Wiring the Siemens Logo 8.4 — Digital Inputs

The Goal

You have a Siemens Logo 8.4 PLC. You want it to report whether a machine is running or stopped. That data will flow into your Unified Namespace via MQTT, where it drives dashboards, KPIs, and alerts.

But the Logo doesn't magically know if a machine is running. You need to give it a signal — an electrical input that represents the machine's state. This guide covers how to wire those signals into the Logo's digital inputs.

What Counts as "Running"?

There is no universal definition. "Running" depends on your machine and your process. Common approaches:

Prerequisites

Hardware

• ☐ Siemens Logo 8.4 PLC (Master unit — 8 digital inputs I1–I8)
• ☐ 24VDC power supply (e.g. Siemens LOGO!Power, as shown in the wire diagram)
• ☐ Signal source — stack light tap, door sensor, relay contact, or proximity switch
• ☐ Wire — 0.5–1.5 mm² stranded, rated for 24VDC (typically brown/blue or as per your site standard)
• ☐ Terminal block / Klemmleiste (for clean distribution of power and signals)
• ☐ Optional: Siemens Logo DM8 24R expansion module (Slave) for additional inputs I1–I4

Tools

• ☐ Flat-blade screwdriver (3mm — for Logo screw terminals)
• ☐ Wire strippers
• ☐ Multimeter (for verifying 24VDC and continuity)
• ☐ Cable ties and labels
• ☐ Ferrules and crimping tool (recommended for stranded wire)

Software (for testing)

• ☐ Siemens Logo Soft Comfort — for I/O status monitoring
• ☐ MQTT broker and client — see the Siemens Logo 8.4 → MQTT guide

Understanding the Logo Hardware

Before you wire anything, you need to understand what the Logo's terminals actually expect electrically. This section is the foundation — skip it at your peril.

Logo 8.4 Master Unit — Terminal Layout

The Master unit has terminals along the top and bottom edges:

# Top row — Power and Digital Inputs
L+    — 24VDC positive supply input
M     — 0V common (ground/negative)
I1    — Digital input 1
I2    — Digital input 2
I3    — Digital input 3
I4    — Digital input 4
I5    — Digital input 5
I6    — Digital input 6
I7    — Digital input 7 (can be analogue 0–10V)
I8    — Digital input 8 (can be analogue 0–10V)

# Bottom row — Relay Outputs
Q1    — Relay output 1 (NO contact)
Q2    — Relay output 2 (NO contact)
Q3    — Relay output 3 (NO contact)
Q4    — Relay output 4 (NO contact)

Digital Input Specifications

How a Digital Input Works

This is the key concept. A Logo digital input reads ON when 24VDC is applied between the input terminal and the M (common) terminal. That's it.

# Input ON — 24V applied
L+ (24V) ──→ through your switch/sensor ──→ I1
M  (0V)  ──→ common return path

# Input OFF — no voltage
Switch open → no 24V reaches I1 → Logo reads 0

Every wiring scenario in this guide follows the same pattern: you're routing 24VDC through some kind of switch or contact to a Logo input terminal, with the return path through M.

Wire Diagram

This is a complete evaluation board wiring diagram for the Siemens Logo 8.4. It shows the Master unit with 8 digital inputs (I1–I8), a Slave expansion module (DM8 24RC) with 4 additional inputs, power supply, switches, LEDs, and terminal distribution.

Reading the Diagram

If you're from an IT background, electrical diagrams can look intimidating. Here's what each section means:

Power Distribution

The red lines in the diagram show 24VDC positive distribution. The blue lines show 0V (common/ground). Notice how both are distributed via the terminal block (Klemmleiste) — this is best practice. Never daisy-chain power from terminal to terminal on the Logo itself.

# Power flow
230V Mains → F1 (4A fuse) → T1 (LOGO!Power PSU) → 24VDC out
                                                          ↓
                                                  Terminal Block
                                                    ↓         ↓
                                              Logo L+/M   All switches & sensors

Common Signal Sources

In practice, you won't be using toggle switches. You'll be tapping into existing machine signals. Here are the four most common scenarios and how to wire each one.

Stack Light Wiring — Detailed

This is the most common scenario, so let's walk through it in detail. A typical stack light has a green lamp that illuminates when the machine is in automatic cycle. The lamp is driven by a 24VDC signal from the machine's own controller.

# Option A: Tap the existing 24V signal (parallel connection)
# The green lamp wire carries 24V when the machine is running.
# Connect a wire from this point to the Logo input.

Machine Controller Output ──┬──→ Green Lamp
                             │
                             └──→ Logo I1

M (0V) ──────────────────────→ Common to both Logo and machine

# Option B: Use an interposing relay (isolated connection)
# If the machine runs on a different voltage (e.g. 48V, 110V),
# or you cannot share a common 0V, use a relay.

Machine Signal ──→ Relay Coil (machine voltage)
                              │
                    Relay Contact (volt-free NO)
                              │
Logo L+ (24V) ──→ Relay Contact ──→ Logo I1
Logo M  (0V)  ──→ Common return

Relay Contact Wiring — Detailed

When another controller or system provides a "machine running" signal via a volt-free relay contact (NO — normally open), the wiring is straightforward:

# Volt-free contact from external system
# The relay provides a dry contact — no voltage of its own.
# You supply 24V from the Logo's own power supply.

Logo L+ (24V) ──→ External Relay NO Contact ──→ Logo I1
Logo M  (0V)  ──→ Common return

# When the external system activates its relay:
#   Contact closes → 24V reaches I1 → Logo reads ON
# When the external system deactivates:
#   Contact opens → no 24V at I1 → Logo reads OFF

This is the cleanest approach when integrating with existing automation. The volt-free contact provides complete electrical isolation between the two systems.

Wiring Step-by-Step

This procedure assumes you're wiring a single digital input signal (e.g. stack light green) to the Logo. Repeat for additional inputs as needed.

Power Down

Switch off the 24VDC power supply. If the Logo is powered from a DIN-rail PSU, switch off the upstream breaker or disconnect the mains supply to the PSU.

Use a multimeter to verify 0V between L+ and M on the Logo terminals.

Identify Source and Destination

Before you pick up a screwdriver, confirm:

• ☐ Signal source: Where is the 24V signal coming from? (stack light wire, relay contact, sensor)
• ☐ Signal voltage: Is it 24VDC? If not, you need an interposing relay
• ☐ Common reference: Do the signal source and the Logo share the same 0V? If not, use a relay
• ☐ Input terminal: Which Logo input will you use? (I1–I8 on Master, I1–I4 on Slave)
• ☐ Cable route: How will the wire run from the signal source to the Logo?

Cut, Strip, and Ferrule

Cut wire to length with ~50mm spare at each end. Strip 8–10mm of insulation from each end.

Connect Source to Input

For a direct 24V signal (stack light, contactor aux):

1. Route the signal wire from the 24V source to the Logo
2. Insert the wire into the chosen input terminal (e.g. I1)
3. Tighten the screw terminal firmly — give it a gentle tug to confirm it's secure

For a volt-free contact (relay, switch):

1. Run a wire from the Logo's L+ terminal (or the 24V rail on your terminal block) to one side of the contact
2. Run a wire from the other side of the contact to the Logo input terminal (e.g. I1)
3. Tighten both screw terminals

For a 3-wire PNP sensor:

1. Connect brown wire to L+ (24V)
2. Connect blue wire to M (0V)
3. Connect black wire (signal output) to the Logo input terminal (e.g. I4)

Check the 0V Path

The signal source and the Logo must share the same 0V reference. If both are powered from the same 24VDC PSU (via the terminal block), this is automatic. If the signal comes from a different power supply, you need to link the two 0V rails together — or use an interposing relay.

# Correct: shared 0V via terminal block
PSU 0V → Terminal Block → Logo M
PSU 0V → Terminal Block → Signal source 0V

# Wrong: separate 0V references, no common link
PSU-A 0V → Logo M
PSU-B 0V → Signal source 0V  ← floating! Input won't read correctly

Document Your Wiring

Label both ends of every wire with its function and destination:

# Example labels
Wire 1:  "Stack Light Green → Logo I1"
Wire 2:  "Door Sensor NC → Logo I2"
Wire 3:  "Motor Contactor Aux → Logo I3"

This seems tedious now. It will save you (or the next person) hours when troubleshooting at 2am.

Energise and Test

Reconnect the power supply. The Logo should boot and enter Operating Mode (white screen).

Use a multimeter to verify:

• ☐ 24VDC between L+ and M on the Logo
• ☐ 24VDC between the input terminal and M when the signal is active (machine running)
• ☐ ~0V between the input terminal and M when the signal is inactive (machine stopped)

Testing the Inputs

With the wiring complete, verify the Logo is reading the inputs correctly using Logo Soft Comfort and then via MQTT.

Software Verification

Connect to the Logo via Ethernet (see the network configuration guide). In Logo Soft Comfort:

1. Right-click the Logo device → I/O Status
2. Click Yes to enter RUN mode if prompted
3. Watch the input values in the status list

Activate your signal (start the machine, close the door, trigger the sensor):

Deactivate the signal (stop the machine, open the door):

End-to-End Test

If you've already configured MQTT publishing (see the Siemens Logo 8.4 → MQTT guide), subscribe to your Logo's topic and activate the signal:

# Subscribe to all Logo topics
/logo/#

# Expected message when machine starts (signal active)
{
  "timestamp": "2026-12-03T11:30:00Z",
  "I1": true
}

# Expected message when machine stops (signal inactive)
{
  "timestamp": "2026-12-03T11:35:00Z",
  "I1": false
}

Safety & Best Practice

Electrical Safety

Wiring Best Practice

IP Rating Considerations

The Logo 8.4 is rated IP20 — protected against solid objects >12mm (fingers) but not against water or dust. If the Logo is installed in a harsh environment:

• ☐ Mount in an IP65 or IP66 enclosure
• ☐ Use cable glands for all wire entries
• ☐ Ensure adequate ventilation (the Logo generates heat)
• ☐ Keep the enclosure door closed during operation

Troubleshooting

Input Not Triggering

Input stays at 0 even when signal should be active:

• ☐ Measure voltage between the input terminal and M — should be ~24VDC when active
• ☐ If 0V: the signal isn't reaching the input. Check the wire, the source, and any switches in the path
• ☐ If ~24V but Logo still reads 0: check the Logo is in RUN mode (not STOP)
• ☐ Verify a program is loaded — the Logo requires a program to process inputs (see PLC Program section)
• ☐ Check you're looking at the correct input in I/O Status (I1 on Master ≠ I1 on Slave)

Input Flickering / Bouncing

Input rapidly toggles between 0 and 1:

• ☐ Mechanical contacts (relays, switches) naturally bounce. Use a debounce timer in the Logo program — a 200ms on-delay is usually sufficient
• ☐ Loose wiring — check all screw terminals are tight. A loose ferrule or bare wire can make intermittent contact
• ☐ Electrical noise — if the signal wire runs alongside a VFD or motor power cable, it can pick up interference. Re-route or use shielded cable
• ☐ Voltage in the dead zone — if the signal voltage is between 5V and 12V, the input is in an undefined state. Check the source voltage is a clean 24V

Input Always ON

Input reads 1 even when signal should be inactive:

• ☐ Measure voltage at the input terminal — if ~24V with signal inactive, there's a wiring error
• ☐ Check for a short circuit between the 24V rail and the input wire
• ☐ If using a normally-closed (NC) contact, remember the logic is inverted — NC is closed (24V) when inactive
• ☐ Verify the correct wire is connected — trace it physically from end to end

No MQTT Messages

Input changes in Logo Soft Comfort but no MQTT messages appear:

• ☐ The wiring is fine — this is a software/network issue
• ☐ Check the Cloud Data Transfer Settings — is the input mapped?
• ☐ Check the MQTT connection status in Logo Soft Comfort
• ☐ See the MQTT troubleshooting section for broker and network issues

Voltage Too Low

Multimeter shows less than 20V at the input terminal:

• ☐ Check the PSU output voltage — should be 24VDC ±10%
• ☐ Long wire runs cause voltage drop — for runs over 20m, use thicker wire (1.5mm² minimum)
• ☐ Too many devices on the same PSU — check the total current draw doesn't exceed the PSU rating
• ☐ Check for corroded or loose connections in the terminal block

Lessons Learned

Practical insights from real installations — the things that aren't in the Siemens manual.

Next Steps