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Edit: I don’t really make a point of saying it, but the common ground between the modules isn’t nesesarry.

Here’s a video of the software running on the boards.

So for this example, there are 4 bytes being transmitted from the master to the slaves.

Start of Transmission (0xDB)
ID of the slave to talk to
The type of command
The data that is transmitted

A transmission to set the pwm value of the green led on the slave with the ID 1 to 100 would be:

0xDB 0x01 0x01 0x64

1	0xDB 0x01 0x01 0x64

This is the schematic of the circuit for this example:

This is the code running on the master Arduino:

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;
int MAX485_Driver_Input_PIN = 11;
int MAX485_Driver_Output_Enable_PIN = 12;

#define NUMSWITCHES 3

int ID_switches[NUMSWITCHES] = {2, 3, 4};
int mode_switch = 5; 

byte IDs[NUMSWITCHES] = {1, 2, 3};

#define TXSIZE 4

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

byte target_node_ID;

struct transmission {

  byte start = 0xDB;
  
  byte ID;
  byte type;
  byte message;

  const int tx_size = TXSIZE;
};

void setup()
{
  software_serial.begin(9600); // begin software serial
  
  pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);
  digitalWrite(MAX485_Driver_Output_Enable_PIN, HIGH); // this disables Receiver Output Enable and enables Driver Output Enable
  
  for (int index = 0; index < NUMSWITCHES; index++)
  {
    pinMode(ID_switches[index], INPUT); 
  }
  
  pinMode(mode_switch, INPUT);
  
  target_node_ID = 0;
}

void loop()
{
  for (int index = 0; index < NUMSWITCHES; index++)
  {
    if ((byte)digitalRead(ID_switches[index]))
    {
      target_node_ID = IDs[index];
      break;
    }
  }
  
  transmission t;
  
  t.ID = target_node_ID;
  t.type = digitalRead(mode_switch);
  t.message = (byte)map(analogRead(0), 0, 1023, 0x00, 0xFF);
  
  byte message[TXSIZE] = {t.start, t.ID, t.type, t.message};
  
  software_serial.write(message, t.tx_size);
}

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;

int MAX485_Driver_Input_PIN = 11;

int MAX485_Driver_Output_Enable_PIN = 12;

#define NUMSWITCHES 3

int ID_switches[NUMSWITCHES] = {2, 3, 4};

int mode_switch = 5;

byte IDs[NUMSWITCHES] = {1, 2, 3};

#define TXSIZE 4

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

byte target_node_ID;

struct transmission {

byte start = 0xDB;

byte ID;

byte type;

byte message;

const int tx_size = TXSIZE;

};

void setup()

{

software_serial.begin(9600); // begin software serial

pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);

digitalWrite(MAX485_Driver_Output_Enable_PIN, HIGH); // this disables Receiver Output Enable and enables Driver Output Enable

for (int index = 0; index < NUMSWITCHES; index++)

{

pinMode(ID_switches[index], INPUT);

}

pinMode(mode_switch, INPUT);

target_node_ID = 0;

}

void loop()

{

for (int index = 0; index < NUMSWITCHES; index++)

{

if ((byte)digitalRead(ID_switches[index]))

{

target_node_ID = IDs[index];

break;

}

transmission t;

t.ID = target_node_ID;

t.type = digitalRead(mode_switch);

t.message = (byte)map(analogRead(0), 0, 1023, 0x00, 0xFF);

byte message[TXSIZE] = {t.start, t.ID, t.type, t.message};

software_serial.write(message, t.tx_size);

}

This is the code running on the slave Arduinos:

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;
int MAX485_Driver_Input_PIN = 11;
int MAX485_Driver_Output_Enable_PIN = 12;

int led_1_PIN = 5; 
int led_2_PIN = 6; 

unsigned long time = 0;  
unsigned long oldtime = 0; 
int state = 0;
int next_ID_button_PIN = 2;

#define NUMSTATES 3
int states[NUMSTATES] = {7, 8, 9};
byte IDs[NUMSTATES] = {1, 2, 3};

#define MESSAGELENGTH 3

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()
{
  software_serial.begin(9600); // begin software serial
  
  pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);
  digitalWrite(MAX485_Driver_Output_Enable_PIN, LOW);
  
  pinMode(led_1_PIN, OUTPUT);
  pinMode(led_2_PIN, OUTPUT);
  
  pinMode(next_ID_button_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(next_ID_button_PIN), next_ID, RISING); 
  
  set_LEDs();
}

void loop() // run over and over
{
  
  byte start_of_message = 0xDB;
  byte incoming_byte;
  byte message[MESSAGELENGTH];
  
  while (true)
  {
    if (software_serial.available())
    { 
      incoming_byte = software_serial.read();
      
      if (incoming_byte == start_of_message)
      {
        software_serial.readBytes(message, MESSAGELENGTH);
        process_message(message);
      }
    }
  }
}
 
void process_message(byte message[])
{
  byte system_ID = IDs[state];

  byte incoming_ID = message[0];
  byte incoming_mode = message[1];
  byte LED_brightness = message[2];
   
  if (incoming_ID == system_ID)
  {
    byte pin;
    
    if (incoming_mode == 0)
    {  
      pin = led_1_PIN;
    }
    else if (incoming_mode == 1)
    {
      pin = led_2_PIN; 
    }
    analogWrite(pin, LED_brightness);  
  }
}

void next_ID()
{
  time = millis();
  
  //soft debounce
  if (time - oldtime > 200) 
  {
    oldtime = time;
    
    state++;
    
    if (state >= (NUMSTATES))
    {
      state = 0;
    }
  }
  set_LEDs();
}

void set_LEDs()
{
  for (int index = 0; index < NUMSTATES; index++)
  {  
    if (index == state)
    {
      digitalWrite(states[index], HIGH);
    }
    else 
    {
      digitalWrite(states[index], LOW);
    }
  } 
}

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#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;

int MAX485_Driver_Input_PIN = 11;

int MAX485_Driver_Output_Enable_PIN = 12;

int led_1_PIN = 5;

int led_2_PIN = 6;

unsigned long time = 0;

unsigned long oldtime = 0;

int state = 0;

int next_ID_button_PIN = 2;

#define NUMSTATES 3

int states[NUMSTATES] = {7, 8, 9};

byte IDs[NUMSTATES] = {1, 2, 3};

#define MESSAGELENGTH 3

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()

{

software_serial.begin(9600); // begin software serial

pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);

digitalWrite(MAX485_Driver_Output_Enable_PIN, LOW);

pinMode(led_1_PIN, OUTPUT);

pinMode(led_2_PIN, OUTPUT);

pinMode(next_ID_button_PIN, INPUT_PULLUP);

attachInterrupt(digitalPinToInterrupt(next_ID_button_PIN), next_ID, RISING);

set_LEDs();

}

void loop() // run over and over

{

byte start_of_message = 0xDB;

byte incoming_byte;

byte message[MESSAGELENGTH];

while (true)

{

if (software_serial.available())

{

incoming_byte = software_serial.read();

if (incoming_byte == start_of_message)

{

software_serial.readBytes(message, MESSAGELENGTH);

process_message(message);

}

void process_message(byte message[])

{

byte system_ID = IDs[state];

byte incoming_ID = message[0];

byte incoming_mode = message[1];

byte LED_brightness = message[2];

if (incoming_ID == system_ID)

{

byte pin;

if (incoming_mode == 0)

{

pin = led_1_PIN;

}

else if (incoming_mode == 1)

{

pin = led_2_PIN;

}

analogWrite(pin, LED_brightness);

}

void next_ID()

{

time = millis();

//soft debounce

if (time - oldtime > 200)

{

oldtime = time;

state++;

if (state >= (NUMSTATES))

{

state = 0;

}

set_LEDs();

}

void set_LEDs()

{

for (int index = 0; index < NUMSTATES; index++)

{

if (index == state)

{

digitalWrite(states[index], HIGH);

}

else

{

digitalWrite(states[index], LOW);

}

Hey! This post was written a long time ago, but I'm leaving it up on the off-chance it may help someone. Proceed with caution. It may not be a good idea to blindly integrate this code or work into your project, but instead use it as a starting point.

For another project I’m currently working on, I need a way to control multiple microcontrollers in a multi-point, multi-drop network configuration. Luckily, we live in amazing times. As I write this, you you can buy a fully assembled breakout board for the MAX485 chip from Maxim Integrated for a mere $0.45 USD shipped from China.

I bought a 5 pack, here are some of the boards:

RS485 is an old protocol, but is the logical next step for devices I’m already communicating with via RS232. For this example, I’m using 4 Arduino boards of various types.

An Arduino Micro as the master
2 Slave Arduino Leonardos
1 Slave Arduino Pro Mini (5v)

Here is a video of the setup:

The schematic is really straightforward as well. The only tricky bit is that I’m using a software serial port on each of the Arduinos for ease of debugging. Here’s a schematic:

The code to acomplish this is really intuitive as well.

Here is the code for the master Arduino:

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;
int MAX485_Driver_Input_PIN = 11;
int MAX485_Driver_Output_Enable_PIN = 12;

int debug_led = 13;

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()
{
  Serial.begin(9600); // begin hardware serial
  software_serial.begin(9600); // begin software serial
  
  pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);
  digitalWrite(MAX485_Driver_Output_Enable_PIN, HIGH); // this disables Receiver Output Enable and enables Driver Output Enable
}

void loop()
{
  
  byte to_send = 0; // declare the byte to be sent to the slaves though, init as 0
  int rate;
  
  while (true) 
  {
    // invert the byte to be sent
    if (to_send == 1) to_send = 0;
    else if (to_send == 0) to_send = 1; 
    
    Serial.print("Sending: ");
    Serial.println(to_send);
    
    digitalWrite(debug_led, to_send);
    
    rate = map(analogRead(5), 0, 1023, 0, 1000);
    
    software_serial.write(to_send); // send our byte out to the MAX485
    
    delay(rate);
  }
}

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;

int MAX485_Driver_Input_PIN = 11;

int MAX485_Driver_Output_Enable_PIN = 12;

int debug_led = 13;

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()

{

Serial.begin(9600); // begin hardware serial

software_serial.begin(9600); // begin software serial

pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);

digitalWrite(MAX485_Driver_Output_Enable_PIN, HIGH); // this disables Receiver Output Enable and enables Driver Output Enable

}

void loop()

{

byte to_send = 0; // declare the byte to be sent to the slaves though, init as 0

int rate;

while (true)

{

// invert the byte to be sent

if (to_send == 1) to_send = 0;

else if (to_send == 0) to_send = 1;

Serial.print("Sending: ");

Serial.println(to_send);

digitalWrite(debug_led, to_send);

rate = map(analogRead(5), 0, 1023, 0, 1000);

software_serial.write(to_send); // send our byte out to the MAX485

delay(rate);

}

This is the code for the slave Arduinos:

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;
int MAX485_Driver_Input_PIN = 11;
int MAX485_Driver_Output_Enable_PIN = 12;

int debug_led_PIN = 9; 

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()
{
  software_serial.begin(9600); // begin software serial
  
  pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);
  digitalWrite(MAX485_Driver_Output_Enable_PIN, LOW);
  
  pinMode(debug_led_PIN, OUTPUT);
  
}

void loop() // run over and over
{
  byte k;
  
  if (software_serial.available() > 0) // make sure there is something to read
  { 
    k = software_serial.read(); // read in a single byte from the serial port    
    digitalWrite(debug_led_PIN, k); 
  }
}

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;

int MAX485_Driver_Input_PIN = 11;

int MAX485_Driver_Output_Enable_PIN = 12;

int debug_led_PIN = 9;

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()

{

software_serial.begin(9600); // begin software serial

pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);

digitalWrite(MAX485_Driver_Output_Enable_PIN, LOW);

pinMode(debug_led_PIN, OUTPUT);

}

void loop() // run over and over

{

byte k;

if (software_serial.available() > 0) // make sure there is something to read

{

k = software_serial.read(); // read in a single byte from the serial port

digitalWrite(debug_led_PIN, k);

}

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;
int MAX485_Driver_Input_PIN = 11;
int MAX485_Driver_Output_Enable_PIN = 12;

int debug_led_PIN = 9; 

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()
{
  software_serial.begin(9600); // begin software serial
  
  pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);
  digitalWrite(MAX485_Driver_Output_Enable_PIN, LOW);
  
  pinMode(debug_led_PIN, OUTPUT);
  
}

void loop() // run over and over
{
  byte k;
  
  if (software_serial .available() &gt; 0) // make sure there is something to read
  { 
    k = software_serial.read(); // read in a single byte from the serial port    
    digitalWrite(debug_led_PIN, k); 
  }
}

#include <SoftwareSerial.h>

int MAX485_Receiver_Output_PIN = 10;

int MAX485_Driver_Input_PIN = 11;

int MAX485_Driver_Output_Enable_PIN = 12;

int debug_led_PIN = 9;

SoftwareSerial software_serial (MAX485_Receiver_Output_PIN, MAX485_Driver_Input_PIN); // RX, TX

void setup()

{

software_serial.begin(9600); // begin software serial

pinMode(MAX485_Driver_Output_Enable_PIN, OUTPUT);

digitalWrite(MAX485_Driver_Output_Enable_PIN, LOW);

pinMode(debug_led_PIN, OUTPUT);

}

void loop() // run over and over

{

byte k;

if (software_serial .available() > 0) // make sure there is something to read

{

k = software_serial.read(); // read in a single byte from the serial port

digitalWrite(debug_led_PIN, k);

}

In subsequent posts, things will start getting more advanced. For now however this should be enough to start from scratch.

Thanks for reading.

esologic

Category: Hardware Networks

RS485 Hardware Network 2 – Multi-Byte Communication

RS485 Hardware Network 1 – Getting Started