Modbus Serial Link Communication Security Protocol Reference Specification and implementation guide

The Modbus TCP protocol has adopted TLS-based security standards; however, Modbus serial communication over EIA/TIA-485 multi-point systems, commonly used in 2-wire or 4-wire configurations, lacks standardized security mechanisms. These systems support cable lengths exceeding 1000m at baud rates up to 9600 bit/s with AWG26 or thicker cables, while Category 5 cables can reach up to 600m. As an application layer protocol, despite its widespread application, the absence of encryption and authentication in Modbus protocol via serial links exposes plaintext data to risks such as MIM interception, modification under attacks such as side-channel analysis etc., particularly in long-distance or bridged network scenarios. Enhancing Modbus serial link security requires introducing proper encryption and authentication methods tailored to varied deployment environments onsidering the characteristics of serial links. A proposed security standard guide outlines lightweight encryption and authentication mechanisms to improve confidentiality and integrity while maintaining compatibility with existing Modbus devices, offering a practical upgrade path for secure industrial control systems.