You can use Open Serial Protocol features of S7-200 PLC. I don't remember now exactly, but it works with XMRT and RXT mnemonics of Statement List language.
Go through its System Manual and you will find examples there.
HI,
If you want to transmit/receive data to or from another equipment you should have a cable (PPI cable, RS232-RS485 cable).
In PLC lader, you init SMB30 cell with rev./trans configure, for example: value 16#09 in SMB30: baud rate: 9600bps, 8 bit data, 1 stop bit, no parity
after that, you using XMT for transmit (using help on program) or interrupt 8 to receive data
It is easy for you to do this, good luck!
Check at S7-200 documentation for freeport configuration. Take a look at special memory bytes SMB30 for Port 0 and SMB130 for Port 1. After port configuration you can use the XMT and RCV function to send and receive a number of data bytes. Check these functions how they work, in help.