how to correctly assign to these variables

Hi,

These are very basic things. I recommend you find a good VHDL book and learn the syntax.

constant ff_tx_mod : IN STD_LOGIC_VECTOR (1 DOWNTO 0) := '00';

should be:

constant ff_tx_mod : STD_LOGIC_VECTOR (1 DOWNTO 0) := "00";

You use IN to define an input to an entity.

variable gm_tx_d2 :STD_LOGIC_VECTOR (7 DOWNTO 0) := '00';

should be:

variable gm_tx_d2 :STD_LOGIC_VECTOR (7 DOWNTO 0) := "00000000";

because your vector has eight elements.

constant ff_tx_mod : IN STD_LOGIC_VECTOR (1 DOWNTO 0) := '00';

get rid of the IN since this is not an input to the entity.

constant preamble : STD_LOGIC_VECTOR (7 DOWNTO 0) := '5555555555555555';

You define an eight bit vector and then try to stuff a huge number into it. The biggest unsigned number that can be represented by eight bits is 255.

Mark.