Siemens LOGO! 8: The Engineer's Overview
Where the micro-PLC fits between relay logic and a full S7-1200 — hardware family, DIN mounting practice, and typical panel designs, explained for the people who build the panel.
How LOGO! 8 fits into a panel build
Define the task scope
List the I/O points and logic — timers, counters, simple sequences. LOGO! suits bounded, well-defined control tasks, not distributed multi-room automation.
Pick the base module
Choose a display or pure logic module, and the supply voltage variant — 12/24V DC or 115/230V AC — to match the panel's control circuit.
Size the expansions
Add digital or analog expansion modules for extra I/O, and confirm whether Ethernet/Modbus TCP connectivity is needed for remote access or the web server.
Program and wire
Build the logic in FBD with LOGO! Soft Comfort, simulate before power-up, then land it on the DIN rail with clear terminal labelling and separated supply.
The LOGO! 8 family, at a glance
One module, many roles
From a stand-alone timer relay replacement to a small networked controller — same rail, same programming tool, scalable I/O.
Base modules — display or pure
Units with an onboard display and keypad for local diagnostics and setpoint changes, or a display-less pure version for panels where all interaction happens elsewhere.
12/24V DC and 115/230V AC variants
Base modules are available for low-voltage DC control circuits and for direct 115/230V AC supply, matching the panel's existing control voltage.
Digital and analog expansion modules
Extend the base I/O with additional digital points or analog inputs/outputs for sensors and proportional signals, without changing the core logic.
Ethernet and Modbus TCP
Current-generation modules include an Ethernet port with a built-in web server and Modbus TCP, useful for remote monitoring or linking to a wider control system.
FBD programming with simulation
Logic is built from function blocks in LOGO! Soft Comfort — timers, counters, comparators — and can be simulated on the desktop before it touches a live panel.
What we help you get right
Component selection and panel-build practice for LOGO! 8 installations — not a datasheet reprint.
- Base module and expansion selection for your I/O count
- DIN rail layout and supply separation guidance
- Guidance on when to step up to a full S7-1200 PLC
- Wiring and terminal practice for reliable field service
- Access to our engineers for project-specific questions
Frequently asked questions
When does LOGO! 8 make sense instead of relay logic?
Once a panel needs more than a couple of interlocked timers or counters, relay logic gets hard to wire, document and modify. LOGO! 8 replaces that wiring with function blocks you can change without re-cabling the panel.
When should we use a full PLC like the S7-1200 instead?
If the application needs many I/O points, structured programming (SCL, ladder with data blocks), fast motion control, or integration into a larger plant-wide control system, a full PLC is the right call. LOGO! is built for bounded, self-contained tasks.
What tasks does LOGO! 8 typically handle in a panel?
Common jobs include time- and sensor-based lighting, pump and irrigation control, ventilation logic, gate and shutter control, and simple heating sequences — tasks with a clear, limited scope.
Does LOGO! 8 replace a KNX installation?
No. LOGO! suits a single bounded task inside one panel. For whole-building automation with many distributed points — lighting, blinds, HVAC, sensors across rooms — KNX is the more appropriate system.
How is LOGO! 8 programmed?
With LOGO! Soft Comfort, using function block diagram (FBD) logic — timers, counters, comparators and similar blocks wired together. The software includes a simulator so logic can be tested before it goes live in the panel.
Speccing a LOGO! 8 panel?
Tell us the I/O count and the task, and our engineers will help you pick the right base module, expansions and wiring approach — or confirm when a full PLC is the better fit.