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RS232 to RS485 Converter specification

• DB9 RS232 connector compatible with PC
• TxD and RxD translated from RS232 to RS485
• Direction of transmission controlled by PC RTS line
• Handshake loop the PC connection so it works with all software
• RS485 signals output on 6 way modular jack
• Indicator LED(s) to show communications traffic
• Power supplied by external unregulated 9v plugpack
• ... and a few extra features, not essential, but desirable
  • RS485 Termination resistor, jumper selectable
  • Pullup/Pulldown resistors on RS485 to establish line-idle condition

As the design proceeded we added a couple of extra features over and above the specification, for the sake of completeness they were:

• RS485 Transmission not just controlled by RTS, but also switched to 'Transmit' whenever the RS232 TxD line goes to space (active). While special software running on the PC should use the RTS line to control RS485 Tx, passive mark pulling and RS485 Tx activated by RS232 space allows a quick and easy test of RS485 serial COMs with existing software.
• Take RI and ground through to spare pins on the RS485 jack
  
  
  The basic RS485 converter schematic

The RS485 converter layout

Reference:
http://www.airborn.com.au/layout/232to485.html
http://www.sixca.com/eng/articles/rs485/index.html