Before any data or command exchange can begin, the communication link between the controlling station (master) and controlled stations (RTUs or substation gateways) must be properly established.
This process — called link initialization — ensures both sides start from a synchronized state with correct frame sequencing, clear buffers, and ready communication paths.
Link initialization is part of the link layer defined in IEC 60870-5-101, the serial telecontrol standard widely used in power systems, substations, and SCADA automation.
Table of Contents
Understanding Link Initialization
In IEC 101, link initialization prepares the communication channel after power-up, a restart, or any loss of connection.
It verifies the remote station’s availability, resets internal frame counters, and confirms that addressing, parity, and transmission parameters match at both ends.
This ensures that when normal data transfer begins, neither device is out of sync or holding residual data from a previous session.
Two link types exist:
- Unbalanced Transmission: A single master communicates with multiple RTUs. Only the master initiates messages; outstations respond when polled.
- Balanced Transmission: Both devices can initiate communication independently, typically used in point-to-point or redundant controller setups.
Both modes require link initialization before exchanging Application Service Data Units (ASDUs) — the payloads carrying real measurement or control information.
Link Initialization Procedure
Unbalanced Transmission
In unbalanced systems, the master controls the process. The sequence usually follows these steps:
- Link Status Request – The master sends a frame requesting the link’s current status.
- Status of Link Response – The RTU confirms it is active and reachable.
- Reset Remote Link – The master issues a reset command to clear buffers and synchronize the Frame Count Bit (FCB) state.
- Acknowledgment (ACK) – The remote station confirms receipt.
- Link Active – Normal polling and data exchange begin.
This ensures that the master and all connected slaves start with clean communication states and matching frame sequences.
Balanced Transmission
In balanced mode, both ends of the link can act as primary or secondary. Initialization therefore happens at both sides:
- Each station sends a Link Status Request to the other.
- Once responses are received, both issue a Link Reset command to align sequencing bits.
- After mutual acknowledgments, the link is considered synchronized and ready for two-way communication.
Balanced operation is ideal for peer-to-peer communication, redundant control centers, or substation gateways, where both devices need to exchange commands and data autonomously.
Best Practices for Reliable Initialization
A properly initialized link ensures predictable and error-free operation of the entire IEC 101 network.
Here are some best-practice recommendations:
- Verify link settings: Match baud rate, parity, stop bits, and address lengths (1 or 2 octets) on both ends.
- Use proper reset logic: Always issue a Reset Remote Link after restarts or loss of synchronization.
- Handle FCB/FCV bits correctly: Misalignment causes repeated retransmissions or ignored messages.
- Monitor Access Demand (ACD) and Data Flow Control (DFC) bits: They indicate pending data or buffer overflow at the RTU.
- Log initialization frames: Capture serial traces to confirm start bytes (
0x10or0x68), control codes, and checksums are valid.
Unlike IEC 60870-5-104, which defines formal T0–T3 timers for link supervision, IEC 60870-5-101 does not standardize specific timing parameters.
Timeouts and retry intervals are typically implementation-dependent — set by device configuration or SCADA system design.
After successfully initialization, both devices operate in a synchronized link state, ready to exchange ASDUs for commands, measurements, and event data.
Comparing Link Initialization in IEC 60870-5-101 and IEC 60870-5-104
Although IEC 60870-5-101 and IEC 60870-5-104 share the same application layer, their link initialization mechanisms differ significantly due to the underlying communication medium.
| Feature | IEC 60870-5-101 | IEC 60870-5-104 |
|---|---|---|
| Communication Type | Serial (RS-232/RS-485) | TCP/IP (Ethernet) |
| Link Layer | Uses FT1.1/FT1.2 frame formats with control fields for link management | Relies on TCP/IP for transport; no link-layer frame control |
| Initialization Method | Explicit Link Status and Reset Link frames (function codes 9 & 0) | Connection-based: TCP session establishment performs link initialization |
| Sequencing Control | Frame Count Bit (FCB/FCV) synchronization | Sequence numbers in the APDU header (Send/Receive counters) |
| Timers | Not standardized — timing handled by device implementation | Standardized timers (T0–T3) for connection supervision and idle link testing |
| Error Recovery | Requires manual Reset Link and re-initialization commands | Automatic reconnection via TCP retries and timer expiry |
Final Tip
To observe how initialization frames work in practice, try the Free IEC 60870-5-101 Frame Decoder.