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DP83867IS: Link failure at particular cable length

Part Number: DP83867IS
Other Parts Discussed in Thread: TMUXHS4212


We are using DP83867IS in 100M mode and are experiencing problems to establish a link in a particular test setup. The unit is connected to a patch panel by ca. 2m cable, connecting to a device on the patch panel with a ca. 2m cable as well. Link is only established if connection on the patch panel is made by a cable of 1+ m. Connection with patch cable of 20cm does not result in a stable link.

We already tried changing the DSP Feedforward Equalizer Configuration (changing register 0x012C to 0x0E81), the Receiver's Equalizer Configuration (changing register 0x008A to 0x010F) and the Receiver's LPF (changing 0x00B3 to 0x000C) but without any improvement. 

Note: the ethernet signal in the unit is multiplexed by TMUXHS4212 and magnetics are 45cm away from the PHY.

Please advise on how to approach this issue.