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Sunday, October 20, 2019
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Querying a Modbus Slave with an Arduino
Having trouble trying to query a Modbus slave with an Arduino through RS485

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 library, 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);
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);


if (result == node.ku8MBSuccess) {
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);



Arduino request signal:
USB/RS485 converter signal:
Overlap of the two signals:

#include <ModbusMaster.h>

#define MAX485_RE_NEG 9
#define MAX485_DE 7

//int myInts[6];

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);


node.begin(10, Serial);


void loop()
// node.clearResponseBuffer();
float finalresult;
//unsigned long long int finalresult;
// uint32_t finalresulta;

// uint16_t result;
// uint16_t resulta;

finalresult = node.readInputRegisters(1,1);

if (finalresult == node.ku8MBSuccess)


Serial.print("\nPV0: ");
float dat=node.getResponseBuffer(0);
// uint64_t data1=node.getResponseBuffer(1);

// Serial.println(data1);

else {
Serial.print("\nError ");