Building Custom Protocol Gateways for Legacy Industrial Equipment

As factories and infrastructure systems undergo digital transformation, a major challenge remains: how to integrate legacy industrial equipment into modern networks, cloud platforms, and automation systems. These machines, often decades old, use outdated or proprietary communication protocols that don’t support IP-based or standard industrial fieldbus networks.

The solution? Custom protocol gateways — embedded systems designed to translate, route, and normalize data between old and new technologies.

In this article, we explore what these gateways do, how they are architected, and how Promwad helps clients bridge the gap between legacy machinery and modern IIoT ecosystems.

What is a protocol gateway?

A protocol gateway is a hardware or software device that allows two or more systems using different communication protocols to exchange data in real time. It acts as a translator and router between:

Legacy serial-based protocols (e.g., RS-232/485, Modbus RTU, CANopen)
Proprietary machine interfaces
Modern industrial protocols (e.g., OPC UA, MQTT, EtherNet/IP, Modbus TCP)
SCADA systems, cloud platforms, or REST APIs

In industrial settings, gateways enable brownfield equipment to become part of modern control, monitoring, or predictive maintenance systems—without requiring full hardware replacement.

Common use cases

Connecting CNC machines to cloud dashboards
Legacy machines output serial data via RS-232. A custom gateway can parse it, convert to MQTT or OPC UA, and forward it to a factory data server.
Integrating Modbus RTU devices into a BACnet/IP building automation system
Bridging proprietary PLC protocols with industrial Ethernet or TSN
Migrating legacy substations or energy meters to IEC 61850 or DNP3 networks
Data normalization for AI/ML analytics
Gateways can pre-process and normalize diverse field data for cloud or edge-based inference.

Core architecture of a custom protocol gateway

While the exact design depends on the project, most gateways share a layered architecture:

1. Hardware layer

MCU or SoC with multiple serial/Ethernet/fieldbus interfaces
Industrial-grade IO: RS-232/485, CAN, USB, digital IO
Isolation circuits (galvanic isolation for serial ports or power domains)
Optional FPGA for high-speed or deterministic tasks
Optional secure elements for encryption and authentication

2. Protocol handling layer

Parsers for legacy protocols: Modbus RTU, IEC 60870-5-101/103, Profibus, etc.
Modern protocol stacks: OPC UA, MQTT, JSON/REST, BACnet, PROFINET
Buffering and reassembly for fragmented or irregular transmissions

3. Conversion and routing logic

Message mapping and transformation rules
Data normalization (unit scaling, tag naming, timestamp insertion)
Filtering, aggregation, or priority management

4. Application layer

Local dashboard or web UI (optional)
Device configuration, diagnostics, and OTA update mechanisms

Key engineering considerations

Developing protocol gateways involves numerous hardware and firmware design trade-offs:

Interface reliability

Industrial environments demand robust signal integrity, ESD protection, and fault tolerance.

Protocols like RS-485 require termination resistors and careful impedance matching.

Real-time behavior

For time-sensitive systems (e.g., motor control or protection relays), latency and determinism are critical.

RTOS or bare-metal implementations may be required for microsecond-level response times.

Security

Gateways often operate between trusted and untrusted domains.

Implement secure boot, encrypted communication (TLS), role-based access control, and OTA signing.

Maintainability

Protocols evolve, and gateway firmware must be updateable in the field.

Configurable mapping rules via JSON/XML or user-friendly interfaces improve long-term usability.

Design example: Promwad’s custom protocol gateway for power quality monitoring

In one project, Promwad designed an embedded gateway that connected legacy power meters with RS-485 output to a cloud analytics platform using MQTT over TLS.

Our solution included:

STM32-based industrial hardware with RS-485, Ethernet, and isolated power
FreeRTOS + LwIP stack with secure MQTT client
Modbus RTU master for polling meters
Real-time buffer with timestamping and retransmission logic
Secure OTA update system

The gateway enabled seamless integration of low-cost legacy meters into an advanced energy analytics dashboard — with full encryption and real-time responsiveness.

How Promwad helps clients modernize with protocol gateways

We offer:

Hardware design for industrial-grade embedded systems
Firmware development with support for legacy and modern protocols
Driver development for serial, fieldbus, and Ethernet stacks
Integration with SCADA, cloud, or edge platforms (AWS IoT, Azure IoT Hub, etc.)
Long-term support for firmware maintenance and OTA update strategies
Enclosure design, IP protection, and certification assistance

Our experience covers smart grid, factory automation, transportation, and energy systems — where legacy and next-gen systems must coexist.

Conclusion

Custom protocol gateways are essential tools in any modernization strategy for industrial equipment. By bridging old and new protocols, they unlock data, extend the life of critical infrastructure, and enable next-generation analytics, control, and remote management.

If you're looking to connect legacy devices to your modern automation or IIoT stack, Promwad can help you design a robust, secure, and scalable gateway solution tailored to your technical and business goals.