Explained 'red' signal level in Eth tx serial data.

libraries/io/eth/src/vhdl/eth_statistics.vhd

@@ -35,6 +35,8 @@
 --   simulation delta cycle.
 -- . The eth_serial_in becomes active /= '0' when the Tx Eth is actually
 --   sending data, so this depends on the application or tb stimuli.
+-- . The transceiver sim model use 'U' to signal eop in tx_serial_out, so
+--   this is why then 'red' signal level appears in Wave window.
 
 LIBRARY IEEE, common_lib, work, technology_lib, dp_lib, tech_tse_lib;
 USE IEEE.STD_LOGIC_1164.ALL;