mitsubishi module

i have a A1SJ71UC-24-R2-S2 communication module with RS-232 connection
how do i go about adding extra I/0 modules to connect to my module

I'm not sure if I understand correcly. It sounds to me that
you would like to install few additional I/O points and
control them through your serial card. If this is the case,
I would rather use something like CC link instead of RS232
(much simpler to use and far better performance).
If you do have to stay with RS232, you are looking for
trouble. I guess, your best chances would be to stay with
devices like ADAM4000 series modules.
Maybe someone has better idea...

See more detail from A-series manual

I agree with Panic ..Also ,in some case you have to read user manual
Some external modul need power supply 24 VDC ..(relay module)
About communication RS232 cable just need to tranfer your's program
from plc to main CPU and you can modify or monitor
I will try to attach some picture for more detail.