This project was written
in December 1998,
prototyped in 1999,
and reviewed in 2009.


RS232 to RS485 Converter

Please take note This version of the device is RTS controlled. Windows does not handle
that well, unless you use custom software. If you use this unit in
conjunction with dedicated hardware, it performs admirably and is
fully controllable. We have a second version of RS232 to 485 converter
which is Data controlled, that unit works well with windows.
We sell this second version.

RS232 to RS485 Converter specification

  • DE9 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 a couple of extra features were added over and above the specification, for the sake of completeness they are added back in to the spec now:

  • 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

For just the RS485 converter, the main two diagrams are below - the circuit and PCB layout. For the complete article, to 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

©2013 AirBorn - Last updated 01 May 2013