» RS-485 04: Modbus Read Coils (v12)
This example shows the use of the function readCoils. This function requests the ON/OFF status of discrete coils from the slave device.
Required Materials
1 x Waspmote
1 x Battery
1 x RS-485
Notes
- Waspmote must be powered with a battery
- There are many variants of Modbus protocols, but Waspmote implements the RTU format.
- Modbus RTU is the most common implementation available for Modbus.
- This example can be executed in Waspmote v12
Code
/*
* ------ [RS-485_04] Modbus Read Coils --------
*
* This sketch shows the use of the Modbus communication protocol over
* RS-485 standard, and the use of the main funcions of the library.
*. Modbus allows for communication between many devices connected
* to the same network. There are many variants of Modbus protocols,
* but Waspmote implements the RTU format. Modbus RTU is the most
* common implementation available for Modbus.
*
* This example shows the use of the function readCoils. This function
* requests the ON/OFF status of discrete coils from the slave device.
*
* Copyright (C) 2014 Libelium Comunicaciones Distribuidas S.L.
* http://www.libelium.com
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 2 of the License, or
* (at your option) any later version.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* Version: 0.1
* Design: David Gascon
* Implementation: Ahmad Saad
*/
// Include these libraries for using the RS-485 and Modbus functions
#include <Wasp485.h>
#include <ModbusMaster485.h>
// Instantiate ModbusMaster object as slave ID 1
ModbusMaster485 node(1);
// Define one addres for reading
#define address 1
// Define the number of bits to read
#define bitQty 1
void setup()
{
// Power on the USB for viewing data in the serial monitor
USB.ON();
// Initialize Modbus communication baud rate
node.begin(9600);
// Print hello message
USB.println("Modbus communication over RS-485");
delay(100);
}
void loop()
{
// This variable will store the result of the communication
// result = 0 : no errors
// result = 1 : error occurred
int result = node.readCoils(address, bitQty);
if (result != 0) {
// If no response from the slave, print an error message
USB.println("Communication error");
delay(1000);
}
else {
// If all ok
USB.print("Read value: ");
// Print the read data from the slave
USB.print(node.getResponseBuffer(0));
delay(1000);
}
USB.print("\n");
delay(1000);
// Clear the response buffer
node.clearResponseBuffer();
}
Output
E#
Modbus communication over RS-485
Read value : 223
Read value : 224