This library allows your Arduino to communicate via Modbus protocol. The Modbus is a master-slave protocol used in industrial automation and can be used in other areas, such as home automation.

The Modbus generally uses serial RS-232 or RS-485 as physical layer (then called Modbus Serial) and TCP/IP via Ethernet or WiFi (Modbus IP).

In the current version the library allows the Arduino operate as a slave, supporting Modbus Serial and Modbus over IP. For more information about Modbus see:

http://pt.wikipedia.org/wiki/Modbus http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf http://www.modbus.org/docs/Modbus_Messaging_Implementation_Guide_V1_0b.pdf

Author's note (motivation and thanks):

It all started when I found the Modbus RTU Arduino library of Juan Pablo Zometa. I had extend the library to support other Modbus functions.

After researching several other Modbus libraries I realized strengths and weaknesses in all of them. I also thought it would be cool have a base library for Modbus and derive it for each type of physical layer used.

I appreciate the work of all the authors of the other libraries, of which I used several ideas to compose the modbus-arduino. At the end of this document is a list of libraries and their authors.

• Operates as a slave (master mode in development)
• Supports Modbus Serial (RS-232 or RS485) and Modbus IP (TCP)
• Reply exception messages for all supported functions
• Modbus functions supported:
• 0x01 - Read Coils
• 0x02 - Read Input Status (Read Discrete Inputs)
• 0x03 - Read Holding Registers
• 0x04 - Read Input Registers
• 0x05 - Write Single Coil
• 0x06 - Write Single Register
• 0x0F - Write Multiple Coils
• 0x10 - Write Multiple Registers

Notes:

1. When using Modbus IP the transport protocol is TCP (port 502) and, by default, the connection is terminated to each transmitted message, that is, is not a keep-alive type connection. If you need a TCP keep-alive connection you have to remove comments of this line in ModbusIP.h header (or ModbusIP_* headers):

#define TCP_KEEP_ALIVE

2. The offsets for registers are 0-based. So be careful when setting your supervisory system or your testing software. For example, in ScadaBR (http://www.scadabr.com.br) offsets are 0-based, then, a register configured as 100 in the library is set to 100 in ScadaBR. On the other hand, in the CAS Modbus Scanner (http://www.chipkin.com/products/software/modbus-software/cas-modbus-scanner/) offsets are 1-based, so a register configured as 100 in library should be 101 in this software.

3. Early in the library Modbus.h file there is an option to limit the operation to the functions of Holding Registers, saving space in the program memory. Just comment out the following line:

#define USE_HOLDING_REGISTERS_ONLY

Thus, only the following functions are supported:

• 0x03 - Read Holding Registers
• 0x06 - Write Single Register
• 0x10 - Write Multiple Registers

4. When using Modbus Serial is possible to choose between Hardware Serial(default) or Software Serial. In this case you must edit the ModbusSerial.h file and comment out the following line:

#define USE_SOFTWARE_SERIAL

Now, You can build your main program putting all necessary includes:

#include <Modbus.h>
#include <ModbusSerial.h>
#include <SoftwareSerial.h>

And in the setup() function:

SoftwareSerial myserial(2,3);
mb.config(&myserial, 38400);   // mb.config(mb.config(&myserial, 38400, 4) for RS-485

There are five classes corresponding to five headers that may be used:

• Modbus - Base Library
• ModbusSerial - Modbus Serial Library (RS-232 and RS-485)
• ModbusIP - Modbus IP Library (standard Ethernet Shield)
• ModbusIP_ENC28J60 - Modbus IP Library (for ENC28J60 chip)
• ModbusIP_ESP8266AT - Modbus IP Library (for ESP8266 chip with AT firmware)

#include <Modbus.h>
#include <ModbusSerial.h>

Modbus Jargon

In this library was decided to use the terms used in Modbus to the methods names, then is important clarify the names of register types:

Notes:

1. Input Status is sometimes called Discrete Input.
2. Holding Register or just Register is also used to store values in the slave.
3. Examples of use: A Coil can be used to drive a lamp or LED. A Holding Register to store a counter or drive a Servo Motor. A Input Status can be used with a reed switch in a door sensor and a Input Register with a temperature sensor.

#include <Modbus.h>
#include <ModbusSerial.h>

Inclusion of the necessary libraries.

const int LAMP1_COIL = 100;

Sets the Modbus register to represent a lamp or LED. This value is the offset (0-based) to be placed in its supervisory or testing software. Note that if your software uses offsets 1-based the set value there should be 101, for this example.

ModbusSerial mb;

Create the mb instance (ModbusSerial) to be used.

mb.config (&Serial, 38400, SERIAL_8N1);
mb.setSlaveId (10);

Sets the serial port and the slave Id. Note that the serial port is passed as reference, which permits the use of other serial ports in other Arduino models. The bitrate and byte format (8N1) is being set. If you are using RS-485 the configuration of another pin to control transmission/reception is required. This is done as follows:

mb.config (& Serial, 38400, SERIAL_8N1, 2);

In this case, the pin 2 will be used to control TX/RX.

mb.addCoil (LAMP1_COIL);

Adds the register type Coil (digital output) that will be responsible for activating the LED or lamp and verify their status. The library allows you to set an initial value for the register:

mb.addCoil (LAMP1_COIL, true);

In this case the register is added and set to true. If you use the first form the default value is false.

mb.task ();

This method makes all magic, answering requests and changing the registers if necessary, it should be called only once, early in the loop.

digitalWrite (ledPin, mb.Coil (LAMP1_COIL));

Finally the value of LAMP1_COIL register is used to drive the lamp or LED.

In much the same way, the other examples show the use of other methods available in the library:

void addCoil (offset word, bool value)
void addHreg (offset word, word value)
void addIsts (offset word, bool value)
void addIreg (offset word, word value)

Adds registers and configures initial value if specified.

bool Coil (offset word, bool value)
bool Hreg (offset word, word value)
bool Ists (offset word, bool value)
bool IREG (offset word, word value)

Sets a value to the register.

bool Coil (offset word)
Hreg word (word offset)
bool Ists (offset word)
IREG word (word offset)

Arduino Modbus RTU
Author: Juan Pablo Zometa, Samuel and Marco Andras Tucsni
Year: 2010
Website: https://sites.google.com/site/jpmzometa/

Simple Modbus
Author: Bester.J
Year: 2013 Website: https://code.google.com/p/simple-modbus/

Arduino-Modbus slave
Jason Vreeland [CodeRage]
Year: 2010
Website: http://code.google.com/p/arduino-modbus-slave/

Mudbus (Modbus TCP)
Author: Dee Wykoff
Year: 2011
Website: http://code.google.com/p/mudbus/

ModbusMaster Library for Arduino
Author: Doc Walker
Year: 2012
Website: https://github.com/4-20ma/ModbusMaster
Website: http://playground.arduino.cc/Code/ModbusMaster

http://github.com/andresarmento/modbus-arduino
prof (at) andresarmento (dot) com

The code in this repo is licensed under the BSD New License. See LICENSE.txt for more info.