options.

Add VSMQTT

Open VS Code, click the Extensions icon (left sidebar — four squares), search for MQTT, and install the VSMQTT extension. Click Trust Publisher and Install.

Set Up a Folder

In VS Code: File → Open Folder. Create a new folder (e.g., MQTT), select it, and trust the authors.

Connect to the Broker

Click the MQTT icon in the left sidebar, then Add Profile:

Profile Name: localhost
Host:         localhost  # or 127.0.0.1
Port:         1883

Click the Connect icon.

Subscribe and Verify

Subscribe to topics: In the MQTT tab, click Subscribe and add:

/logo/#  # wildcard — catches all subtopics

Publish a test message (optional): Click Publish, set the topic and payload:

Topic:   /logo/test
Payload: Hello MQTT!

Testing

Hardware Setup

Use a jumper wire to connect P1 (24V power) to I1 (Input 1).

Method 1: Screw Terminal

1. Unplug Logo (safety)
2. Unscrew P1 terminal
3. Insert jumper wire alongside power cable
4. Screw down securely
5. Plug in Logo

Method 2: Quick Test

Touch exposed copper wire to the top of the screw heads — P1 screw → I1 screw. You should hear a small relay click.

Verify in Logo Soft Comfort

Right-click Logo → I/O Status. Click Yes to change to RUN mode. Watch the status list.

Verify in VS Code MQTT Client

Ensure you're subscribed to /logo/#. When the input changes, you'll see JSON messages like:

{
  "timestamp": "...",
  "I1": true
}

Network Architecture & Topic Structure

Network Diagram

MQTT Topic Structure

Recommended Format (UNS Framework):

# Full UNS hierarchy
/{site}/{area}/{line}/{cell}/{device}/{datapoint}

# Example
/filton/hydraulics/pipebender/logo01/input1
/filton/hydraulics/pipebender/logo01/status

Simple Format (Used in This Tutorial):

/logo/test
/logo/demo

Troubleshooting

Connection Issues

Logo not found on network:

• ☐ Check Ethernet cable connection
• ☐ Verify PC IP: 10.1.10.1
• ☐ Verify Logo IP: 10.1.10.2
• ☐ Test with: ping 10.1.10.2
• ☐ Ensure Logo is powered on

"Server Connection Error":

• ☐ Check Mosquitto service is running
• ☐ Verify mosquitto.conf has bind_address 0.0.0.0
• ☐ Check firewall rules for port 1883
• ☐ Restart Mosquitto service

"Not Authorized":

• ☐ Add allow_anonymous true to mosquitto.conf
• ☐ Restart Mosquitto service

Logo Configuration Issues

"Unable to access remote device":

• ☐ Logo must be in Stop mode before downloading programs
• ☐ Click Stop button (red square) in Logo Soft Comfort
• ☐ Then attempt download again

"No programme exists in Logo":

• ☐ This is expected! The Logo requires a program
• ☐ Create the basic I/O program as described in Step 12
• ☐ Download the program to the Logo

Logo screen stays orange:

• ☐ Logo is in Admin mode
• ☐ Navigate to: Setup → Switch to OP
• ☐ Screen should turn white in Operating mode

Input not triggering:

• ☐ Ensure wire has good contact with screw terminals
• ☐ Verify you're connecting to P1 (positive 24V), not P2 (ground)
• ☐ Check Logo is in RUN mode (not STOP)
• ☐ Verify program has been downloaded

MQTT Data Issues

No messages in VS Code:

• ☐ Check Logo shows "Connected" status
• ☐ Verify subscription topic matches publish topic
• ☐ Use # wildcard: /logo/# to catch all messages
• ☐ Check Cloud Data Transfer Settings configured correctly

Messages only sent once:

• ☐ Ensure both "On Change" AND "Periodically" are checked
• ☐ This provides redundancy

Important Discoveries & Lessons Learned

These insights came directly from the training session — they're the things that aren't obvious from the documentation.

Security Considerations

Next Steps

Quick Reference

Network

# Test connection
ping 10.1.10.2
ipconfig

Mosquitto Config

# Location
C:\Program Files\mosquitto\mosquitto.conf

# Required lines
bind_address 0.0.0.0
allow_anonymous true
listener 1883

# Restart
Services → Mosquitto Broker → Restart

IP Addresses