I'm working with a 32-Input Modbus RTU module using RS485. I'm trying to read/poll the inputs to check for ON(1) states. I have tested this module with Arduino UNO, Radzio! Modbus Master simulator, and Modbus Poll -- all worked as expected, (displaying 0=off, 1=on). Using an Arduino Nano with a TTL to RS485 Module like this.

So using the below code, the output is all 0s regardless of input state. It compiles and runs without errors.

#include <ModbusRtu.h>
#include <SoftwareSerial.h>

// data array for modbus network sharing
uint16_t au16data[32];
uint8_t u8state;

 *  Modbus object declaration
 *  u8id : node id = 0 for master, = 1..247 for slave
 *  u8serno : serial port (use 0 for Serial)
 *  u8txenpin : 0 for RS-232 and USB-FTDI 
 *               or any pin number > 1 for RS-485
Modbus master(0); // this is master and RS-232 or USB-FTDI via software serial

 * This is an structe which contains a query to an slave device
modbus_t telegram;

unsigned long u32wait;

SoftwareSerial mySerial(2, 3);//Create a SoftwareSerial object

void setup() {
  master.begin( &mySerial, 9600 ); // begin ModBus object 9600baud
  master.setTimeOut( 2000 ); // if no answer in 2000 ms, roll over
  u32wait = millis() + 1000;
  u8state = 0; 

void loop() {
  switch( u8state ) {
  case 0: 
    if (millis() > u32wait) u8state++; // wait state
  case 1: 
    telegram.u8id = 1; // slave address
    telegram.u8fct = 3; // function code (this one is registers read)
    telegram.u16RegAdd = 128;// start address in slave 0x0080-0x009F(128-159)
    telegram.u16CoilsNo = 32; // number of elements to read  0x00 0x01
    telegram.au16reg = au16data; // pointer memory array in Arduino

    master.query( telegram ); // send query (only once)
  case 2:
    master.poll(); // check incoming messages
    if (master.getState() == COM_IDLE) {
      u8state = 0;
      u32wait = millis() + 2000; 
        Serial.println(au16data[0]);//Or do something else!
        Serial.println(au16data[1]);//Or do something else!
        Serial.println(au16data[2]);//Or do something else!
        Serial.println(au16data[3]);//Or do something else!
        Serial.println(au16data[4]);//Or do something else!
        Serial.println(au16data[5]);//Or do something else!
        Serial.println(au16data[6]);//Or do something else!
        Serial.println(au16data[7]);//Or do something else!
        Serial.println(au16data[8]);//Or do something else!
        Serial.println(au16data[9]);//Or do something else!
        Serial.println(au16data[10]);//Or do something else!
        Serial.println(au16data[11]);//Or do something else!
        Serial.println(au16data[12]);//Or do something else!
        Serial.println(au16data[13]);//Or do something else!
        Serial.println(au16data[14]);//Or do something else!
        Serial.println(au16data[15]);//Or do something else!
        Serial.println(au16data[16]);//Or do something else!
        Serial.println(au16data[17]);//Or do something else!
        Serial.println(au16data[18]);//Or do something else!
        Serial.println(au16data[19]);//Or do something else!
        Serial.println(au16data[20]);//Or do something else!
        Serial.println(au16data[21]);//Or do something else!
        Serial.println(au16data[22]);//Or do something else!
        Serial.println(au16data[23]);//Or do something else!
        Serial.println(au16data[24]);//Or do something else!
        Serial.println(au16data[25]);//Or do something else!
        Serial.println(au16data[26]);//Or do something else!
        Serial.println(au16data[27]);//Or do something else!
        Serial.println(au16data[28]);//Or do something else!
        Serial.println(au16data[29]);//Or do something else!
        Serial.println(au16data[30]);//Or do something else!
        Serial.println(au16data[31]);//Or do something else!

Could this be caused by the 'Number of elements" to read? Any suggestions greatly appreciated.

This is the 32-input module manual and programming guide

Code is based on this example.

  • Your code would be more readable with proper indentation. Can you clarify what you mean with "never displayed"? What is actually displayed? Is the actual amount of received bytes correct?
    – StarCat
    Mar 21, 2023 at 6:47
  • What is actually displayed is a 0, not a 1 when the input is ON.
    – Ashton
    Mar 21, 2023 at 12:55
  • If you change your code completely, you should added it as an UPDATE than total replace your original code.
    – hcheung
    Mar 22, 2023 at 7:21
  • What RS485/Modbus interface board you have? other than the Pin2 for RX, and P3 for TX, How do you wire the RE(Receiver Enable) and DE(Driver Enable) pins?
    – hcheung
    Mar 22, 2023 at 8:12
  • 2
    "Code courtesy of: [He who shall not be named]" No, what that guy does is leech off these sites with an automatic video generation system. Real credit goes to (or more to) one of the authors here of an ESE Q&A.
    – timemage
    Mar 22, 2023 at 13:16


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