[Paper Review] Inter-communication between Programmable Logic Controllers using IoT technologies: A Modbus RTU/MQTT Approach

Marios Tyrovolas, Tibor Hajnal|arXiv (Cornell University)|Feb 11, 2021

Network Time Synchronization Technologies11 references4 citations

TL;DR

This paper proposes an IoT-based communication architecture between two Programmable Logic Controllers (PLCs) using Modbus RTU for local PLC-gateway communication and MQTT for inter-gateway data exchange. The solution leverages Siemens Simatic IoT2020 gateways to enable real-time, scalable, and reliable inter-PLC communication in distributed control systems, demonstrating effective integration of industrial protocols with modern IoT messaging protocols in an experimental setup.

ABSTRACT

Internet of Things (IoT) can be widely used in various applications such as manufacturing industry,achieving high operational efficiency and increased productivity.The exploitation of IoT paradigm made more feasible the use of distributed control systems (DCS) where more than one PLCs implement an industrial application.In case of having more than one PLC,it is obligatory to ensure their inter-communication.In this paper,a solution for PLCs' communication using Message Queuing Telemetry Transport (MQTT) IoT-protocol is presented.An experimental industrial plant was used,controlled by two PLCs.For the process' implementation,two Siemens gateways Simatic IoT2020 were integrated and communicated with the PLCs through Modbus Remote Terminal Unit (RTU) industrial protocol.Finally,data exchange between the two gateways was established using MQTT protocol.

Motivation & Objective

• To address the challenge of inter-communication between multiple PLCs in distributed control systems (DCS) using modern IoT technologies.
• To integrate legacy industrial protocols like Modbus RTU with lightweight IoT messaging protocols such as MQTT for scalable and real-time data exchange.
• To validate the feasibility and performance of a hybrid Modbus RTU/MQTT communication architecture in a real industrial control setup.
• To demonstrate the practical implementation of IoT-enabled PLC coordination using off-the-shelf industrial gateways.

Proposed method

• Two Siemens Simatic IoT2020 gateways were used as intermediaries between PLCs, enabling protocol translation and data forwarding.
• Modbus RTU was employed for direct, low-level communication between each PLC and its respective gateway.
• MQTT was implemented as the inter-gateway communication protocol to enable asynchronous, event-driven data exchange over a network.
• The system was configured to publish and subscribe to MQTT topics, allowing real-time data sharing between the two gateways.
• The experimental setup included two PLCs controlling a physical industrial plant, with data exchanged via the gateway-to-gateway MQTT channel.
• The architecture was tested for reliability and latency in a real-world industrial control environment.

Experimental results

Research questions

• RQ1 How can Modbus RTU and MQTT be effectively combined to enable communication between multiple PLCs in a distributed control system?
• RQ2 What is the performance and reliability of using IoT gateways to mediate PLC communication via MQTT while maintaining Modbus RTU for local control?
• RQ3 Can a hybrid protocol architecture reduce complexity and improve scalability in industrial automation systems compared to traditional PLC networking?
• RQ4 What are the practical challenges and limitations of integrating legacy industrial protocols with modern IoT messaging protocols in real-time control applications?

Key findings

• The proposed Modbus RTU/MQTT architecture successfully enabled real-time data exchange between two PLCs via intermediate gateways.
• The use of MQTT for inter-gateway communication provided low-latency, reliable, and scalable communication suitable for industrial IoT applications.
• The Siemens Simatic IoT2020 gateways effectively handled protocol translation and network communication, demonstrating their suitability for industrial edge computing.
• The system maintained consistent data synchronization and communication integrity across the network during testing.
• The integration of Modbus RTU with MQTT proved feasible and effective in a real experimental industrial plant setup.