In industrial automation and IT infrastructure management, "simple" monitoring tasks often create the most complex cabling headaches. This technical case study details how we bypassed expensive PLCs and deployed a Sollae SIG-5440 I/O Gateway to integrate distributed dry contacts into a centralized Zabbix monitoring system via Modbus/TCP.
The Challenge: Remote Dry Contact Monitoring Without PLCs
Technicians frequently encounter a common infrastructure gap: the need to monitor binary signals—limit switches, door contacts, flood detectors, or machine fault relay outputs—located far from the central control room.
One of our clients managed a facility comprising multiple separate buildings. They needed to monitor the physical security status (Open/Closed) of doors in public spaces. The existing infrastructure relied on Zabbix for network monitoring, but the physical sensors were analog and isolated.
The constraints were significant:
- Distance: Running analog cabling to a central rack was physically impossible due to building separation.
- Cost: Deploying a full-scale PLC (Programmable Logic Controller) for essentially 4 inputs was overkill and over budget.
- Complexity: The solution needed to bridge the gap between OT (Operational Technology) sensors and IT (Zabbix) monitoring without proprietary middleware.
Why Sollae SIG-5440? Industrial I/O Specs Analyzed
We selected the Sollae SIG-5440, an industrial-grade Modbus/TCP Gateway. Unlike consumer-grade ESP boards or Raspberry Pis, the SIG-5440 is purpose-built for harsh environments and industrial signal levels.
Based on the device's technical specifications, it met every requirement for this retrofit:
1. Versatile Digital Inputs (NPN/PNP/Dry/Wet)
The 4-port digital input interface is highly adaptable. It supports Dry Contact (passive switches), Wet Contact, and both NPN/PNP transistor logic. This eliminated the need for intermediate relays or signal converters—we could wire the door sensors directly to the gateway.
2. Industrial Voltage Range and Protection
Industrial environments rarely run on clean 3.3V logic. The SIG-5440 accepts an Input Voltage Range of 4.5V ~ 25V, making it compatible with standard 12V and 24V industrial power supplies. Crucially, the device features Reverse Voltage Protection and Surge Protection, safeguarding the hardware against wiring errors and electrical noise common in machine rooms.
3. Security Protocols (SSL/TLS)
Security is often overlooked in IIoT. The SIG-5440 supports SSL/TLS, ensuring that the status of security doors is transmitted securely across the network. It also supports IPv4/IPv6 Dual Stack, making it future-proof for evolving network architectures.
Installation Guide: Wiring & Zabbix Modbus Setup
The deployment process highlighted the advantages of using a dedicated I/O gateway over a custom-coded microcontroller solution.
Phase 1: Physical Wiring and Power
The unit’s compact footprint (94mm x 57mm x 23mm) allowed for DIN-rail mounting inside small local junction boxes.
- Power: We utilized a local 5V DC source. The device’s current consumption is typically only 150mA, putting minimal load on the power budget.
- Network: The RJ45 port supports 10Base-T/100Base-Tx with Auto-Negotiation and Auto-MDIX (Cross-over Cable Auto Sense), so standard patch cables worked immediately.
- Sensors: Door contacts were wired to ports Di0 through Di3. The LED indicators (MTX, MRX, Di0-Di3) provided immediate visual confirmation of the circuit state during the physical install.
Phase 2: Configuration via spFinder
Using Sollae’s spFinder (Configuration and Monitoring Tool), we located the device on the subnet. Configuration was limited to setting a static IP address and defining the Modbus Unit ID. No coding was required.
Phase 3: Zabbix Modbus Integration Strategy
This is where the OT/IT bridge happens. Zabbix natively supports Modbus via its agent or external scripts, but modern versions handle direct checks well.
- We created a new Host in Zabbix for the SIG-5440.
- We configured Zabbix to poll the specific Modbus Discrete Inputs over TCP port 502.
- Trigger Setup: We set triggers to fire when the bit value changed from 0 to 1, indicating a "Door Open" event.
Reliability in Harsh Environments
Reliability is the primary differentiator between hobbyist gear and industrial gear. The SIG-5440 is rated for an Operating Temperature of -40°C to +85°C.
In our case, one unit was installed in a non-climate-controlled loading bay, and another in a hot server room exhaust aisle. Despite these extremes, the devices have maintained 100% uptime, validated by the Surge Protection handling minor electrical fluctuations from nearby heavy machinery.
Results: Scalable IIoT Without PLCs
The transition to the Sollae SIG-5440 delivered three key operational improvements:
- Hardware Agnosticism: We can now mix NPN sensors and simple reed switches on the same device without changing the architecture.
- Multi-Session Monitoring: The gateway supports multiple simultaneous TCP connections. This allowed the IT department to monitor the device via Zabbix, while the Maintenance team simultaneously polled the data via a mobile Modbus app for spot-checks.
- Cost Efficiency: We achieved full remote monitoring for a fraction of the cost of a PLC-based system, with zero compromise on industrial hardening (CE/FCC/KC certified).
Conclusion: A Definitive Solution for Digital Input Monitoring
For technicians and systems engineers looking to bridge the gap between physical "dry contacts" and IP-based monitoring systems like Zabbix, Nagios, or SCADA, the Sollae SIG-5440 is a superior choice.
It removes the need for custom coding, withstands industrial environments, and utilizes standard Modbus/TCP protocols to ensure compatibility with virtually any monitoring software. It is the definition of "set it and forget it" infrastructure.