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RS485 Converter

Please take note There is a nasty little "bug" in the circuit board described here - one of those horrible things that you can't do much about:
Windows doesn't control the serial port handshaking lines very well. Actually, compared with DOS, it is very poor in its timing. Any RTS controlled RS485 converter (and, like ours, that is how most of them work!) will only have mediocre performance under windows - and with some RS485 devices at higher baud rates it may well fail to operate. Our advice is test it completely before you commit to making any quantity...having said that this page is still useful for teaching principles of CAD, and for those who want an RS485 converter controlled by their own specially written drivers -

If you want to use this design, please contact us for schematics

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

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 will be the PCB layout and the schematic, and they are both reproduced below. If you want to continue and see how these diagrams were put together (ie go through the tutorial) click here.


Schematic - 232 to 485 converter
The basic RS485 converter schematic


PCB - 232 to 485 converter

The RS485 converter layout

[Graphic: Small schematic]

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

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