RS485 Hardware, Software, Direction control Example

Hi, There have been many requests for RS485 How-To.

A working example is here: http://arduino-info.wikispaces.com/SoftwareSerialRS485Example

Any comments, critique, suggestions welcome...

What's the purpose of the 4 - 10K resistors and the 2 - 20K resistors on the RS485 board?

Hi,

The 10K's are pullups that might be needed on some devices that could use this module. It's not Arduino-specific.

The 20K's are to define the state of the RS485 bus when no module is driving it. see http://arduino-info.wikispaces.com/RS485Info (near the end)..

So if my MAX485 is on the same board as the arduino I don't need the pullups? The MAX485 would be connected at all times to the arduino pins.

But I need the 20Ks no matter what. Good to know.

So if my MAX485 is on the same board as the arduino I don't need the pullups? The MAX485 would be connected at all times to the arduino pins.

Yes, I believe so, the MAX485 Data sheet shows the outputs as regular TTL/CMOS levels..

http://arduino-info.wikispaces.com/file/view/MAX485.pdf/329206482/MAX485.pdf