Troubleshooting LOGO! 8 Network Input Stuck ON with KTP HMI

Overview

Siemens LOGO! 8 (firmware 8.0 through 8.4) exposes up to 64 network inputs (NI1–NI64) and 64 network outputs (NQ1–NQ64) that can be written and read by external controllers, SCADA systems, and SIRIUS / SIMATIC HMI panels such as the KTP400 Basic, KTP700 Basic, KP400 Comfort, and TP700 Comfort. When configured inside a TIA Portal project, those network bits become writable from a button, switch, or script on the HMI.

The most common commissioning defect with LOGO! 8 and TIA Portal V13 SP1 through V18 is that the network input remains latched ON (always RED in the online test) after the operator releases the button. The bit never returns to 0, the LOGO! program reacts to a permanent high, and the user concludes that LOGO! or the HMI is broken. In almost every case the problem is not in the LOGO! 8 base module at all; the problem is in the WinCC event configuration of the HMI button.

This reference documents the root cause, the three correct event configurations (Set Bit + Reset Bit, Toggle Bit, and Edge-triggered Pulse), the relevant LOGO! 8 tag mapping table, and a verification procedure for panels programmed with WinCC Comfort / WinCC Basic from TIA Portal V13 SP1 onward.

Problem Description

Symptom report from the field (typical):

• LOGO! 8.3 base module (6ED1052-1MD08-0BA0 or 6ED1052-1CC08-0BA0) connected to a KTP400 Basic mono PN (6AV2123-2DB03-0AX0) or KTP700 Basic PN (6AV2123-2GB03-0AX0) over Ethernet.
• LOGO! Soft Comfort V8.3 / V8.4 program uses a network input, e.g. NI1, as a control bit (start, enable, manual override, etc.).
• Operator presses a button on the HMI screen. The button is configured with the event "Set Bit" on Press and "Set Bit" on Release.
• NI1 in the LOGO! online test goes RED and stays RED even after the finger is lifted. The LOGO! program behaves as if a hardware switch is permanently closed.
• Result: motor, valve, or function controlled by NI1 cannot be turned off from the HMI without cycling power or using LOGO! Soft Comfort to clear the bit.

Root Cause Analysis

Three distinct mistakes produce the "always ON" symptom. Identify which one applies before changing the configuration.

Cause A — Both events are "Set Bit"

The user configured the button to SetBit NI1 = 1 on Press and SetBit NI1 = 1 on Release. The HMI is therefore issuing the same write at both transitions. The bit is correctly set high; the bit is never set low. This is the most common mistake and is explicitly the case described in the source report ("I have try to use 'set bit' on both Press and Relase, but Its the same").

Cause B — No event on Release at all

The button has an event only on Press and nothing on Release. The HMI writes 1 to NI1 on press and never writes 0. The bit stays high.

Cause C — Animation reads the network input back as visual feedback

The button appearance is animated by the same network input that the button writes. Once the HMI writes 1, the network input reads 1, the button animates as "pressed" and stays pressed even after the operator lifts the finger, because no release event ever fires (the HMI considers the button logically held by the network input). The cure is to either decouple the animation from the tag, or use Toggle Bit which inverts the visual state as well.

LOGO! 8 Network I/O Architecture

LOGO! 8 base modules communicate with TIA Portal HMI panels over S7 communication (PUT/GET) on TCP port 102. The network inputs and outputs are bit-addressable memory locations exposed by the LOGO! firmware.

Reference: LOGO! 8 system manual (Siemens Support entry ID 109751654).

The exact count depends on the LOGO! 8 variant:

• LOGO! 8.3 (0BA8) — base module 6ED1052-1xxx08-0BA0 — supports the full NI1–NI64 / NQ1–NQ64 set.
• LOGO! 8.2 (0BA7) — same range, older firmware.
• LOGO! 7 (0BA6) and earlier — different address model, do not mix with this article.

Solution 1 — Set Bit on Press, Reset Bit on Release (Pushbutton Behavior)

Use this configuration when the operator must hold the button to keep the LOGO! action active (jog, manual override, inching).

1. Open the TIA Portal project that contains the HMI device (KTP400 / KTP700).
2. Open the screen that contains the button.
3. Select the button element.
4. Open Properties → Events → Press.
5. Add a system function: SetBit.
6. Tag: select the LOGO! network input that you declared in the HMI tag table, e.g. "HMI → LOGO!".NI1 (HMI tag of PLC type Bool, connection "LOGO_8_Connection", address DB / NI area / NI 1 or simply the symbolic tag NI1).
7. Value to set: 1.
8. Open Properties → Events → Release.
9. Add a system function: ResetBit.
10. Same tag, value 0.

Result: while the button is physically held, NI1 = 1; the moment the operator releases, NI1 = 0. The LOGO! program sees a true pushbutton.

Solution 2 — Toggle Bit on Press (Latching Switch Behavior)

Use this when the button should behave like a maintained switch (light, fan, mode selector) — the operator taps once for ON, taps again for OFF.

1. Open the button's Properties → Events → Press.
2. Add the system function ToggleBit.
3. Tag: the LOGO! network input, e.g. NI1.
4. Leave Release empty.

Every press inverts the bit in the LOGO!. The animation of the button (Appearance → "Pressed" state) can be tied to the same network input — the button will visually toggle because the bit toggles.

Solution 3 — One-Click Impulse (Pulse from a Single Press)

The follow-up question from the original report is explicit: "If i Just want impuls, What to do then? How do i make impuls with Just one click?" A true one-shot pulse from a single HMI press requires a flag inside the LOGO! program itself, because the HMI cannot natively emit a fixed-width pulse through S7 communication.

Method 3a — LOGO! pulse generator block

1. In LOGO! Soft Comfort, add a Pulse generator (Off-delay) or Wiping relay (pulse) block.
2. Wire the network input (e.g. NI1) to the trigger input of the block.
3. Set the pulse width TH to the desired duration, e.g. T = 00:00:01s for a 1 second impulse.
4. Wire the block output to the action (counter increment, conveyor start, etc.).

Behavior: the HMI button uses Set Bit on Press and Reset Bit on Release (Solution 1). The LOGO! sees a 0 → 1 → 0 transition on the rising edge and emits a single pulse of the configured width. This is the most reliable method.

Method 3b — Rising edge + flag in LOGO!

1. LOGO! program: network input NI1 → Edge-triggered flag (positive edge) → coil B001 (latching flag).
2. Add a second network input NI2 wired to the reset of B001.
3. Wire B001 into the controlled function.
4. HMI: button 1 → SetBit NI1 on Press, ResetBit NI1 on Release. Button 2 (or long press) → ResetBit NI2 to clear the flag.

Method 3c — HMI script with SetBit/ResetBit scheduled

For Comfort Panels that support VBScript (TP700 Comfort, KP400 Comfort):

Sub OnClick(ByVal Item)
  ' Pulse NI1 for 500 ms
  SmartTags("NI1") = 1
  Dim t
  Set t = CreateObject("Scripting.Timer") ' not available on Basic; use scheduled task
End Sub

On Basic panels (KTP400 Basic, KTP700 Basic), scripting is not available. Use Method 3a or 3b exclusively.

Step-by-Step Configuration in TIA Portal

Complete walk-through for LOGO! 8.3 + KTP700 Basic PN on TIA Portal V17.

Prerequisites

• LOGO! 8 base module with firmware 8.0 or newer (recommended: 8.3 or 8.4 for full function set).
• LOGO! Soft Comfort V8.3 or V8.4 installed (download at Siemens Support 109751654).
• TIA Portal V13 SP1, V14 SP1, V15.1, V16, V17, or V18.
• Ethernet connectivity between HMI and LOGO!, same subnet, S7 communication enabled on LOGO! (Tools → Ethernet Connections → S7 Connections Allowed).
• LOGO! configured to permit external access: in the LOGO! menu Settings → Network → Access Control, the option "Allow S7 communication" must be enabled (firmware 8.3 and later — see LOGO! 8 operating instructions (entry 109741041)).

Procedure

1. Define the network input in LOGO! Soft Comfort. Open the program, add a network input block (coil side) and assign it to NI1 (or any free NI). Connect the output of the block to the LOGO! logic you want to control.
2. Build and download the program to the LOGO! 8 base module.
3. Create the TIA Portal HMI project. Add a KTP700 Basic device, add a connection to the LOGO! 8 (S7 connection, default port 102).
4. Create HMI tags. In the HMI tag table add a Bool tag NI1_HMI linked to the LOGO! connection, address space Network inputs, bit 0 (or the symbolic NI1 if you exported the LOGO! symbol table).
5. Add a button to the screen. From the toolbox, drag a Button onto the root screen.
6. Configure the Press event. Properties → Events → Press → Add function → SetBit → tag = NI1_HMI, value = 1.
7. Configure the Release event. Properties → Events → Release → Add function → ResetBit → tag = NI1_HMI, value = 0.
8. (Optional) Animate the button. Properties → Appearance → "Pressed" state appearance → link the visibility or color to the tag NI1_HMI. Verify the animation does not feed back into the same Press event.
9. Compile and download the HMI project to the KTP700.
10. Run LOGO! Soft Comfort online test (Tools → Online Test or press F11) and observe NI1: it should be RED only while the HMI button is physically held; it should turn GREY immediately on release.

Verification

Network I/O Limits and Specifications

Troubleshooting Matrix

Commissioning Tips

• Always declare the network input in LOGO!Soft Comfort first, even if the program does not yet use it. An orphan network input block in the LOGO! diagram is required for the HMI to write to it.
• Use symbolic addressing in the HMI tag table: import the LOGO! symbol table from LOGO!Soft Comfort (Export → .sdf / symbol table) and link HMI tags to the symbolic names. This eliminates address-shift errors after program modifications.
• For complex panels, separate the command tag (write-only from HMI, not used for animation) from the status tag (read-only, used for animation). This is the only way to avoid Cause C in the root cause analysis above.
• LOGO!Soft Comfort V8.4 added support for direct import of TIA Portal project symbols and for the LOGO! Web Editor V2.0 — see LOGO! 8 software downloads on Siemens Support.
• When using WinCC Comfort/Advanced on a PC runtime, prefer the system function InvertBit for self-latching commands, because ToggleBit in PC runtime can be subject to double-trigger on accidental double-click events.
• The HMI cycle (acquisition cycle for the tag) must be set to "On demand" for write events to be sent immediately. The default 1 s cycle delays every button press by up to 1 s.

FAQ