Electronic – Arduino as modbus master with MAX485 doesn’t get any response

I'm having some troubles trying to query a modbus slave with an Arduino through RS485.

I've already succeeded in querying a software modbus slave running on my PC through the USB/COM port using the ModbusMaster libray, hence it shouldn't be a software issue.

I read about TTL and level conversions and I put on a circuit like this on a breadboard:

Using the same firmware/sketch that worked for the software slave, I connected the arduino pin TX and RX to the max485 and A and B to the modbus slave and I issued several requests.

I can see the signals converted by the MAX485 (CPA1114) though the oscilloscope and it seems to be right. The led on the modbus slave lights on as it sees a modbus transaction. Still, what I read as result of the request is always 0xE0 (invalid slave id) or 0xE2 (timeout).

I queried the slave with the same equal request using another tool (a RS485/USB converter and CAS Modbus Scanner), and it gives the expected results, that is data 0x01.

This is the code I'm running on an Arduino Ethernet (with a display for debug purpose):

#include <ModbusMaster.h>
#include <LiquidCrystal.h>
LiquidCrystal lcd(12, 11, 4, 5, 6, 7);

ModbusMaster node(1);

void setup() {

  pinMode(3, OUTPUT);
  node.begin(19200);
  lcd.begin(16, 2);

}

void loop() {

  uint16_t m_startAddress=1;
  uint8_t m_length=1;
  uint8_t result;

  digitalWrite(3, HIGH); // TX
  result = node.readHoldingRegisters(m_startAddress, m_length);

  lcd.clear();

  if (result == node.ku8MBSuccess) {
    lcd.print("DATA:");
    digitalWrite(3, LOW); // RX
    for (uint8_t j = 0; j < m_length; j++) lcd.print( node.getResponseBuffer(j), HEX );
  } else {
    lcd.print("ERR ");
    lcd.print(result, HEX);
  }

  delay(500);

}

These are the request signals emitted by the Arduino, that always fail to get a data response, and the other tool, that always succeed:

Arduino request signal

USB/RS485 converter signal

Overlap of the two signals

Is there something wrong with the request signal? Am I making any mistakes in the circuit or the code?

Any pointers would be greatly appreciated.

EDIT:

As suggested by Kvegaoro, I got it working by editing the ModbusMaster library in order to switch the D3 pin to the right status at the right moment. For doing so I used some code I found on this post (italian) in the Arduino forum.

This is the edit I've done in ModbusMaster.cpp, function ModbusMasterTransaction, starting at line 746:

  // code edited to work with MAX485:
  // transmit request
  UCSR0A=UCSR0A |(1 << TXC0);  
  Serial.flush();
  digitalWrite(3, HIGH);

  for (i = 0; i < u8ModbusADUSize; i++)
  {
#if defined(ARDUINO) && ARDUINO >= 100
    MBSerial.write(u8ModbusADU[i]);
#else
    MBSerial.print(u8ModbusADU[i], BYTE);
#endif
  }
  while (!(UCSR0A & (1 << TXC0)));
  digitalWrite(3, LOW);
  // --

  u8ModbusADUSize = 0;

Note that the D3 pin is then hard-coded in the library so this is not a good design, if someone is going to need this would adjust it better. It works though!