RS485 Working only in one direction.

Projects Networking, Protocols, and Devices
1

Hi.
I´m trying to replace our old (broken) doorbell with a few Arduinos.
Because of the long distance and needed communication in both ways for the electrical door opener, I thought of using RS485 for this.

So I´ve got a few Arduino Nanos laying around and got some MAX485 breakout boards and put them together on a breadboard.

Both Arduinos have currently the same code. They listen for Serial Input and send it with SoftwareSerial to the MAX485 module. Then listen for the input from the MAX485 and printing it back on the Serial connection.
But the communication seems only to be working in one direction.

I can send the serial input from one device to the other and it´s printed out on the serial port.
But in the other direction nothing is received.

I first thought either the sending part on one RS485 module or the receiving part on the other one is broken, so I replaced both. But it´s still the same.

Maybe someone can tell me whats wrong here.

Here´s how I connected everything. (Of course not the MAX485 directly but a breakout board with the pullup resistors.)

591×923 188 KB

And here the code that´s on both Arduinos. (Only difference is that it prints out Slave on the Serial port instead of Master at Setup)

#include <SoftwareSerial.h>

#define SSerialRX        10  //Serial Receive pin
#define SSerialTX        11  //Serial Transmit pin

#define SSerialTxControl 3   //RS485 Direction control

#define RS485Transmit    HIGH
#define RS485Receive     LOW

#define Pin13LED         13

SoftwareSerial RS485Serial(SSerialRX, SSerialTX); // RX, TX

int byteReceived;
int byteSend;

void setup()
{
  // Start the built-in serial port, probably to Serial Monitor
  Serial.begin(9600);
  Serial.println("Master");
  
  pinMode(Pin13LED, OUTPUT);
  pinMode(SSerialTxControl, OUTPUT);    
  
  digitalWrite(SSerialTxControl, RS485Receive);  // Init Transceiver   
  
  // Start the software serial port, to another device
  RS485Serial.begin(4800);   // set the data rate 
}

void loop()
{
  digitalWrite(Pin13LED, HIGH);  // Show activity
  if (Serial.available())
  {
    byteReceived = Serial.read();
    
    digitalWrite(SSerialTxControl, RS485Transmit);  // Enable RS485 Transmit   
    RS485Serial.write(byteReceived);          // Send byte to Remote Arduino
    
    digitalWrite(Pin13LED, LOW);  // Show activity    
    
    digitalWrite(SSerialTxControl, RS485Receive);  // Disable RS485 Transmit       
  }
  
  if (RS485Serial.available())  //Look for data from other Arduino
   {
    digitalWrite(Pin13LED, HIGH);  // Show activity
    byteReceived = RS485Serial.read();    // Read received byte
    Serial.write(byteReceived);        // Show on Serial Monitor
    
    digitalWrite(Pin13LED, LOW);  // Show activity   
   }
}
2

Just checking, but is there a ground wire also?

3

Yes forgot to draw it in.
Ground is between both connected.

4

As so often I think I resolved it.

I replaced one of the MAX485 modules again and it´s working now.

Seems I got a batch of broken breakout boards.
2 out of 4 can´t receive anything.