Siemens LOGO! Tips: PI Control, Web Server, and Networking

Overview

Siemens LOGO! is the modular logic module (Logic Module) designed for small automation tasks where a full SIMATIC S7 PLC would be over-specified. The official LOGO! Simply ingenious. Simply more. Tips and tricks (Video) entry on the Siemens Industry Online Support portal bundles short, task-focused tutorials around LOGO!Soft Comfort programming, the on-board Ethernet port, the integrated web server, the PI controller special function, function keys on the LOGO! TDE, runtime parameter changes, and the Access Tool used to read logged data back from the module.

This reference expands each of the eight techniques in the video collection with the engineering context, menu paths, parameter tables, and commissioning checks you need to apply the same patterns in your own LOGO! programs. The goal is to shorten the gap between watching the videos and shipping a working installation.

Prerequisites and Engineering Environment

Before opening LOGO!Soft Comfort, verify that the PC and the LOGO! are on the same IP subnet. A factory-fresh LOGO! 8 base module defaults to IP 192.168.0.3 with subnet mask 255.255.255.0. If the PC is on a different subnet, change the PC's address temporarily or assign a new static address to the LOGO! through the on-board menu (Network > IP Address on the LOGO! display).

Network LOGO! Controllers with Drag and Drop

LOGO! 8 modules communicate natively over Ethernet (10/100 Mbit/s) using the Siemens S7 protocol and can be linked into a project from inside LOGO!Soft Comfort without manually configuring subnet IDs, station addresses, or MPI/PB bus parameters. The drag-and-drop method is covered in the first video of the series.

Procedure

1. In LOGO!Soft Comfort, open the network view (menu Tools > Select Hardware > Ethernet) and confirm your PC is the online interface.
2. Right-click the project tree and choose Add New Device > LOGO! 8.... The new device icon appears in the network editor.
3. Click the Ethernet icon of the LOGO! block and drag a connection line to another LOGO! block, to a SIMATIC S7 target, or to a PC with the LOGO!Soft Comfort proxy enabled.
4. Open the device properties of the target and verify the IP Address, Subnet Mask, and TSAP fields. The default TSAP for a LOGO! server is 01.00.
5. Press Tools > Ethernet Connections > Configure Network to make the offline configuration match the online topology before downloading.

A LOGO! can act as a client or a server on the S7 connection. For pure data sharing between two LOGO! 8 modules, configure the client (initiator) with the server's IP and TSAP. Use the Network Input/Output function blocks on the client side to map variables from the server into the client's program space.

Color-Code Connecting Lines in LOGO!Soft Comfort

Long ladder programs in FBD become unreadable quickly. The video series demonstrates the right-click context menu for selecting a connection line and assigning one of the standard colors. This is more than cosmetic: colored signal traces make single-cycle diagnostics on the LOGO! display much faster, and color survives program upload/download round-trips.

Best-practice color scheme

To apply a color, double-click the line, choose Line Color in the properties dialog, and confirm. The same dialog also lets you set the line thickness to Thick for signals that cross several pages, so the editor's page-break view remains traceable.

Exercise 1: Light and Fan Control Logic

The first structured programming exercise walks through a classic presence-detection scenario: a ceiling light and an extraction fan that must follow a PIR sensor with a configurable overrun time. The program is intentionally small (3 blocks on the rung) but introduces three patterns used everywhere else in the series.

Block map

Key learning points

• Connect the TOF output to two outputs by drawing a branch from the connector point above the output coil; the TOF result is reused without an extra block.
• The TOF's parameter Ta (current time) is editable from the LOGO! display and from the web server, which is the bridge to the later parameter-change exercise.
• A pre-purge TON is wired as a parallel branch so the fan starts Tp seconds before the light turns on (positive ventilation) or after (negative ventilation) - swap the branch from a parallel to a series contact to switch mode.

Exercise 2: PI Controller Implementation

The PI controller is one of the special functions in the LOGO! instruction set. It accepts a setpoint (SP) and a process value (PV) on its inputs, drives an analog output (AQ), and supports manual/automatic mode as well as bumpless transfer. The video shows a temperature control loop where the analog input AI1 (0-10 V from a PT1000 conditioner) is held at a target value by modulating a 0-10 V actuator signal on AQ1.

PI controller block parameters

Startup procedure

1. Scale the analog input to engineering units with the Analog Amplifier block (sensor type, min/max in °C, min/max mA or V).
2. Drop the PI controller, set DIR so that an increase in PV produces a decrease in AQ (heating/cooling convention).
3. Start with KC = 1.0, TI = 1:00, AQmax = 1000, AQmin = 0. Observe the response to a 10% step change in SP.
4. Increase KC until the loop just begins to oscillate, then back off to 50% of that value. Add integral action by reducing TI until the steady-state error is removed in roughly 1-2 process time constants.
5. Save the final KC and TI on the LOGO! display or in the web server, then use the comment field on the block to record the tuning date and engineer.

Exercise 3: Web Server Configuration

Every LOGO! 8 base module ships with an integrated web server that exposes the variable table, the message text configuration, and a live trend page. The third programming exercise enables the web server, sets a password, and shows how to access the LOGO! from a smartphone on the plant Wi-Fi.

Activation steps

1. In the LOGO! on-board menu, navigate to Network > Web Server Access and select Enable.
2. Set Web Server > Read-only access to Yes for production panels; set it to No only when the panel is in a controlled engineering network.
3. Open the project in LOGO!Soft Comfort, open Tools > Web Editor, drag the variables to be exposed into the left pane, and click Web Editor > Compile to push the configuration to the connected LOGO!.
4. From the browser, navigate to http://<LOGO-IP> and log in with the user LOGO and the password configured in step 1. Change the default password before commissioning.

Web server URL map

Exercise 4: Function Keys on the LOGO! TDE

The LOGO! TDE is a 6-line text display with four cursor keys, two function keys (F1, F2, F3, F4 on the TDE; F1 and F2 on the LOGO! 8 on-board display), and an OK/ESC pair. The fourth video exercise maps each key to a flag (M-flag) and uses those flags as digital inputs in the program.

Wiring pattern

1. Open Tools > Message Text and create one message text that displays the current state of the four function-key flags.
2. Insert the Cursor Key special function (cursor keys are used in the same way as function keys; on the TDE you also have the F1-F4 hardware keys). Connect its output to a memory flag, for example M8 for F1, M9 for F2, and so on.
3. Use the flag in the FBD as you would any digital input: M8 toggles a manual override, M9 acknowledges an alarm, M10 starts a teach-in sequence.

To make the function key behave as a momentary push-button, leave the output Edge-triggered = No. To make it behave as a toggle (press once = on, press again = off), wire the flag into a self-latching branch with a reset on the second press, or place a Current Impulse Relay block in front of the flag and feed the function-key flag into its trigger input.

Exercise 5: Changing Parameters at Runtime

Several blocks in LOGO!Soft Comfort expose parameters that the operator should be able to change without a download: the run-on time of the off-delay, the setpoint of the PI controller, the threshold of an analog comparator. The fifth video exercise shows three methods to make those values editable from the LOGO! display, the TDE, and the web server.

Parameter access matrix

To make a parameter visible on the LOGO! display, open the block properties and tick Parameter can be changed (in older firmware versions the wording is Operator-visible parameter set). When the block is downloaded, the parameter appears under Card > Program > Parameter on the LOGO! display and can be edited by the operator, optionally protected by a 4-digit parameter protection password.

Using the LOGO! Access Tool for Data Acquisition

The Access Tool is a PC utility bundled with LOGO!Soft Comfort that retrieves the data log from the SD card inserted in the LOGO! and exports it to CSV for analysis in Excel, MATLAB, or Python. The final video exercise configures the data log, runs a recording, and reads it back to a spreadsheet.

Data log configuration

1. Insert a Siemens-branded SD card (FAT16/FAT32, 32 GB max) into the LOGO!. Cards larger than 32 GB are not recognized.
2. In LOGO!Soft Comfort, open Tools > Data Log and enable logging. Add the variable, the sampling interval, and the trigger mode (continuous or on a flag edge).
3. Compile the project and download it to the LOGO!. The LOGO! writes a new CSV to the SD card at each cycle; the file is named LOGO_01.CSV by default.
4. Remove the SD card (the LOGO! continues to log to internal RAM for a short time), insert it into the PC, and open the Access Tool (Start > Siemens Automation > LOGO!Soft Comfort > LOGO! Access Tool).
5. In the Access Tool, select File > Open, choose LOGO_01.CSV, and use File > Export > CSV to create a clean comma-separated file with a timestamp column.

If the SD card is not recognized, check the mechanical insertion: the card must click into the spring-loaded slot. Format the card on a PC as FAT32 with the default allocation unit size if it is still not recognized, and avoid SDXC cards (exFAT) which the LOGO! firmware does not support.

Verification and Commissioning Procedure

Run the following checks before handing the panel to operations.

1. Connection check. Ping the LOGO! from the engineering laptop. A successful reply confirms the IP, subnet, and Ethernet link.
2. Watch table. In LOGO!Soft Comfort, open Tools > Online > Watch Table. Confirm all digital inputs, outputs, memory flags, and analog values update at the expected rate.
3. Web server. Open the web URL in a browser, log in, and check the trend view. The trend should sample at the configured interval and show the expected engineering units.
4. Function keys. Press each function key on the TDE and confirm the corresponding memory flag toggles in the watch table.
5. PI loop. Step the setpoint by 10% and confirm the analog output moves to the expected AQ value within the configured KC and TI, and that the AQ stops integrating when AQmax or AQmin is reached.
6. Data log. Insert an SD card, run a 5-minute test, and read the file back with the Access Tool. Confirm the timestamp column is continuous and the variable column matches the watch-table values.
7. Parameter protection. Lock the parameter protection menu if operators are not allowed to change setpoints, or unlock it for the agreed set of parameters.

Troubleshooting Matrix

Field Notes and Best Practices

• Lock all eight S7 connections with unique TSAPs in a multi-LOGO! project so a maintenance laptop cannot accidentally address the wrong station.
• Adopt a single color convention project-wide (the table above is a good default) and document it in the project header comment.
• For PI control of slow processes (HVAC, water temperature), start with KC = 1.0, TI = 5:00, then tune. For fast processes (flow, pressure), use KC = 3.0, TI = 0:30 as a starting point.
• Use the on-board LOGO! display as a fall-back HMI: even when the web server is unreachable, the operator can still change setpoints and acknowledge alarms.
• Keep one known-good SD card per LOGO! cabinet. Mark the cards with the LOGO! IP address and the date of last format.

FAQ