chore: rewrite to modbus rtu

modbus.h

@@ -0,0 +1,26 @@
+#ifndef __MODBUS_H
+#define __MODBUS_H
+
+#include <stdint.h>
+
+// function codes
+#define MODBUS_FC_READ_HOLDING_REGISTERS 0x03
+#define MODBUS_FC_WRITE_SINGLE_REGISTER 0x06
+#define MODBUS_FC_WRITE_MULTIPLE_REGISTERS 0x10
+
+// errors
+#define MODBUS_ERROR_NONE 0x00
+#define MODBUS_ERROR_FUNCTION 0x01
+#define MODBUS_ERROR_ADDRESS 0x02
+#define MODBUS_ERROR_VALUE 0x03
+
+uint8_t modbus_process_message(uint8_t *buf, uint16_t len, uint8_t slave_addr,
+                               uint16_t *holding_registers,
+                               uint16_t num_registers);
+uint16_t modbus_create_response(uint8_t *response, uint8_t slave_addr,
+                                uint8_t function, uint16_t address,
+                                uint16_t value);
+uint16_t modbus_create_error_response(uint8_t *response, uint8_t slave_addr,
+                                      uint8_t function, uint8_t error_code);
+
+#endif  // __MODBUS_H