RS485 not working

Question

I want to send data between different Arduinos distributed in different places: Something like this: https://forum.arduino.cc/t/arduino-master-slave-communication-using-rs485-and-modbus-protocol/644168

I created my own Arduino shield based on this RS485 board:

When I connect two Arduinos using two of these boards, I can send data from one to the other. But when I use at least one of mine, nothing happens.

I checked:

• cables.
• power in the RS485 with multimeter.
• pins matching Arduino to Shield.
• Disconnecting the GND between the boards as my circuit has R28 and C22 that are not in the original board.
• Connecting the GND of the Arduinos togheter.

I don't know what else I can do. I saw R25 and R27 can be smaller. Any suggestions? Should I remove R28 and C22? Should I reduce R25 and R27?

This is how my schematic:

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SOLUTION

When designing the schematic, I crossed by mistake A and B and it is mentioned in @MF3 answer. With this error, R25 and R27 are pulling up and down the lines in the wrong way and it is not working. After removing them, I can connect my shield and one of the boards mentioned without any problem.

The current configuration is:

Answer 1

I see two problems in your schematic:

• A and B are crossed: The terminal A is connected to the pin B (pin 7 or the MAX485E) and the terminal B is connected to pin A (pin 6 or the MAX485E).
• The fail-safe biasing circuit is wrong: The resistor connected to the pin B (R25) should be a pull-down and it is pull-up. Something similar happens to the pin A, as the R27 shoul be pull-up and it pull-down. This capture may help:

Reference: https://www.analog.com/media/en/technical-documentation/application-notes/an-960.pdf

Regarding the values of R25 and R27, the same link shows you how to obtain them.

I think you should remove the two resistors mentioned (R25 and R27) from the boards, and then wired the boards considering that all the pin B are connected togheter and the same with pin A. One last thing, you should have only two terminator resistors (R26) and in your circuit, it looks like they are always there, so if you connect more than 2 boards, you should remove one.

Answer 2

If all nodes have biasing and termination resistors, it will not work.

Only the two devices at the ends of the bus should have the terminations present. Not all. And the biasing resistors should be present only once on the bus.

Also RS-485 is not a two-wire bus, as the devices must share a common ground reference. Your design does not connect grounds between RS-485 boards. So if the are powered from different, unrelated power sources, there may be too much voltage between grounds and RS-485 will not work.