Protocol Conversion Using a Gateway Device
A gateway device bridges the communication gap between serial-based IEDs (Intelligent Electronic
Devices) and Ethernet-based RTUs (Remote Terminal Units). This is essential in substations where
older IEDs use serial communication protocols, such as Modbus RTU or DNP3 over RS-485, and the
RTU communicates over Ethernet using protocols like Modbus TCP/IP or DNP3 over TCP/IP.
How the Gateway Works
1. Protocol Conversion:
o The gateway translates messages between the serial protocol used by the IEDs and
the Ethernet protocol used by the RTU.
o For example, it converts Modbus RTU (serial) to Modbus TCP/IP (Ethernet).
2. Interfacing:
o On one side, the gateway connects to serial ports of IEDs (e.g., RS-232 or RS-485).
o On the other side, it connects to the RTU's Ethernet interface via an Ethernet port.
3. Data Mapping:
o The gateway maps data from serial registers (e.g., coil or holding registers in
Modbus) to Ethernet packets.
o It ensures the integrity and correct sequence of data during the conversion.
4. Network Addressing:
o Each IED is assigned a unique address on the serial network.
o The gateway uses IP addressing on the Ethernet side to identify and communicate
with the RTU.
Example Setup
Scenario:
A substation uses legacy protection relays (IEDs) that communicate over RS-485 using Modbus RTU.
The substation’s RTU is designed for modern Ethernet-based communication using Modbus TCP/IP.
Components:
1. IEDs (Protection Relays):
o Connected using RS-485 serial cables.
o Each IED uses Modbus RTU protocol.
2. Gateway Device:
o Converts Modbus RTU to Modbus TCP/IP.
3. RTU:
o Communicates over Ethernet using Modbus TCP/IP.
o Interfaces with the control center for SCADA integration.
Workflow:
1. Data Collection:
o The IED measures parameters like voltage, current, and fault status and sends this
data via RS-485 using Modbus RTU.
2. Protocol Conversion:
o The gateway receives the Modbus RTU messages from the IEDs, converts them into
Modbus TCP/IP format, and forwards them to the RTU over Ethernet.
3. Data Transmission to RTU:
o The RTU processes the received data for monitoring and control. It can send the
information to a SCADA system at the control center.
Real-World Devices
Example Gateway Devices:
o Schneider Electric EGX150: Converts Modbus RTU to Modbus TCP/IP.
o Siemens RUGGEDCOM RX1400: Supports multiple protocols, including DNP3 and
Modbus.
o Moxa MGate MB3180: Specifically designed for Modbus protocol conversion.
Benefits of Using a Gateway
1. Extending System Life:
o Allows integration of legacy IEDs with modern RTUs without replacing older
equipment.
2. Protocol Interoperability:
o Facilitates communication between devices using different protocols.
3. Cost-Effective:
o Avoids the need for large-scale hardware upgrades.
4. Scalability:
o Adds flexibility to expand and upgrade the system over time.
Conclusion
A gateway device is critical for ensuring seamless communication between legacy IEDs and modern
RTUs. By converting protocols, it enables interoperability, preserves existing infrastructure
investments, and ensures compatibility with modern Ethernet-based systems. This setup is
especially valuable in substations transitioning to IEC 61850-based automation.
