Arduino Mega and RS485 Modbus Sensor - arduino

Question

I'm trying to connect a soil sensor using RS485 communication to arduino mega and I can't get it to work. I'm using the SparkFun RS485 breakout: https://www.sparkfun.com/products/10124
I've connected TX to pin 18, RS to pin 19 and RTS to pin 8.
I've tried to adapt the code from here: https://www.youtube.com/watch?v=tBw15SfmuwI using the sensor's manufacturers default setting:
Modbus address fixed to 0
The communication configuration is 9600,N,8,1（9600bps， no check bit, 8 data bits,
1 stop bit）
Communication protocol is Modbus-RTU
While the addresses I need to read are 0x0000-0x0002.
However, I get random characters as output when I open the serial monitor "?", any idea why? I'd appreciate any help reading the sensor's output.
This is the code I've used:
#include <ModbusMaster.h>
#define MAX485_DE 8
#define MAX485_RE_NEG 8
ModbusMaster node;
void preTransmission () {
digitalWrite(MAX485_RE_NEG, 1);
digitalWrite(MAX485_DE, 1);
}
void postTransmission () {
digitalWrite(MAX485_RE_NEG, 0);
digitalWrite(MAX485_DE, 0);
}
void setup() {
pinMode(MAX485_RE_NEG, OUTPUT);
pinMode(MAX485_DE, OUTPUT);
digitalWrite(MAX485_RE_NEG, 0);
digitalWrite(MAX485_DE, 0);
Serial.begin(9600);
node.begin(0,Serial);
node.preTransmission(preTransmission);
node.postTransmission(postTransmission);
}
void loop() {
uint8_t resultMain;
resultMain = node.readInputRegisters(0x0000, 3);
if (resultMain == node.ku8MBSuccess) {
Serial.println("-------");
Serial.print("Temp: ");
Serial.println(node.getResponseBuffer(0x00) /100);
Serial.print("VWC: ");
Serial.println(node.getResponseBuffer(0x01) /100);
Serial.print("EC: ");
Serial.println(node.getResponseBuffer(0x02) /100);
}
}

Answer 1

The arduino mega has 3 serial ports:
Serial
Serial1
Serial2
You could think of it as Serial as Serial0 (the zero is never written). This port is hardwired to the USB port on the arduino mega.
Your RS485 breakout board is connected to Serial1. You may notice the screenprinting on the mega next to pins 18 and 19 says TX1 and RX1.
So when you initialize your node at this line:
node.begin(0,Serial);
you should pass it Serial1 instead of Serial
eg
node.begin(0,Serial1);