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This is Modbus RTU slave library written in C99. It is intended to be used on a bare-metal device. Hardware communication interface needs to be implemented by the user. The usual communication interface is UART/USART. Modbus itself needs to be configured by setting the number and type of wanted registers. Supported Modbus registers are:

• Holding Register
• Input Register
• Coils
• Input Bits

Usage

driver_init.h lists and describes interfaces that user needs to provide for establishing communication. This file needs to be included in user space. Three hardware interfaces are needed from MCU:

• timer - with counter(IRQ) on every 1ms
• UART/USART - receiving bytes one by one. It is recommended to use both interrupts - for Tx and Rx.
• Two signal diodes(optional) - used to visualize bytes received and bytes transmited.

modbus_configuration.h requires setup of Modbus interfaces. The following needs to be defined:

• Number of registers - separatly for holding registers, input registers, coils and input bits. Eg: #define HOLDING_REGISTER 1
• A unique name for each position in each register type. If you define 3 holding registers then you will define 3 different names in enumeration. Eg: HOLDING_REGISTER_1
• For each register type, define unique address in struct modbus_variables_map_t. Eg: modbus_variable_t holding_register_1;
• Map all defined registers in modbus_register_mapping_init(). Eg: modbus_variable_init(&modbus_variables_map.holding_register_1, &HoldingRegister[HOLDING_REGISTER_1]);

User space

These API functions need to be called before starting the manipulation of Modbus registers:

//init all HW interfaces
rs485_interface_init();

//Init Modbus, set slave address
modbus_init(SLAVE_ADDRESS);

//Init your own reigsters
modbus_register_mapping_init();

To set the value of any modbus reigster use:

modbus_variable_set_value()

To read the current value of any register check the register's array by navigating to wanted position:

• HoldingRegister[HOLDING_REGISTER]
• InputRegister[INPUT_REGISTER]
• Coils[COILS]
• InputBits[INPUT_BITS]

This function must to be called periodically in loop(as often as it can):

modbus_process()