Connecting PC as a master and stepper motor as a slave using rs485

Hi,
I have a arcus stepper motor which supports rs232/rs485. I am able to connect and work with motor using rs232 (using DB9 cable) but i am confused how to connect this motor for RS485 mode with same DB9 cable.

In motor specs they mentioned i need to connect +485 pin of motor to PC +485 and -485 to -485.But I am confused what are the pin numbers i have to connect in the DB9 serial cable.

rs485 is needed to implement modbus protocol.This might be a simple.

thanks

Are you using an RS232-to-RS485 converter?

thanks for the reply.
No. I am not using.I am not suer.Should I use?