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Without adding the 'intelligence' of a controller you can't completely convert one to the other. RS485 is bi-directional along two wires using two anti-phase signals, RS232 is uni-directional using independent single transmit and receive wires. The electrical conversion is easy as mentioned in the previous posts but you have to consider how you change the data direction on the RS485 side of the link.
Coneversion as suggested in the posts 2 & 3 will not solve your query. RS485 is about differential communication. A single RX or TX line from RS232 will get converted into two lines (+ and -) of RX & TX respectively. A single IC of MAX485 or SN75176 will just give you a half duplex communication so you need to use pair of suggested IC's to attain the conversion. But before doing the design please go through clearly regarding the RS485 communication and then start proceeding.