220.127.116.11.1. TX Preamble, Start, and SFD Insertion
In the TX datapath the MAC appends an eight-byte preamble that begins with a Start byte (0xFB) to the client frame.
The source of the preamble depends on whether you enable the preamble pass-through feature by turning on Enable preamble passthrough in the E-Tile Hard IP for Ethernet Intel FPGA IP parameter editor.
If the preamble pass-through feature is turned on, the client must provide 8 preamble bytes (including an SFD byte) on the data bus. The MAC will automatically replace the Start Control byte.