Also see: What are the 1000-T PMA Test Modes? and our PMA Test Suite.
Subclause 126.96.36.199 of the IEEE 802.3-2002 Standard specifies that the four transmitters, BI_DA, BI_DB, BI_DC, and BI_DD, shall be driven by the same 125MHz transmit clock, TX_TCLK. It is the clock that generates the 5 level Pulse Amplitude Modulated signal being driven onto the line. A sample configuration and waveform capture is shown below.
Sample TX_TCLK configuration
Sample TX_TCLK waveform, with Test Mode 2 waveform