The best lib for ModBus over RS485: libmodbus (Link)
Installation:
Search already-done package for raspbian
sudo apt-cache search libmodbus
Install your favorite version. Dev if you want .h file:
sudo apt-get install libmodbus-dev
or standard for end-user
sudo apt-get install libmodbus5
Code Example
//ModBusRequest Example #include <modbus.h> modbus_t *mb; uint16_t tab_reg[32]; mb = modbus_new_tcp("127.0.0.1", 1502); modbus_connect(mb); /* Read 5 registers from the address 0 */ modbus_read_registers(mb, 0, 5, tab_reg); modbus_close(mb); modbus_free(mb);