rs485
I'm trying to do communicating an Arduino Mega to an Arduino Nano through RS485 protocol.
In this way, I'm using two MAX485 boards like this :
If I apply this wiring :
I get this signal :
As you can see, the signal output from A and B are same and a little distorded at the beginning.
Would you have an idea why ?
Regards
I think you have an issue with you line driving RE and DE because you set the D3 line high to transmit, then you issue a modbus function 3 (read holding registers). After this you check if the request was a success and after this you set the D3 line low and you read the response from the library buffer from the node.
The issues stands that a modbus transaction includes the master querying the slave and the slave responding which seems to be taken care of by the node.readHoldingRegisters() fucntion therefore the master releasing the RS485 bus should happen within this fucntion not after it.
What I guess its the problem, its that the modbus library you used is intended for RS232 as the physical layer therefore you might have to modify the library so the RS485 bus is released as soon as the master completely finishes sending the query
Hope this helps
Martin,
I'm by no means an expert at RS485, but I have run into some of the same problems you've had with independent power supplies on multi-drop RS485 networks.
Most frequently, in spite of claiming fail-safe operation, they don't play nice with ground loops/DC offsets. For me, in the past, the most direct route is to use fail-safe biasing on the two lines. E.g. 
.
By the way: Most of my learning has come from others here as well as some pretty good application notes from Texas Instruments (PDF), Maxim (HTML), and Analog Devices (PDF).
Hope to hear if this gets you in business.
Josh
Best Answer
It is probably due to the fact that you have not connected up the VCC and GND to the two modules. The initial communications transitions are back feeding the VCC and GND through the input protection diodes of the MAX485 until the C1 bypass capacitor gets charged up to a stable value.
Make sure not to run your RS485 transceivers without power applied.