PLC Connectivity Options
How to get data from a Siemens Logo 8.4 on the factory floor into your MQTT broker — from simple wired Ethernet to wireless LoRaWAN. An architectural guide to the real-world trade-offs.
The Problem
The Siemens Logo 8.4 can publish MQTT natively. The protocol side is solved — the PLC speaks your language. The real challenge is the transport layer: getting a network connection to the machine in the first place.
In a manufacturing facility, running Ethernet cable to each machine is often the single most expensive part of a UNS deployment. It's not the PLC, not the software, not the broker — it's the cable drop.
Why Ethernet Drops Are Expensive
- £ Cable runs through trunking, across ceilings, through walls — typically £1,500–2,000+ per drop
- £ Fire barrier penetrations require certified contractors and re-certification
- £ Production downtime during installation — you can't always cable a live factory floor
- £ Moving a machine means re-cabling from scratch
- £ Legacy buildings were never designed for network infrastructure
- £ At scale (20+ machines), cabling costs can exceed the entire software budget
This guide covers two approaches: wired Ethernet (when it makes sense) and wireless LoRaWAN (when it doesn't).
Option 1: Wired Ethernet
If you can run a cable, run a cable. Wired Ethernet is the simplest, most reliable, and lowest-latency way to connect a Logo 8.4 to your MQTT broker. There is no protocol translation, no bridge device, no wireless complexity.
Architecture
The Logo connects directly to your LAN via Ethernet. It publishes MQTT messages to the broker. No intermediate devices, no protocol conversion. This is the architecture described in the Siemens Logo 8.4 MQTT guide.
When Wired Ethernet Works Well
✅ Short Cable Runs
Machine is within 10–20 metres of an existing network switch or patch panel. A single cable run with minimal trunking.
✅ New Builds
Network infrastructure is being installed alongside the machines. Cable drops are planned into the build cost from day one.
✅ Fixed Machines
Equipment that will never move. Permanent installations where the cable investment is a one-time cost.
✅ Real-Time Requirements
Applications needing sub-second latency or high-frequency data. Wired Ethernet has no duty cycle limits or airtime constraints.
Advantages
- ✓ Simplest architecture — no bridge devices, no protocol translation
- ✓ Lowest latency — sub-millisecond, suitable for real-time control
- ✓ Most reliable — no wireless interference, no duty cycle limits
- ✓ Native MQTT — Logo publishes directly to broker, nothing in between
- ✓ No ongoing costs — no subscriptions, no airtime, no licence fees
Disadvantages
- ✗ Cable drop cost — £1,500–2,000+ per machine in an existing facility
- ✗ Installation disruption — may require production downtime
- ✗ Inflexible — moving a machine means re-cabling
- ✗ Fire barriers — penetrations need certified contractors
- ✗ Scales poorly — 20 machines × £2,000 = £40,000 in cabling alone
Option 2: Wireless LoRaWAN
When Ethernet drops are impractical, LoRaWAN provides a wireless transport layer. It's long-range, low-power, licence-free, and designed for exactly this kind of industrial IoT use case.
Why LoRa?
With WiFi and 4G/5G excluded, the viable wireless options narrow quickly. LoRaWAN stands out for industrial environments:
| Property | LoRaWAN |
|---|---|
| Range | 2–5 km urban, up to 15 km rural |
| Frequency | EU868 (Europe), US915 (USA) — licence-free ISM bands |
| Power | Very low — battery operation possible (years) |
| Payload | 51–242 bytes per message |
| Cost | No subscriptions, no SIM cards, no airtime fees |
| Interference | Designed for noisy RF environments (spread spectrum) |
The Fundamental Issue
This means you need a bridge device at each machine. The bridge sits between the Logo's Ethernet port and the LoRa radio. It receives MQTT messages from the Logo over a short local Ethernet cable, repackages the data into LoRa packets, and transmits wirelessly to a central gateway.
At the gateway end, the process reverses: LoRa packets are received, decoded, and republished as MQTT messages to your broker.
LoRa System Architecture
At Each Machine: Bridge Device
Each Logo 8.4 connects via a short Ethernet cable (1–2 metres) to a bridge device mounted alongside it on the DIN rail. The bridge:
- Connects to the Logo's Ethernet port as an MQTT client
- Subscribes to the Logo's MQTT topics
- Repackages messages into compact LoRa packets
- Transmits wirelessly on EU868 or US915
Recommended device: Advantech WISE-6610 V2
| Specification | Details |
|---|---|
| Manufacturer | Advantech (Taiwan) |
| Interface | Ethernet RJ45 (10/100 Mbps) |
| Protocols | MQTT, Modbus TCP, BACnet/IP, OPC UA |
| LoRa Chipset | Semtech SX1302 |
| Frequency | EU868, US915, AS923, AU915 |
| Power | 12–24V DC or PoE |
| Mounting | DIN rail or wall mount |
| Operating Temp | -40°C to +75°C |
| Software | Node-RED, EdgeLink |
Central Hub: LoRaWAN Gateway
One gateway in the building receives LoRa transmissions from all bridge devices and publishes the data as MQTT messages to your broker over Ethernet.
Recommended device: MultiTech Conduit 300
| Specification | Details |
|---|---|
| Manufacturer | MultiTech Systems (USA — Minnesota) |
| LoRaWAN | 8-channel (via mCard) |
| Frequency | EU868 or US915 |
| Backhaul | Ethernet + optional 4G LTE |
| Protocols | MQTT, HTTP/HTTPS |
| Software | Built-in network server, Node-RED |
| Power | PoE (802.3af) or DC |
| Mounting | DIN rail or wall mount |
| Operating Temp | -40°C to +70°C |
EU alternative: Kerlink Wirnet iStation — manufactured in France, 10-channel, IP67 outdoor rated, carrier-grade. A strong choice if you prefer EU-manufactured equipment.
LoRa Limitations
LoRaWAN is not a drop-in replacement for Ethernet. It has real constraints that affect what you can and cannot do.
| Parameter | Value | Impact |
|---|---|---|
| Data Rate | 0.3–50 kbps | Sufficient for PLC telemetry, not for streaming |
| Latency | 1–10 seconds | Not suitable for real-time control loops |
| Duty Cycle | 1% (EU868) | Max 36 seconds of airtime per hour |
| Payload | 51–242 bytes | Enough for register data, not for large payloads |
| Direction | Primarily uplink | Downlink (control commands) is limited |
✅ Suitable For
Monitoring and data collection. Alarm notifications. Slow-changing process values (temperature, pressure, cycle counts). Remote status checking. Non-critical control commands. Polling intervals of 30 seconds or more.
❌ Not Suitable For
Real-time control applications. High-frequency polling (< 10 seconds). Large data transfers. Safety-critical operations. Closed-loop control systems. Applications requiring guaranteed sub-second delivery.
Decision Matrix
| Dimension | Wired Ethernet | Wireless LoRaWAN |
|---|---|---|
| Cost per Machine | £1,500–2,000 (cable drop) | ~£400 (bridge device) |
| Latency | < 1 ms | 1–10 seconds |
| Reliability | Very high | High (with retries) |
| Setup Complexity | Low (cable + config) | Medium (bridge + gateway + config) |
| Flexibility | Low (fixed cable) | High (wireless, machines can move) |
| Protocol Translation | None — native MQTT | MQTT → LoRa → MQTT |
| Ongoing Costs | None | None |
| Real-Time Control | Yes | No |
| Devices in Path | 0 (direct) | 2 (bridge + gateway) |
| Best For | Short runs, new builds, real-time | Retrofit, many machines, monitoring |
Cost Analysis
10 Machines — Wired Ethernet
| Component | Qty | Unit Cost | Total |
|---|---|---|---|
| Ethernet cable drops | 10 | £2,000 | £20,000 |
| Network switch | 1 | £200 | £200 |
| Configuration | 10 | £50 | £500 |
| Total | £20,700 | ||
10 Machines — LoRaWAN
| Component | Qty | Unit Cost | Total |
|---|---|---|---|
| WISE-6610 V2 bridge | 10 | £400 | £4,000 |
| Short Ethernet cables (1m) | 10 | £10 | £100 |
| MultiTech Conduit 300 | 1 | £1,200 | £1,200 |
| LoRaWAN mCard | 1 | £170 | £170 |
| Antenna | 1 | £80 | £80 |
| Installation & config | — | — | £1,000 |
| Total | £6,550 | ||
Approved Manufacturers
All recommended equipment is manufactured in the EU, UK, USA, or Taiwan. No products sourced from China.
| Manufacturer | Country | Product | Role |
|---|---|---|---|
| Siemens | 🇩🇪 Germany | Logo 8.4 | PLC — MQTT publisher |
| Advantech | 🇹🇼 Taiwan | WISE-6610 V2 | MQTT-to-LoRa bridge (at machine) |
| MultiTech | 🇺🇸 USA | Conduit 300 | LoRaWAN gateway (central hub) |
| Kerlink | 🇫🇷 France | Wirnet iStation | LoRaWAN gateway (EU alternative) |
| Eclipse | Open Source | Mosquitto | MQTT broker |
Next Steps
| Step | Description |
|---|---|
| Wired Setup | Follow the Siemens Logo 8.4 → MQTT guide for the full Ethernet configuration |
| Pilot Project | For LoRa — start with 2–3 machines to validate range, reliability, and data flow before scaling |
| Site Survey | Test LoRa coverage in your facility — walls, metal structures, and EMI affect range |
| UNS Topics | Adopt the UNS Framework topic hierarchy for consistent MQTT topic structure |
| fn-uns Integration | Connect to the fn-uns data pipeline for KPIs, historian, and analytics |
Guide Version: 1.0 · Applies To: Siemens Logo 8.4, Advantech WISE-6610 V2, MultiTech Conduit 300
Last updated March 2026.