yaacov / arduinomodbusslavetcp

Modbus slave library for ESP8266-Arduino

ModbusSlaveTCP

ModbusSlave library for ESP8266-Arduino

This modbus slave library uses callbacks to handle modbus requests. Handler functions are called on modbus request, and the users can implement them in their sketch.

ModbusSlave is fun and easy to use

Register a handler function:

slave.cbVector[CB_READ_REGISTERS] = ReadAnalogIn;

Implement it:

void ReadAnalogIn(uint8_t fc, uint16_t address, uint16_t length) {
    for (int i = 0; i < length; i++)
        slave.writeRegisterToBuffer(i, analogRead(address + i));
}

And thats it, your sketch is modbus enabled. (see the full examples for more detail)

---

• Install
• Competabilty
• Callback vector - Slots - Handler function - Function codes - Reading and writing to the request buffer
• Examples - handle "Force Single Coil" as arduino digitalWrite - handle "Read Input Registers" as arduino analogRead

---

Install

Download the zip package, and install it into your Arduino IDE.

See the Arduino tutorial about installing 3rd party libraries: https://www.arduino.cc/en/Guide/Libraries#toc4

Competabilty

This class implements:

• FC1 "Read Coil Status"
• FC2 "Read Input Status"
• FC3 "Read Holding Registers"
• FC4 "Read Input Registers"
• FC5 "Force Single Coil"
• FC16 "Preset Multiple Registers"

Callback vector

Users register handler functions into the callback vector.

Slots

The callback vector has 4 slots for request handlers:

• slave.cbVector[CB_READ_COILS] - called on FC1 and FC2
• slave.cbVector[CB_WRITE_COIL] - called on FC5
• slave.cbVector[CB_READ_REGISTERS] - called on FC3 and FC4
• slave.cbVector[CB_WRITE_MULTIPLE_REGISTERS] - called on FC16

Handler function

handler functions must return void and take:

• uint8_t fc - request function code
• uint16_t address - first register / first coil address
• uint16_t length / status - length of data / coil status

Function codes

• FC_READ_COILS = 1
• FC_READ_DISCRETE_INPUT = 2
• FC_READ_REGISTERS = 3
• FC_READ_INPUT_REGISTERS = 4
• FC_WRITE_COIL = 5
• FC_WRITE_MULTIPLE_REGISTERS = 16

---

Reading and writing to the request / response buffer

• uint16_t readRegisterFromBuffer(int offset) : read one register value from the request buffer.
• void writeCoilToBuffer(int offset, uint16_t state) : write one coil state into the answer buffer.
• void writeRegisterToBuffer(int offset, uint16_t value) : write one register value into the answer buffer.

---

Examples

---

handle "Force Single Coil" as arduino digitalWrite

#include <ESP8266WiFi.h>
#include <ModbusSlaveTCP.h>

const char* ssid = "....";
const char* pass = "....";

Modbus slave(1); // slave id = 1

void setup() {
    // connect to WiFi
    WiFi.begin(ssid, pass);
    while (WiFi.status() != WL_CONNECTED) delay(500);
    
    // register one handler functions
    // if a callback handler is not assigned to a modbus command 
    // the default handler is called. 
    // default handlers return a valid but empty replay.
    slave.cbVector[CB_WRITE_COIL] = writeDigitlOut;
    
    // start slave at port 502
    slave.begin(); // start listen on port 502
}

void loop() {
    // listen for modbus commands con serial port
    slave.poll();
}

// Handel Force Single Coil (FC=05)
void writeDigitlOut(uint8_t fc, uint16_t address, uint16_t status) {
    if (status == HIGH) {
        digitalWrite(address, HIGH);
    } else {
        digitalWrite(address, LOW);
    }
}

---

handle "Read Input Registers" as arduino analogRead

#include <ESP8266WiFi.h>
#include <ModbusSlaveTCP.h>

const char* ssid = "....";
const char* pass = "....";

Modbus slave(1); // slave id = 1

void setup() {
    // connect to WiFi
    WiFi.begin(ssid, pass);
    while (WiFi.status() != WL_CONNECTED) delay(500);
    
    // register handler functions
    slave.cbVector[CB_READ_REGISTERS] = ReadAnalogIn;
    
    // start slave at port 502
    slave.begin(); // start listen on port 502
}

void loop() {
    // listen for modbus commands con serial port
    slave.poll();
}

// Handel Read Input Registers (FC=04)
void ReadAnalogIn(uint8_t fc, uint16_t address, uint16_t length) {
    // we only answer to function code 4
    if (fc != FC_READ_INPUT_REGISTERS) return;
    
    // write registers into the answer buffer
    for (int i = 0; i < length; i++) {
      slave.writeRegisterToBuffer(i, analogRead(address + i));
    }
}

arduinomodbusslavetcp's Issues

Simple Request for new library

Hi, I'm Vincenzo and I'm new to arduino word.
In my first job with Arduino I used your Modbus Slave RTU library and it is truly exceptional, simple and effective; compliments.
I have to implement the ModBus TCP slave communication and I found your library for ESP32, but unfortunately I can't adapt it for arduino with ethernet (actually I use Teensy 4.1 with "NativeEthernet.h" but it is similar to arduino "Ethernet.h") . Could you help me?
I tried to search on the internet for other libraries (for example "ArduinoModbus.h") but in my opinion they are too complicated, especially as regards the data management of the Modbus server. You in your libraries ask that there are dedicated functions for each modbus function and so it is simple and intuitive to analyze / update the data.
Thanks in advance

PS: sorry for my elementary english

program esp8266 or arduino sheild?

Hello
can i use this library for esp8266 stand alone or i should use it with arduino board?

bug on writeCoilToBuffer

i think i found a bug in writeCoilToBuffer:

void ModbusTCP::writeCoilToBuffer(int offset, uint16_t state) {
int address = MLEN + 3 + offset / 8;
int bit = offset % 8;

if (state == HIGH) {
    bitSet(bufOut[address], bit);
} else if (state) {                                 <-- this is wrong ! and must be (!state)
    bitClear(bufOut[address], bit);
}

}
are you ok ?

Incorrect sanity check?

Hi. I was reading your library to see how it works and I've foud what I think it's an error, tell me if I'm right.

You declare buffOut as an array of uint8_t and size MAX_BUFFER which is 64 (bytes).
When a client requests a function 3 read, you have to fill the buffOut array with the user data (width writeRegisterToBuffer). What I don't get, in the modbus protocol, the "length" is specified in registers (16 bits = uint16_t) but when you performe the "sanity" check you compare it againts MAX_BUFFER which is specified in bytes. So, if a modbus requests asks for address 0 length 64. You will need a buffer of 128 bytes (+ modbus header) to return the answer. If you compare against MAX_BUFFER it will return that it's OK to proceed but it is not!

Thanks