Modbus Esp8266AT Library for Arduino

Over TCP/IP implementation for ESP8266 AT controller

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 TCP).

In the current version the library allows the Arduino operate as a slave, supporting Modbus over TCP.

How to

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

• Modbus-Arduino - Base Library
• Modbus-Serial - Modbus Serial RTU Library
• Modbus-Ethernet - Modbus TCP Library (standard Ethernet Shield)
• Modbus-EtherCard - Modbus TCP Library (for ENC28J60 chip)
• Modbus-Esp8266AT - Modbus IP Library (for ESP8266 chip with AT firmware)

By opting for Modbus Serial or Modbus TCP you must include in your sketch the corresponding header and the base library header, eg:

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

Modbus-Esp8266AT

Modules based on ESP8266 are quite successful and cheap. With firmware that responds to AT commands (standard on many modules) you can use them as a simple wireless network interface to the Arduino.

The firmware used in the module (at_v0.20_on_SDKv0.9.3) is available at: http://www.electrodragon.com/w/ESP8266_AT-command_firmware

(Other AT firmwares compatible with ITEAD WeeESP8266 Library should work)

Warning: Firmware such as NodeMCU and MicroPython does not work because libraries used here depend on a firmware that responds to AT commands via serial interface. The firmware mentioned are used when you want to use ESP8266 modules without the Arduino.

You will need the WeeESP8266 library (ITEAD) for the Arduino. Download from:

https://github.com/itead/ITEADLIB_Arduino_WeeESP8266 and install in your IDE.

Notes:

Remember that the power of ESP8266 module is 3.3V.

For Modbus IP (ESP8266 AT) there is four examples that can be accessed from the Arduino interface. Let's look at the example Lamp.ino (only the parts concerning Modbus will be commented):

#include <ESP8266.h>
#include <Modbus.h>
#include <ModbusEsp8266AT.h>

ESP8266 wifi(Serial, 9600);

Create the wifi object (ESP8266) specifying the rate in bps.

Continuing with our example:

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.

ModbusIP mb;

Create the mb instance (ModbusSerial) to be used.

mb.config(wifi, "your_ssid", "your_password");

Configure ESP8266 module. The values quoted correspond to the network name (SSID) and security key. By default IP configuration is received via DHCP. See the end of the section how to have an Static IP (important so you do not need to change the master / supervisor if the IP changes).

Folowing, we have:

mb.addCoil (Lamp1Coil);

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 (Lamp1Coil, 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 (Lamp1Coil));

Finally the value of Lamp1Coil 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 setCoil (offset word, bool value)
bool setHreg (offset word, word value)
bool setIsts (offset word, bool value)
bool setIReg (offset word, word value)

Sets a value to the register.

bool coil (offset word)
word hreg  (word offset)
bool ists (offset word)
word ireg (word offset)

Returns the value of a register.

Using a static IP on the ESP8266 module

We are aware today of two options:

1. In your router configure the MAC address of the module so that the IP address provided by DHCP is always the same (Most routers have this feature).

2. In your code, include two lines to change the IP address after the module configuration:

   mb.config(wifi, "your_ssid", "your_password"); delay(1000); Serial.println("AT+CIPSTA="192.168.1.44"");

Note: For the module to receive IP via DHCP again you will need to remove the lines and run (at least once) the command: AT + CWDHCP = 1.1 via direct connection to the module, either:

Serial.println("AT+CWDHCP=1,1");

License

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