rs 485 communication problem between 2 Arduinos - arduino

Question

I am a novice to Arduino. I came across an article on rs 485 and currently trying to implement the same between 2 arduino unos. I copied code from internet which is pretty simple but I am unable to make communication work properly. Here is the code for master arduino with explanation of what I am trying to do.
**Master Arduino Code**
/* master will send a message I1LF where I shows start of master message by master.
* 1 shows the slave device No. from whom data is to be fetched and L
* shows that master wants to read slave's sensor data
* and F indicates finish of message by master.
* slave will respond by sending Serial.println(i1) where i is slave no and 1 is to let master know that
* correct slave is sending data. then slave would send sensor_value and then f indicates end of slave message.
* master will display data read from rs 485 bus.
*/
#include<LiquidCrystal.h> //Library for LCD display
//function
LiquidCrystal lcd(4, 5, 6, 7, 8, 9);
const int Enable = 2;
int value1=0;
void setup()
{
Serial.begin(9600);
pinMode(Enable, OUTPUT);
digitalWrite(Enable, HIGH); // put master arduino into transmission mode.
lcd.begin(16,2);
lcd.clear();
}
void loop()
{
Serial.print('I'); // I indicates master has started communication.
Serial.print('1'); // this is the address of slave from whom data is to be fetched.
Serial.print('L'); // this means master wants to read sensor data.
Serial.print('F'); // this means end of message.
Serial.flush(); // wiat untill all data has been pushed to the serial.
digitalWrite(Enable, LOW); // put master into receiving mode.
if(Serial.find('i')) // i will be sent (to indicate start of slave
//message , by the slave that was prompted by
//master.
{
int slave = Serial.parseInt(); // check which slave is sending
//data.
value1 = Serial.parseInt();
if(Serial.read() == 'f' && slave == 1) // f indicates end of message
//sent by slave and 1 is the
//address of the slave sent by
// the slave so that master can
//recognize which slave is this.
{
lcd.setCursor(0,0);
lcd.print("Slave : ");
lcd.setCursor(11,0);
lcd.print(value1);
}
}
digitalWrite(Enable, HIGH);
delay(300);
}
This is the slave arduino code
#include<LiquidCrystal.h>
LiquidCrystal lcd(4, 5, 6, 7, 8, 9);
const int Enable = 2;
const int SlaveNumber = 1;
void setup()
{
Serial.begin(9600);
pinMode(Enable, OUTPUT);
digitalWrite(Enable, LOW); // initially put slave in receiving mode
pinMode(A0, INPUT);
lcd.begin(16,2);
lcd.clear();
}
void loop()
{
if(Serial.available())
{
if(Serial.read()=='I') // all slaves will detect I on rs 485 bus which //means master has started commmunication.
{
int Slave = Serial.parseInt(); // read next character on rs 485 bus
//which would definitely be a digit
//indicating slave no to whom master
//wishes to communicate.
if(Slave == SlaveNumber) // check if master wants to commnucate
//with you.
{
char command = Serial.read(); // read next character in
//serial buffer which would be
//command L
delay(100);
if(command == 'L')
{
if(Serial.read()=='F') // if master's message has
//finished.
{
int AnalogValue = analogRead(0); // read sensor
digitalWrite(Enable, HIGH); // put slave in
//transmission mode.
Serial.print("i"); // i indicates start of
//slave message
Serial.println(SlaveNumber); // send slave no so that
//master can identify which
//slave he is receiving
//data from.
Serial.print(AnalogValue);
Serial.print("f"); // indicates end of message by the
//slave.
Serial.flush();
digitalWrite(Enable, LOW); // put slave in
//listening ( receivig ) mode
}
}
}
}
}
}
This is screen shot of proteous simulation showing Master unable to read slave1's sensor data
enter image description here
I assessed that problem arose because of master and slave both being in transmission mode so I attached a not gate between master's enable pin and slave's enable pin and then master is working fine. but practically attaching a not gate wont be possible. that's why I want to know how to ensure that enable of both master and slave is not high simultaneously. I have tried a couple of strategies and introduced delays at different locations in the code but nothing seems to work. by hit and trial it starts to work sometimes but I want to know the correct way of doing so.
This is screen shot of proteous simulation which shows master reading correctly slave1 data only after I attach a not gate between master Enable pin and Slave's Enable pin.
Code for both master and slave and circuit is exactly the same as before. only a not gate has been used between master's enable pin and slave's enable pin.
enter image description here
I tried another library madleech/Auto485 but that i also showing the same problem. have tried a couple of times adding delays at different locations in master code but to no use. i know problem is because of enable pins of both MAX 485 modules connected to 2 arduinos going high simultaneously but i am unable to figure out how to address this issue.

Answer 1

You need to wait for each byte to be available before reading the serial port.
In your code:
if(Serial.available()) // you wait
{
if(Serial.read()=='I') // all slaves will detect I on rs 485 bus which //means master has started commmunication.
{
// parseInt() does wait for a byte to be availeble, so that works...
int Slave = Serial.parseInt();bus
if(Slave == SlaveNumber)
{
// Here serial.read() will most likely return -1, since it is very
// unlikely that a byte is already available at this point.
char command = Serial.read();
// ...
You should wait for the serial port to have received bytes before reading them.
You can try inserting a loop before calling read(), as in:
while (!Serial.available()) {} // wait for something to read
char byteRead = Serial.read();
This blocking code should help fix the code as you have it now. You should probably explore non-blocking ways of reading serial commands, such as using some sort of finite state machine algorithm. As a rule of thumb, embedded software should avoid blocking.