IEC 101 and IEC 104 are both widely used in power system automation and SCADA networks, but they were designed for very different communication environments.
IEC 101 comes from the serial communication era. IEC 104 was developed to bring the same type of telecontrol data into modern TCP/IP networks. Understanding the difference is particularly useful when upgrading substations or integrating older RTUs into newer control systems.
IEC 101: Built for Serial Communication
IEC 60870-5-101 is mainly used for communication between control centers and RTUs over serial links such as RS-232 and RS-485.
Because serial networks often operate at relatively low speeds and may face noise or unstable transmission conditions, IEC 101 includes its own link-layer control mechanisms. These handle frame checking, acknowledgments, retransmission, and link supervision.
IEC 101 uses the FT1.2 frame format and supports both balanced and unbalanced communication modes. Operational data such as measurements, device status, and control commands are carried in ASDUs (Application Service Data Units).
Despite being considered a legacy protocol, IEC 101 is still widely found in existing substations and power facilities.
IEC 104: Moving Telecontrol onto IP Networks
IEC 60870-5-104 was designed for TCP/IP networks.
Instead of relying on serial communication, IEC 104 can run over Ethernet and other IP-based infrastructure. This makes it more suitable for geographically distributed SCADA systems and modern substation networks.
One important point is that IEC 104 keeps the same ASDU-based application data model as IEC 101. The main change is how that data is transported.
IEC 104 uses TCP for reliable transport and adds APCI for application-level control and sequence management. Its I, S, and U frames handle data transfer, acknowledgments, and connection control.
IEC 101 vs IEC 104: The Main Differences
The biggest difference is the communication infrastructure.
IEC 101 works primarily over serial links and relies heavily on its own data link layer to manage communication. IEC 104 runs over TCP/IP, allowing it to use modern Ethernet infrastructure and TCP-based transmission.
IEC 101 is generally better suited to older serial-based systems and installed RTUs. IEC 104 offers higher bandwidth, full-duplex communication, persistent TCP connections, and more flexible network deployment.
Another practical difference is event handling. IEC 101 commonly relies on master-controlled communication and polling, while IEC 104 can support spontaneous transmission once a TCP connection is established.
Can IEC 101 and IEC 104 Work Together?
Yes. The key reason is that both protocols use the same ASDU structure.
Fields such as Type Identification, Cause of Transmission, Common Address, and Information Object Address can remain consistent between IEC 101 and IEC 104. In other words, the application data can be preserved even though the underlying communication method changes.
However, the protocols cannot simply connect directly. An IEC 101 device communicates through serial links and FT1.2 framing, while IEC 104 operates over TCP/IP with APCI-based control. A protocol conversion gateway is therefore needed between the two systems.
How IEC 101 to IEC 104 Conversion Works
A typical IEC 101 to IEC 104 gateway works on both sides of the network.
On the serial side, it communicates with IEC 101 devices, processes the serial frames, and extracts the ASDU data.
On the Ethernet side, it establishes IEC 104 communication over TCP/IP and packages the ASDU into IEC 104 messages.
The important part is that the operational data itself does not need to change. The gateway mainly replaces the communication and transport mechanism around the ASDU.
This approach allows utilities and system integrators to keep existing IEC 101 RTUs in service while connecting them to newer IEC 104-based SCADA networks.
For projects involving older substation equipment, industrial IEC 101-to-IEC 104 protocol gateways—such as those available from COME-STAR and other industrial communication suppliers—can provide a practical migration path without requiring a complete RTU replacement. When selecting a gateway, it is worth checking the number of serial ports, supported IEC 101 communication mode, electrical isolation, EMC performance, operating temperature, and power redundancy requirements.
Conclusion
IEC 101 and IEC 104 represent two different generations of power system communication.
IEC 101 remains important because of its large installed base and proven use in serial communication environments. IEC 104 provides the network flexibility and performance needed for modern IP-based SCADA systems.
Fortunately, their shared ASDU structure makes protocol conversion relatively straightforward. For modernization projects, an IEC 101 to IEC 104 gateway can provide a practical bridge between existing serial RTUs and modern Ethernet-based control networks.
Source: Adapted from COME-STAR Industrial Networking Technical Blog → https://www.come-star.com/blog/iec-101-vs-iec-104-protocol-converter/
