After completing this cross-over connection, you have a cable that is capable of loopback at Ethernet speeds of 10, 100, or 1000 Mbps.
The more difficult task of GigE loopback is making sure the NEXT (Near End Cross Talk) cancellation is turned off in the transceiver. GigE must use all four (4) pairs of wires at a base baud rate of 125Mhz in order to function. This configuration creates a lot of crosstalk1, so all GigE PHYs must incorporate some sort of crosstalk cancellation. However, when you are performing loopback and have the wires directly connected to each other, the crosstalk will appear to be at 100% even though this configuration is intentional. Therefore, the NEXT canceller needs to be disabled for loopback testing.
Some GigE PHYs do not have an option to disable the NEXT canceller, but most do. You will need to consult the datasheet for your transceiver to determine the specific sequence. Some can be as simple as enabling a single bit in a control register -- others require a more complicated sequence such as:
1. Force the PHY to “Master” mode
2. Perform PHY reset
3. Force the PHY into GigE mode
4. Enable the Gigabit stub (i.e. - the loopback connector)
5. Disable NEXT cancellation
Once the NEXT canceller is disabled, you should be able to perform loopback testing using the GigE loopback cable.
After completing this
After completing this cross-over connection, you have a cable that is capable of loopback at Ethernet speeds of 10, 100, or 1000 Mbps.
The more difficult task of GigE loopback is making sure the NEXT (Near End Cross Talk) cancellation is turned off in the transceiver. GigE must use all four (4) pairs of wires at a base baud rate of 125Mhz in order to function. This configuration creates a lot of crosstalk1, so all GigE PHYs must incorporate some sort of crosstalk cancellation. However, when you are performing loopback and have the wires directly connected to each other, the crosstalk will appear to be at 100% even though this configuration is intentional. Therefore, the NEXT canceller needs to be disabled for loopback testing.
Some GigE PHYs do not have an option to disable the NEXT canceller, but most do. You will need to consult the datasheet for your transceiver to determine the specific sequence. Some can be as simple as enabling a single bit in a control register -- others require a more complicated sequence such as:
1. Force the PHY to “Master” mode
2. Perform PHY reset
3. Force the PHY into GigE mode
4. Enable the Gigabit stub (i.e. - the loopback connector)
5. Disable NEXT cancellation
Once the NEXT canceller is disabled, you should be able to perform loopback testing using the GigE loopback cable.
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