NRZS

VHDL

NRZ(S)	NRZS	Non-return-to-zero space	Serializer mapping {0: toggle, 1: constant}.

Encoder

library ieee;
use ieee.std_logic_1164.all;

entity nrzs_encoder is
port(
    clk :in  std_logic;
    d   :in  std_logic;
    q   :out std_logic
);
end entity;

architecture rtl of nrzs_encoder is

    function nrzi_s(
        signal curr :in std_logic;
        signal prev :in std_logic
    ) return std_logic is

        if (curr = '0') then
            return not prev;
        else 
            return prev;
        end if;
    end function;

begin

    process(clk)   
    begin
        q <= nrzi_s(d, q);
    end process;

end architecture;

Decoder

entity nrzi_decoder is
port(
    clk :in  std_logic;
    d   :in  std_logic;
    q   :out std_logic
);
end entity;

architecture rtl of nrzi_decoder is
    signal lastd :std_logic := '0';
begin

    process(clk)
    begin
        if (rising_edge(clk)) then
            if (d = lastd) then
                q <= '0';
            else 
                q <= '1';
            end if;
            lastd <= d;
        end if;
    end process;

end architecture;