Siemens LOGO

What is Siemens LOGO!?

Siemens LOGO! is a compact, programmable logic module from Siemens that automates simple control tasks such as lighting, pumps, gates and ventilation without the need for a full-scale PLC. It was launched in 1996 to replace hard-wired circuits built from individual relays, timers and counters, which is why it is often called a smart relay or nano-PLC. The current generations are LOGO! 8.4 and LOGO! 9, which Siemens introduced in 2026.

🧠 What does a LOGO! logic module do?

A LOGO! reads digital and analogue inputs, processes them with pre-built function blocks and switches its outputs accordingly. Where you once built a control cabinet full of relays and time switches, you now draw the logic in software and download it to the module.

• Basic functions — AND, OR, NOT, XOR and edge detection
• Special functions — on-delay and off-delay timers, latching (set/reset) relay, pulse relay, weekly and yearly timers, up/down counter, analogue threshold trigger and a PI controller
• Message texts — status messages on the built-in display or on an external text display
• Communication — since LOGO! 8, every base module has Ethernet, a web server and support for S7 communication and Modbus TCP

🕰️ How has LOGO! evolved?

A fully expanded LOGO! 8 handles 24 digital inputs, 20 digital outputs, 8 analogue inputs and 8 analogue outputs. LOGO! 9 raises this to 64 DI, 60 DQ and 16 analogue inputs and outputs.

🔧 Which variants and expansion modules exist?

The type designation tells you the supply voltage and output type: R means relay outputs, C a real-time clock, E Ethernet, and the suffix o a variant without display (the “pure” version).

💻 How do you program a LOGO!?

You can make small changes directly using the keys and display on the base module. For real work you use LOGO! Soft Comfort (LSC). It lets you program in a function block diagram (FBD), the module’s native language, or in a ladder diagram (LAD). Reusable sub-circuits are stored as User Defined Functions (UDF). The built-in simulator lets you force inputs and test behaviour before anything touches the hardware.

LOGO! 8.4 requires LSC V8.4, while LOGO! 9 uses LOGO! Soft Comfort V9. Like its predecessors, LSC runs on Windows, macOS and Linux, and you migrate an existing LOGO! 8 project simply by opening it in V9 and changing the hardware. LOGO! is not programmed in TIA Portal, although an S7 PLC can exchange data with a LOGO! over S7 communication.

🔄 How does LOGO! compare with an S7-1200 or an Arduino?

Comparable logic modules include the Eaton easyE4, the Schneider Electric Zelio Logic (SR2/SR3) and the Crouzet em4. An Arduino or Raspberry Pi is cheap and flexible, but lacks the robustness, documentation and product lifecycle that industrial control demands.

🏭 Where is LOGO! used?

• Building automation — lighting on timers and twilight switches, roller shutters, access gates
• HVAC — fans, heating circuits and air handling based on temperature
• Water — pumps on level switches, wells, rainwater and pond systems
• Small machines — conveyors, mixers, packaging and dosing units
• Agriculture and utilities — greenhouses, livestock barns, charging points and energy monitoring

🌐 How does LOGO! communicate with other systems?

A LOGO! 8 from function set FS04 onwards acts as a Modbus TCP server and client on port 502 and speaks S7 communication on port 102. This makes it easy for a SCADA or HMI system to read its variables. With the LOGO! Web Editor you can build your own pages on the integrated web server. From version 8.3, LOGO! publishes data to the cloud over MQTT. A LOGO! 8 has no OPC UA server of its own; to bring it into an OPC UA environment you use a gateway or an external OPC UA server that reads the LOGO! over S7 communication.

🔐 How secure is Siemens LOGO!?

LOGO! was designed for convenience, not for a hostile network. Siemens ProductCERT has published several advisories:

• 2017 — CVE-2017-12734 (web session hijacking) and CVE-2017-12735 (man-in-the-middle on network traffic)
• 2019 — CVE-2019-10919, -10920 and -10921: via TCP port 10005 an attacker could reconfigure the device without authentication, read project files, decrypt them thanks to a hard-coded key and recover passwords stored in plain text; fixed in LOGO! 8.3, which requires new hardware
• 2022 — CVE-2022-36361 (buffer overflow allowing code execution, CVSS 9.8) and two related CVEs
• 2025 — CVE-2025-40815 to -40817 in all LOGO! 8 base modules: a buffer overflow plus unauthenticated changes to the IP address and clock; no fix is planned for the last two, leaving only network-level mitigations

So never connect a LOGO! directly to the internet. Openly reachable controllers are a favourite target; see Internet-Exposed OT. Place the module behind a firewall in a separate segment following network segmentation principles, use a VPN for remote access and restrict ports 10005/TCP, 8443/TCP and 10006/UDP to trusted addresses. LOGO! 9 improves the baseline with Secure Boot, secure LOGO! communication, user management with four roles and an administrator password you must set on first start-up.

🛠️ How do you build a first project: a pump control?

A classic first application is a sump or rainwater well that is pumped out between two level switches.

1. Wire the inputs — I1 = high level (normally open), I2 = low level (normally open, closed while water is present), I3 = fault contact of the motor protection
2. Choose the output — Q1 switches the pump through a contactor; the internal relays are rated for at most 10 A resistive load, not for heavy inductive motors
3. Draw the logic in LSC — a latching relay with S = I1 and R = NOT I2 OR I3
4. Add a delay — an on-delay of 2–5 seconds on I1 prevents chattering caused by ripples on the water surface
5. Protect against dry running — a maximum run time (for example 15 minutes) switches Q1 off and shows a message on the display
6. Simulate and download — test every combination in the simulator, download the program over Ethernet and set a password

❓ Frequently asked questions

Is Siemens LOGO! a real PLC?

Siemens LOGO! is a logic module: a simplified PLC with fixed function blocks instead of freely programmable code. For simple control tasks with up to a few dozen I/O, LOGO! does the same job as a PLC, but for complex machines with fast motion control you should choose an S7-1200 or higher.

Is LOGO! Soft Comfort free?

LOGO! Soft Comfort is paid software, but Siemens offers a free demo version for programming and simulation. Downloading a program to a physical LOGO! requires the full licence, which is also included in the LOGO! starter kits.

Can a LOGO! communicate with an S7-1200?

Yes, a LOGO! 8 or LOGO! 9 exchanges data over Ethernet with an S7-1200 or S7-1500 using S7 communication or Modbus TCP. This lets a small LOGO! controller feed data into a larger PLC or SCADA environment.

Can I control a LOGO! over the internet?

You should never make a LOGO! reachable over the internet through an open port, because it has known vulnerabilities for which no patch exists. For remote operation, use a VPN, a CMR module for SMS alerts and commands or a secured cloud connection over MQTT.

What is the difference between LOGO! 8 and LOGO! 9?

LOGO! 9 doubles the number of function blocks to 800, supports more I/O and calculates with floating-point numbers. LOGO! 9 also adds a colour touchscreen, Secure Boot, user management with four roles and a mandatory administrator password, while existing LOGO! 8 programs can be migrated in LOGO! Soft Comfort V9.

Which LOGO! variant do I need?

The right LOGO! variant depends on the control voltage in your cabinet and the load on the outputs. A 12/24 RCE with relay outputs is the most common choice; a 230 RCE suits a 230 V control circuit, and the “o” variants without display fit applications where nobody needs to operate the device on site.

📌 In summary

Siemens LOGO! is the compact logic module that has replaced relay circuits since 1996, making simple control tasks in buildings, water systems and small machines programmable. With Ethernet, Modbus TCP and cloud connectivity, LOGO! is also an OT device with known vulnerabilities: keep it off the internet and place it in a protected network segment.