Guide to legacy industrial communication protocols still widely used in South African plants. Profibus DP, Modbus RTU, AS-Interface for connecting field devices to PLC systems.
Despite the rapid adoption of industrial Ethernet, legacy fieldbus protocols including Profibus DP, Modbus RTU, and AS-Interface remain widely deployed in South African plants. Understanding these protocols is essential for maintaining, upgrading, and integrating legacy equipment into modern control architectures.
Profibus DP — The Fieldbus Standard
Profibus DP (Decentralized Periphery) is still the most common fieldbus in Siemens-based installations across South Africa. It operates at up to 12 Mbit/s and supports up to 126 devices per segment. Use Profibus repeaters for long-distance runs exceeding 100m and fibre optic converters for electrically noisy environments common in mining and heavy industry.
Modbus RTU — Universal Device Connectivity
Modbus RTU over RS-485 remains the most widely supported protocol for connecting field instruments — flow meters, pressure transmitters, temperature sensors, and analytical instruments. Use RS-485 repeaters for extended distances (up to 1200m) and surge protectors on all field wiring to protect against lightning damage.
AS-Interface — Sensor/Actuator Level
AS-Interface (AS-i) is designed for connecting binary sensors and actuators at the field level. Its flat cable with insulation displacement connection (IDC) technology simplifies wiring and reduces installation time for applications with many proximity switches, photoelectric sensors, and solenoid valves.
Gateway Integration for Modernization
Modern PLCs can communicate with legacy fieldbus devices through gateway modules. Siemens ET200SP and IM155 modules support Profibus to Profinet bridging. Anybus gateways connect Modbus RTU, CANopen, or DeviceNet devices to Profinet or EtherNet/IP networks.