Reliable Custom Design Click here to contact AirBorn
 
--AirBorn Electronics-- US SITE · AUS SITE
ABOUT US
Company Detail
Enquiry Form
Sitemap
WHAT WE MAKE
Our Design method
Spec'ing your job
Hardware design
Firmware design
Examples: Circuits
Examples: Photos
Subject Overview     Next Subject     Exit Site
Example project
OUR CIRCUITS
Serial to Parallel
Navy SMD Trainer
Danfoss tester
89C2051 project
Weighscale
Light Key Tone Gen
Index - Main Library
OUR DESIGN STEPS
Design step-by-step
Circuit diagrams
PCB Layout
PCB Etching
Prototyping
Firmware design
Documentation
Manufacturing
Economies of scale
Test Procedures
ECOs - changes
SPECIFY YOUR WORK
Overview
Specification Intro.
Writing a Spec
Tech Ingredients
Example Specs
Engineer employment
R&D Economics
Design Inspiration
EXAMPLE BOARDS
A PCB Factory
Example projects
...more ...more
A real design Lab
REFERENCE
Our PicoBus IO Std
RS232 RS485 FR4 Autotrax CAD Library P89LPC932 Program Header
Past products
Using a Multimeter
FURTHER INFO
Complete site index
Offsite Links (15000+)
Google search
©2010 AirBorn
This project was written in December 1998, prototyped in 1999, retested recently.

RS485 Converter


Please take note There is a nasty little "bug" in the circuit board described here - it affects just some applications of the board. And without totally replacing the design, we can't do much about this:

Windows doesn't control the serial port handshaking lines very well, - it's timing is (by default) poor and imprecise at the millisecond level. 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.

We have a newer version of this device that uses a microprocessor to avoid the RTS-controlled bug

Our advice is test it completely before you commit to making any quantity. Really this design is most likely to be useful if you are writing your own PC RS485 software - then you can completely control the RTS line on our board and there will be no problem at all.

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