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©2010 AirBorn
This project was written in December 1998, prototyped in 1999, retested recently.

RS485 Converter


Please take note This version of the device is RTS controlled. Windows does not handle that well, unless you use custom software (or dedicated hardware). We have a second version of RS232 to 485 converter which is Data controlled to overcome this. We sell this second version.

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
  • Fix the Rx / Tx wiring fault that was in the first prototypes

If you are just interested in the RS485 converter, there is no reason to go through the whole tutorial - the two diagrams you will need to see are the PCB layout and the schematic, both shown below. If you want to continue and see how this design was put together (ie go through the tutorial) click here.


Final circuit for 232 to 485 converter
The basic RS485 converter schematic


PCB - 232 to 485 converter
The RS485 converter PCB layout

[Graphic: Small schematic]

Click the Continue arrow below for "PCB Tutorial - how these diagrams were put together" or go on to:

Subject Overview Subject Tutorial