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IEC 62406 TCnet: Time-Critical Control Network
Understanding IEC/PAS 62406 for Industrial Communication
1. Overview
IEC/PAS 62406 defines TCnet, a real-time Ethernet protocol designed for industrial control systems. Developed by Yokogawa and Toshiba, TCnet was standardized as an IEC PAS in 2005.
TCnet achieves cycle times as low as 1 ms with jitter below 1 microsecond.
The standard specifies the complete communication profile across the Data Link Layer (DLL), Application Layer (AL), and the associated protocol state machines. It is structured around a ring topology where each node acts as a repeater, forwarding frames to the next node, eliminating the need for managed switches.
| Parameter | Specification |
|---|---|
| Physical Layer | 100BASE-TX |
| Topology | Ring |
| Min Cycle Time | 1 ms |
| Jitter | < 1 microsecond |
| Max Nodes | 100 |
2. Data Link Layer
The TCnet DLL is responsible for deterministic medium access control. Unlike CSMA/CD, TCnet employs a token-ring method where a SYN frame establishes cycle timing. The cycle has High-speed (HTP) and Low-speed (LTP) periods.
The dual-period approach guarantees real-time performance for control data.
- SYN – Synchronization frame
- REQ – In-Ring Request
- CLM – Claim frame
- DT – Data frame
- CMP – Transmission Complete
3. Application Layer
TCnet uses a Common Memory (CM) model where nodes publish data into shared memory synchronized across all ring members. The CM ASE defines object types including Boolean, Integer, Unsigned, and Float. The AR ASE manages application relationships.
Pay attention to CM configuration – improper allocation causes data inconsistency.
From an engineering perspective, TCnet is simple: no managed switches needed, ring topology provides redundancy, and deterministic timing enables worst-case latency calculation.
4. FAQs
Q: Max cable length?
100 m for 100BASE-TX, 2 km for fiber.
100 m for 100BASE-TX, 2 km for fiber.
Q: How are node failures handled?
Adjacent nodes detect and reconfigure the ring automatically.
Adjacent nodes detect and reconfigure the ring automatically.
Q: Can TCnet coexist with standard Ethernet?
No – all nodes must be TCnet-compatible.
No – all nodes must be TCnet-compatible.
Q: Is TCnet still used?
Yes – in Japanese industrial markets, especially Yokogawa DCS installations.
Yes – in Japanese industrial markets, especially Yokogawa DCS installations.
