This 2006 design is commercially
sold, the circuit is provided here

 

RS232 to RS485 Converter

RS232 to RS485 Converter specification Data (vs RTS) controlled - does not rely on PC handshake timing TxD and RxD translated from RS232 to RS485 DB25 RS232 connector compatible with PC Direction of transmission controlled by Data Handshake loop the PC connection so it works with all software RS485 signals output on D9 male with lots of Ground pins Indicator LED(s) to show communications traffic Line powered - powered by RS232 handshake lines RS485 Termination resistor not included Baud rate is dipswitch selectable 1200 - 115.2k Pullup/Pulldown resistors on RS485 to establish line-idle condition $256 Australian 2-49pc $99 Australian 50pc and up Manufacturing Rights to the design (including source code) available for the cost of 50 units

We have a very simple RS232 converter design that is well documented and free. The direction of data communication is controlled directly by the RTS line in that design - it works really well with custom software or hardware, but windows does not operate RTS with the precision required for reliable operation. This model of RS485 converter overcomes the RTS issue by using a microprocessor and switching on Data transfer. There is a greater complexity to the design, however. We sell this unit (mostly to) local Australian companies. This design has been relatively trouble free, and if you require quantities we can adapt the connector pinout to suit your application - however then the device will no longer fit the simple little D25 case in which you see it pictured.

The RS232 to RS485 converter

The RS485 converter circuit diagram