using when...else statment in port map - vhdl

Question

i can't find anything about using when...else statment in port map. It seems to be a correct form but when i compile i see a error like this :
Error (10500): VHDL syntax error at Device.vhd(68) near text "when";
expecting ")", or ","
It's probably a silly mistake cause i'm still fresh in vhdl. Could you give me a hint about this? I would be very thankful for any help.
Here is the code and the SDA port is inout type:
com : I2C_com port map (
reset_en => reset_en,
I2C_clock_port => SCL,
clk => clk,
sda_read_data <= SDA when RD ='1' else 'Z',
sda_write_data => SDA
);

Answer 1

For one, your arrow points in the wrong direction. For port associations, always use =>, regardless of input or output ports.
Second: the when/else construct is not an expression like a?x:y is in C. It is specific to a when/else signal assignment: http://www.sigasi.com/content/signal-assignments-vhdl-withselect-whenelse-and-case
You need to either use an intermediate signal, or use an adaptor function:
com : I2C_com port map(
reset_en => reset_en,
I2C_clock_port => SCL,
clk => clk,
sda_read_data => SDA_or_z,
sda_write_data => SDA
);
SDA_or_z <= SDA when RD = '1' else 'Z';
or:
com : I2C_com port map(
reset_en => reset_en,
I2C_clock_port => SCL,
clk => clk,
sda_read_data => myFunction(SDA,RD),
sda_write_data => SDA
);

Answer 2

Take one signal temp<= SDA when RD='1' else 'Z';
then assign sda_read_data <= temp;