# nanoMODBUS - A compact MODBUS RTU/TCP C library for microcontrollers

nanoMODBUS is a small C library that implements the Modbus protocol. It is especially useful in resource-constrained
system like microcontrollers.  
Its main features are:

- Compact size
  - Only ~1000 lines of code
  - Client and server code can be disabled, if not needed
- No dynamic memory allocations
- Transports:
    - RTU
    - TCP
- Roles:
    - Client
    - Server
- Function codes:
    - 01 (0x01) Read Coils
    - 02 (0x02) Read Discrete Inputs
    - 03 (0x03) Read Holding Registers
    - 04 (0x04) Read Input Registers
    - 05 (0x05) Write Single Coil
    - 06 (0x06) Write Single Register
    - 15 (0x0F) Write Multiple Coils
    - 16 (0x10) Write Multiple registers
- Platform-agnostic
    - Requires only C99 and its standard library
    - Data transport read/write function are implemented by the user
- Broadcast requests and responses

## At a glance

```C
#include "nanomodbus.h"
#include "my_platform_stuff.h"
#include <stdio.h>

int main(int argc, char* argv[]) {
    // Set up the TCP connection
    void* conn = my_connect_tcp(argv[1], argv[2]);
    if (!conn) {
        fprintf(stderr, "Error connecting to server\n");
        return 1;
    }

    // my_transport_read_byte, my_transport_write_byte and my_sleep are implemented by the user 
    nmbs_platform_conf platform_conf;
    platform_conf.transport = NMBS_TRANSPORT_TCP;
    platform_conf.read_byte = my_transport_read_byte;
    platform_conf.write_byte = my_transport_write_byte;
    platform_conf.sleep = my_sleep_linux;
    platform_conf.arg = conn;    // Passing our TCP connection handle to the read/write functions

    // Create the modbus client
    nmbs_t nmbs;
    nmbs_error err = nmbs_client_create(&nmbs, &platform_conf);
    if (err != NMBS_ERROR_NONE) {
        fprintf(stderr, "Error creating modbus client\n");
        return 1;
    }

    // Set only the response timeout. Byte timeout will be handled by the TCP connection
    nmbs_set_read_timeout(&nmbs, 1000);

    // Write 2 holding registers at address 26
    uint16_t w_regs[2] = {123, 124};
    err = nmbs_write_multiple_registers(&nmbs, 26, 2, w_regs);
    if (err != NMBS_ERROR_NONE) {
        fprintf(stderr, "Error writing register at address 26 - %s", nmbs_strerror(err));
        return 1;
    }

    // Read 2 holding registers from address 26
    uint16_t r_regs[2];
    err = nmbs_read_holding_registers(&nmbs, 26, 2, r_regs);
    if (err != NMBS_ERROR_NONE) {
        fprintf(stderr, "Error reading 2 holding registers at address 26 - %s\n", nmbs_strerror(err));
        return 1;
    }
    
    // Close the TCP connection
    my_disconnect(conn);
    
    return 0;
}
```

## Installation

Just copy `nanomodbus.c` and `nanomodbus.h` inside your application codebase.

## API reference

API reference is available in the repository's [GitHub Pages](https://debevv.github.io/nanoMODBUS/nanomodbus_8h.html).

## Platform functions

nanoMODBUS requires the implementation of 3 platform-specific functions, which are passed as function pointers when creating a
client/server instance.

### Transport read/write

```C
int read_byte(uint8_t* b, int32_t, void* arg);
int write_byte(uint8_t b, int32_t, void* arg);
```

These are your platform-specific functions that read/write data to/from a serial port or a TCP connection.  
Both functions should block until the requested byte is read/written.  
If your implementation uses a read/write timeout, and the timeout expires, the function should return 0.  
Their return values should be:

- `1` in case of success
- `0` if no data is available immediately or after an internal timeout expiration
- `-1` in case of error

### Sleep

```C
void sleep(uint32_t milliseconds, void* arg);
```

This function should sleep for the specified amount of milliseconds.

### Platform functions argument

Platform functions can access arbitrary user data through their `void* arg` argument. The argument is useful, for
example, to pass to read/write function the connection they should operate on.  
Its initial value can be set inside the `nmbs_platform_conf` struct when creating the client/server instance,
and changed at any time via the `nmbs_set_platform_arg` API method.

## Platform endianness

nanoMODBUS will attempt to detect the endianness of the platform at build time. If the automatic detection fails, you can
manually set the endianness of the platform by defining either `NMBS_BIG_ENDIAN` or `NMBS_LITTLE_ENDIAN` in your build
flags.

## Tests and examples

Tests and examples can be built and run on Linux with CMake:

```sh
mkdir build && cd build
cmake ..
make
```

## Misc

- To reduce code size, you can define the following `#define`s:
  - `NMBS_CLIENT_DISABLED` to disable all client code
  - `NMBS_SERVER_DISABLED` to disable all server code
  - `NMBS_STRERROR_DISABLED` to disable the code that converts `nmbs_error`s to strings
- Debug prints about received and sent messages can be enabled by defining `NMBS_DEBUG`