Siemens LOGO! PLC Programming: Beginner Guide

Siemens LOGO! is a programmable logic module for small, bounded control jobs such as lighting, ventilation, simple pumps, gates and educational panels. It is not a miniature S7-1200: it has its own hardware limits, engineering workflow and software. This tutorial takes one defensible route from requirements to a simulated program and then to witnessed commissioning.

Current-product note — reviewed July 25, 2026: Siemens's 2024 LOGO! Soft Comfort V8.4.1 online help documents the current tool family. A Siemens 2024 LOGO! application example lists LOGO! Soft Comfort V8.4 as a single-license product. Siemens separately publishes demo and update packages. Therefore, this guide does not call the full production tool a free download. Confirm the current license, demo limits, operating-system support, controller firmware and catalog numbers in your region before starting.

The examples are educational patterns, not Siemens-certified project files. They deliberately avoid model-specific I/O counts and fixed prices. Validate block behavior in the help for your installed build, simulate the abnormal cases, and test the real field wiring under the site's electrical and machine-safety procedures.

Practice the same start/stop, timer and feedback ideas in a browser-based PLC simulator. The simulator is a learning tool; it does not emulate LOGO! firmware, electrical outputs, web-server security or safety certification.

What Is the Siemens LOGO!?

LOGO! sits between fixed-function relays and a general machine PLC. A base module combines power supply, digital I/O, logic execution and—on selected catalog numbers—a local display and Ethernet features. Expansion modules add compatible I/O within the limits of the selected generation.

The useful distinction is the requirement envelope:

• choose LOGO! when the logic is compact, the supported blocks cover the sequence, and the exact base/expansion combination meets the I/O and environment;
• choose a larger PLC when the project needs a richer data model, extensive diagnostics, motion, large distributed I/O, complex recipes, team-scale software architecture or a lifecycle path that LOGO! cannot meet;
• never infer a required safety integrity or performance level from ordinary LOGO! logic. Required safety functions need an approved, validated safety architecture.

Siemens product names, generations and catalog suffixes matter. Record the full catalog number, supply voltage, output type, display option, expansion modules and firmware. Two units described casually as “LOGO! 8” may not be interchangeable for a real panel.

LOGO! Hardware and Fit Boundary

Build a short selection sheet before opening the programming tool:

Requirement	Evidence to record	Why it matters
Supply and outputs	Exact base-module catalog number and datasheet	Prevents a DC/AC or relay/transistor mismatch
Inputs and analog signals	Channel type, voltage/current range and common reference	Avoids treating every terminal as the same signal type
Expansion	Approved module order, power budget and generation compatibility	A physical fit does not prove electrical or firmware support
Communications	Required peer, protocol, port and loss-of-comms response	“Has Ethernet” is not a communication design
Environment	Temperature, enclosure, pollution, EMC and approvals	Determines whether the controller can be installed in the area
Maintainability	Spare availability, backup process and local skills	A cheap controller can be expensive to recover

Do not design around a remembered maximum I/O number. The supported module mix depends on the base device, generation and expansion rules. The current Siemens manual and catalog are the source of truth.

LOGO! or S7-1200?

Use a matched proof project rather than a price slogan. Create the same I/O list, sequence, alarm list and network requirement for both candidates. Then verify:

1. whether the required blocks and data fit cleanly;
2. whether a technician can diagnose the expected failures;
3. whether expansion and communications are supported;
4. how the project is backed up and restored;
5. the complete quoted cost for controller, I/O, software entitlement, panel hardware, training and support.

LOGO! often wins a small, stable task. S7-1200 becomes the better engineering choice when the apparent saving would be consumed by workarounds or a foreseeable migration.

LOGO! Soft Comfort: Install and Verify

LOGO! Soft Comfort is the desktop engineering environment used to create, simulate, document and transfer LOGO! switching programs. Treat the installer and license as controlled project dependencies.

Installation evidence checklist

1. Obtain the installer, demo or update from Siemens—not a third-party mirror.
2. Record the product version, service pack, file source and license entitlement.
3. Check the operating system against the help or release notes for that build.
4. Confirm the build supports the target LOGO! hardware and firmware.
5. Create a blank project for the exact device, compile it and open simulation.
6. Store the installer reference, project file and as-left backup in the controlled project repository.

Siemens's V8.4.1 help covers the editor and simulator. An update package does not by itself grant a new full license, and a demo is not proof of production entitlement. These distinctions belong in purchasing and contractor handover records.

Languages and program representation

LOGO! Soft Comfort centers on graphical switching logic. Depending on the supported device and project, the workflow uses:

• Function Block Diagram (FBD): logic, timers, counters and special functions connected as a signal flow;
• Ladder Diagram (LAD): a contact-and-coil representation familiar to relay and PLC technicians.

Do not promise Structured Text portability from LOGO! to an S7 controller. A later migration may preserve the control intent, but the project structure, blocks, addressing and diagnostics must be rebuilt and retested in the target engineering system.

Your First Function-Block Program

Use a ventilation fan as a first program because it reveals more than a bare output coil. The educational requirement is:

• the fan may start only in Auto with a valid demand and healthy ordinary permissives;
• Stop or loss of an ordinary permissive resets the run request;
• after the command, airflow feedback must appear within an approved time;
• a failed start removes the command and latches an actionable fault;
• Reset clears the fault only after the condition is healthy.

Suggested signal contract

Signal	Type	Meaning	Fail behavior
AutoMode	BOOL input	Automatic operation selected	False disables auto start
FanDemand	BOOL internal/input	Ventilation requested	False removes run request
FreezeStatOK	BOOL input	Ordinary process permissive healthy	False blocks/resets ordinary command
FanRunFeedback	BOOL input	Auxiliary/airflow proof	Missing proof triggers start failure
FanCommand	BOOL output	Command to interposing device	Off on stop or failed start
FanStartFault	BOOL internal	Start was not proven in time	Latched until approved reset

Build order in FBD

1. Combine AutoMode, FanDemand, FreezeStatOK and NOT FanStartFault into the run permit.
2. Feed Start and Reset/Stop conditions into a reset-dominant latch.
3. Drive FanCommand from the latched request.
4. Start an on-delay timer while FanCommand is true and feedback is false.
5. When the timer completes, set FanStartFault and reset the command.
6. Require a deliberate Reset and healthy permissives before another attempt.

The exact block names and input polarity must be checked in the installed LOGO! Soft Comfort help. The ordinary software chain does not replace a required hardwired or safety-rated stop function.

Simulation cases

Run a truth-table test, not just a successful start:

Initial condition	Stimulus	Expected observation
Auto, healthy, no demand	Demand becomes true	Command turns on; feedback timer begins
Command on, no feedback	Timer reaches approved limit	Command drops; fault latches
Command on, feedback arrives	Feedback before limit	Timer resets/does not fault
Running	Stop or permissive loss	Command drops without waiting
Faulted	Reset with condition still bad	Restart remains blocked
Faulted, condition healthy	Authorized reset	Fault clears; a new request is permitted

This test set transfers well to larger PLCs because it expresses state and evidence instead of depending on a vendor screenshot.

Worked Application 1: Scheduled Lighting with Override

A schedule-only lighting example is deceptively incomplete. A maintainable design must resolve schedule, occupancy sensor, manual override, time validity and restart state.

Control intent

• ScheduleOn enables normal occupied-hours lighting.
• OccupancyPulse may extend lighting for a bounded duration outside the schedule.
• ManualOverride is authorized, visible and expires automatically unless the requirements explicitly call for a maintained override.
• an invalid clock or failed time synchronization produces an approved fallback and alarm/maintenance indication;
• power restoration returns to a documented state rather than relying on an accidental retained bit.

Build the logic as explicit modes: Off, AutoOn, TimedOccupancy, ManualOverride and ClockFault. Give the override timer and occupancy timer independent reset rules. Display the active mode so a technician can explain why the lights are on.

What to test

• schedule boundary before and after midnight;
• daylight-saving or time correction behavior where applicable;
• override activation and expiry;
• repeated occupancy pulses;
• power cycle during each mode;
• invalid clock/time source;
• manual switch or output-device failure.

Do not treat an astronomical-clock block as evidence that the site latitude, time zone or daylight-saving configuration is correct. Those are commissioning data.

Worked Application 2: Ventilation Fan Permissives

Extend the first program into a small HVAC sequence:

1. open the outside-air damper when demand is valid;
2. wait for a damper-open proof or approved travel time;
3. start the fan if the freeze stat, overload and maintenance inputs are healthy;
4. prove airflow within the start window;
5. on loss of airflow, remove the ordinary command and raise a diagnostic;
6. on normal stop, apply the specified fan and damper post-run sequence.

The sequence benefits from named state bits even in a small smart relay:

State	Output intent	Transition evidence
Idle	Fan off, damper in approved idle position	Demand becomes valid
OpeningDamper	Damper open command	Open proof or timeout
StartingFan	Fan command on	Airflow proof or timeout
Running	Fan on; monitor permissives	Demand false or fault
Stopping	Fan off; apply post-run	Timer complete
Faulted	Outputs at approved fault state	Condition healthy + authorized reset

A compact controller can implement this bounded sequence, but every field device, output rating and motor-control interface still needs normal electrical design.

Simulation Before Hardware

LOGO! Soft Comfort simulation is useful for logical behavior:

• contact and block states;
• timer/counter transitions;
• mode selection and reset behavior;
• power-up assumptions represented in the program;
• sequence paths and impossible transitions.

It cannot prove:

• terminal wiring, voltage, commoning or input polarity;
• relay/transistor output duty and interposing-device selection;
• sensor response, mechanical travel or motor feedback;
• network latency, firewall rules or web-server exposure;
• electrical safety, machine safety or regulatory compliance.

Use simulation to find logic defects early, then use a point-to-point and functional test on the installed system.

Web Server and Communications

Some current LOGO! catalog numbers provide Ethernet and web-related functions. “Ethernet present” does not establish a secure remote-access design.

Document:

• IP address ownership and change control;
• protocol peers, read/write direction and required ports;
• user accounts and role boundaries;
• time synchronization;
• loss-of-comms and stale-data behavior;
• backup and recovery of web content/configuration;
• how remote support is authorized, logged and revoked.

Never expose an embedded controller web interface directly to the public internet. Follow the asset owner's network segmentation and remote-access standard, and verify the exact functions of the installed generation.

Commissioning and Evidence Checklist

Before energization

• verify catalog numbers, supply, grounding, enclosure and output interface against drawings;
• inspect terminal labels and conductor segregation;
• confirm the controlled software file, version and target hardware;
• review the cause-and-effect and the electrical/safety procedures;
• identify which tests may energize equipment and who authorizes them.

Point-to-point test

For every signal, record the drawing reference, terminal, field state, observed program state and expected polarity. Do not use software forcing as a substitute for proving the field circuit. If forcing is permitted for a controlled test, log it and remove every force before handover.

Functional test

Test each mode, permissive, timer boundary, failed start, feedback loss, power restoration and communication failure. Witness the actual physical result and alarm path. A screenshot of green logic alone is not acceptance evidence.

Handover record

• as-left LOGO! project and export/backup;
• software version and license/installer reference;
• controller and expansion catalog numbers plus firmware;
• I/O check sheets and functional-test results;
• known limitations, temporary overrides and open issues;
• recovery procedure and responsible owner.

Common Mistakes

Calling the full software free

Siemens publishes demos and updates, while official application documentation also lists a single-license LOGO! Soft Comfort product. State exactly which artifact and entitlement you used.

Selecting by a generic I/O maximum

Expansion compatibility and channel types are catalog-specific. Build the module list from the I/O schedule and verify it in current Siemens documentation.

Using an ordinary input as the safety function

PLC status from an E-stop or guard may support indication and sequence control. It does not automatically perform the required safety function.

Hiding a sequence in latch bits

Use explicit states and transition conditions. A future technician should be able to tell whether the sequence is idle, starting, running, stopping or faulted.

Assuming simulation proves commissioning

Simulation validates the modeled logic. It does not validate wiring, field physics, output loading or cybersecurity.

LOGO! Learning Path

1. Build Boolean start/stop logic and a reset-dominant latch.
2. Add on-delay, off-delay and pulse timing with boundary tests.
3. Add counters and explicit reset rules.
4. Build the lighting override state model.
5. Build the fan sequence with permissive and feedback failure.
6. Document an I/O contract and FAT matrix.
7. Rebuild one example in an IEC PLC simulator using ladder or function blocks.
8. Move to an S7-1200 when the requirement needs richer data, diagnostics and program organization.

The browser product connected to this site can help with steps 1–7 using generic training scenarios. It is intentionally not presented as Siemens hardware emulation. Open the simulator and create a free practice account.

Official Sources and Review Boundary

• Siemens, LOGO! Soft Comfort V8.4.1 Online Help, Operating Instructions, August 2024.
• Siemens, LOGO! CIM to VPN application example, component table listing LOGO! Soft Comfort V8.4 single license, 2024.
• Siemens Industry Support, LOGO! Soft Comfort V8.4 update information, describing upgrade/update contents and supported LOGO! generations.

This page was editorially reviewed July 25, 2026. Product availability, catalog numbers, licenses and software support can change; the controlled Siemens catalog, manual and release notes for the installed system take precedence.

Related Reading

• Siemens PLC Programming Tutorial
• PLC Programming Languages
• Motor Start/Stop Ladder Logic
• PLC Timer Programming
• PLC Simulator Software Guide