Say please and I might give you the names of the converters you need.
No kiddin'
For the S7-200 you need a PPI interface. This is a relative simple data-driven RS232/RS485 (or USB/RS485) interface converter operating at speeds of 9.6kBaud or 19.2kBaud.
For the S7-300 it is more complicated. Here you need a more complex RS232/RS485 or USB/RS485 converter with integrated processor, which is capable of buffering and protocol conversion between the two different interface.
Kind regards,