Using RS232 to RS485 converter connections

Hi Dear Experts,

I am going to use Dtech DT900 RS232 to 485 converter to setup modbus communication between two machines.
I can see one end of that converter has terminal strip to connect signal wires coming from RS485 end but other end has only DB9 female port with no terminal strips. I have 3 wires coming from RS232 end as well. How do I connect wires 3 coming from RS232 end in this? Do I need to get additional DB9 male adapter with terminal strip here?

Thank you!

Vijay Gosavi