Sunday, 14 September 2014

Modbus Master implementation

In my last post, I've published my last Arduino Modbus library, which contains a Modbus Master example:

 *  Modbus master example 1:
 *  The purpose of this example is to query an array of data
 *  from an external Modbus slave device. 
 *  The link media can be USB or RS232.
 *  Recommended Modbus slave: 
 *  diagslave
 *  In a Linux box, run 
 *  "./diagslave /dev/ttyUSB0 -b 19200 -d 8 -s 1 -p none -m rtu -a 1"
 *  This is:
 *   serial port /dev/ttyUSB0 at 19200 baud 8N1
 *  RTU mode and address @1

#include <ModbusRtu.h>

// data array for modbus network sharing
uint16_t au16data[16];
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(0,0,0); // this is master and RS-232 or USB-FTDI

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

unsigned long u32wait;

void setup() {
  master.begin( 19200 ); // baud-rate at 19200
  master.setTimeOut( 2000 ); // if there is 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 = 1; // start address in slave
    telegram.u16CoilsNo = 4; // number of elements (coils or registers) to read
    telegram.au16reg = au16data; // pointer to a memory array in the Arduino

    master.query( telegram ); // send query (only once)
  case 2:
    master.poll(); // check incoming messages
    if (master.getState() == COM_IDLE) {
      u8state = 0;
      u32wait = millis() + 100; 

The master requires:
  • A memory array, which is linked to the communication network, and this role is assigned to au16data array.
  • A master object, which sends queries and receives answers through Serial port. This belongs to Modbus class and is assigned to address 0.
  • An object or a set of objects to query, which are modbus_t type;
  • A machine state to make queries and wait for answers in the loop section.

The modbus object

This is the master definition. As in a Modbus slave, there are some parameters to define it:
  • node id u8id is always 0;
  • Serial port u8SerNo must be 0 in an Arduino UNO and could be 0, 1, 2 and 3 in an Arduino Mega;
  • the txenpin is the pin that makes the flow control for the RS-485 transceiver, if any.
For XBEE, look at this.

The memory array

This is the place to link the data pooled through the network with the controller program.

For instance, in a network with an weather station and a cooling system, the master controller must fetch data from the weather station and send commands to the cooling system.

The machine state

The master should consider 3 states:
  • wait state (with no telegrams)
  • send query
  • wait for answer or generate a time-out event
  • restart
This last state should also move the query index if several queries are been processed.


 This is the result when starting an slave with diagslave in a Linux box:

./diagslave /dev/ttyUSB0 -b 19200 -d 8 -s 1 -p none -m rtu -a 1

1 comment:

  1. Good evening.
    I am engineering student and my end of course project, I'm trying to make reading some sensors.
    These sensors are remote, and would like to use Modbus for communication using master and slave Arduino.

    I'm having trouble getting the values sent by the slaves when using your library.

    If possible, I would like your help with an example where a slave sends 3 values and the master shows these three values in a display.



    ps: sorry my English, was translated using google translator