V01012
Port MII
Px4x IED bus
Embedded
Managed Switch
Self Healing
Manager (SHM)
Flash
Memory
Address
Switch
Failsafe
Output
Relays
PHY PHY
Primary Ring
Secondary Ring
Rx secondary (Rs)
Tx primary (Ep)
Tx secondary (Es)
Rx primary (Rp)
100 base FX Ethernet
Key: PHY = Physical port
Primary Ring
Secondary Ring
Figure 62: Internal architecture of IED, C264 bay computer and H36x Ethernet switch
The SHM functions manage the ring. If the fibre optic connection between two devices is broken, the network
continues to run correctly.
Normally the Ethernet packets travel on the primary fibre in the same direction, and only a checking frame (4
octets) is sent every 5
μs on the secondary fibre in the opposite direction.
If the link goes down, both SHMs immediately start the network self-healing. At one side of the break,
received messages are no longer sent to the primary fibre but are sent to the secondary fibre. On the other
side of the break, messages received on the secondary fibre are sent to the primary fibre and the new
topological loop is closed in less than 1 ms.
As well as providing bumpless redundancy for unintentional network failure, this system can also be used to
extend the number of devices, or the size of a sub-station network, without having to disable the network.
First, the loop is opened intentionally and it immediately self heals. Then the new equipment is connected
and it immediately self heals again, closing the loop.
To increase the reliability some additional mechanisms are used:
● The quality of transmission is monitored. Each frame (Ethernet packet or checking frame) is controlled
by the SHM. Even if the link is not broken, but a larger than normal error rate is detected, the
redundancy mechanism is initiated.
● Even if there is no traffic in the primary link, the secondary link is still supervised by sending out
checking frames every 5 μs.
Chapter 8 - Redundant Ethernet MiCOM P747
248 P747-TM-EN-1
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