fido2100 3-Port Industrial Ethernet DLR Switch with IEEE1588
Data Sheet
April 10, 2013
only to the host CPU. The source identifier and sequence identifier fields of received link/neighbor
status frames are captured to identify the received port later.
2.3.1 Network Loops
The fido2100 has a built in safety feature to detect network loops. Undetected, network loops can cause
severe network problems. Network loops may be present when the user inadvertently misconnects the
network in such a way as to create a loop or when the user intentionally creates a DLR topology, but
fails to configure a DLR supervisor. When a frame is received through one of the external ports with
the source MAC address the same as the MAC address of the host CPU, the fido2100 will drop that
frame and will set a port-specific bit in the switch event register indicating that such a frame was
received. These bits can be read and cleared by firmware and can be used to flag the network fault
situation to the user. Alternatively, the fido2100 can be configured to interrupt the host CPU when
those bits are set.
It should be noted that in a properly configured DLR with a DLR supervisor, network loops may be
present for short durations when the network is being reconfigured. Hence firmware should not flag the
user of this fault condition when operating in proper DLR mode. The firmware can detect the proper
DLR mode by the presence of DLR beacons on the network, whereas in non-DLR mode or an
improperly configured DLR mode, DLR beacons would be absent on network.
2.4 Cut Through Forwarding
The fido2100 implements cut through forwarding with store and forward on contention at the
transmitting port. COTS switches and standard switching integrated circuits typically support only the
store and forward approach. With the store and forward approach a frame has to be completely
received before being transmitted. Hence every frame will encounter a delay equal to the length of the
frame and additional internal switching delay. This can become an issue for high performance
applications such as CIP motion when a long chain of cascaded switches are involved, such as in daisy
chain or DLR topology networks.
With cut through forwarding, when frame reception begins and when a sufficient number of bytes of a
frame have been buffered, the frame is queued for transmission, subject to the following forwarding
rules:
1. If the transmitting port is idle and ready for transmission, it will immediately start frame
transmission while the reception is in progress.
2. If the transmitting port is not free immediately, it will start transmitting as soon as the port
becomes ready for transmission, while the reception is in progress.
3. If the transmitting port doesn’t become available before the entire frame is received, the
received frame will be buffered completely for store and forward.
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