The following figure shows the translation of GMII encoding into the code-group stream performed by the PCS Transmit Engine; this stream is transmitted out of the core, either serially using the device-specific transceiver, or in parallel across the TBI.
In this example, the assertion of the gmii_tx_en signal of the GMII
occurs in the odd position; in response, the state machines are
unable to immediately insert a Start-Of-Packet code group /S/ as the Idle character
must first be completed. The Start of Packet code group /S/ is therefore inserted
(in the even position) after completing the Idle. This results
in the /D16.2/ character of the Idle /I2/ sequence being inserted in place of the
first byte of the preamble field, and the Start-Of-Packet /S/ being inserted in
place of the second byte of preamble as shown.