void MODBUS_Master_Init(){
    RCC -> APB2ENR |= RCC_APB2ENR_IOPBEN | RCC_APB2ENR_AFIOEN;
    RCC -> APB1ENR |= RCC_APB1ENR_USART3EN | RCC_APB1ENR_TIM4EN;
    RCC -> AHBENR |= RCC_AHBENR_DMA1EN;

    stat.modbus_busy = 0;

    GPIOB -> CRH &= ~(GPIO_CRH_CNF10 | GPIO_CRH_MODE11);
    GPIOB -> CRH |= GPIO_CRH_CNF10_1 | GPIO_CRH_MODE10 | GPIO_CRH_CNF11_0;
    GPIOB -> CRL &= ~(GPIO_CRL_CNF2);
    GPIOB -> CRL |= GPIO_CRL_MODE2;
    DIR_LOW;

    TIM4 -> PSC = 7200 - 1;
    TIM4 -> ARR = 30000;    //300ms
    TIM4 -> DIER = TIM_DIER_UIE;
    TIM4 -> CR1 = TIM_CR1_OPM;

    DMA1_Channel3 -> CPAR = (uint32_t)&USART3 -> DR;
    DMA1_Channel3 -> CCR = DMA_CCR1_PL_0 | DMA_CCR1_MINC | DMA_CCR1_TCIE;

    USART3 -> BRR = 3750;        	    //9600
    USART3 -> CR1 = USART_CR1_TCIE | USART_CR1_TE | USART_CR1_RE;
    USART3 -> CR2 = USART_CR2_STOP_1;
    USART3 -> CR3 = USART_CR3_DMAR;
    USART3 -> CR1 |= USART_CR1_UE;
    NVIC_EnableIRQ(USART3_IRQn);
    NVIC_EnableIRQ(DMA1_Channel3_IRQn);
    NVIC_EnableIRQ(TIM4_IRQn);
}

void MODBUS_Master_Send(_modbus_request modbus){
    uint16_t crc;

    if (stat.modbus_busy) return;
    send_buf[0] = modbus.id;
    send_buf[1] = modbus.func;
    send_buf[2] = modbus.addr >> 8;
    send_buf[3] = modbus.addr;
    switch (modbus.func){
    case 3:
    case 4:
	send_buf[4] = modbus.length_data >> 8;
	send_buf[5] = modbus.length_data;
	answer_len = 5 + modbus.length_data * 2;
	answ_buf = modbus.answer;
        send_counter = 8;
	break;
    }
    crc = crc_chk(send_buf, send_counter - 2);
    send_buf[send_counter - 2] = crc;
    send_buf[send_counter - 1] = crc >> 8;
    pointer = send_buf;
    stat.modbus_busy = 1;
    DIR_HIGH;
    //Оправляем send_buf
    USART3 -> CR1 |= USART_CR1_TXEIE;
}

void USART3_IRQHandler(){
    if (USART3 -> SR & USART_SR_TXE){
	if (send_counter){
	    USART3 -> DR = *pointer++;
            send_counter--;
	} else {
	    USART3 -> CR1 &= ~(USART_CR1_TXEIE);
	}
    } 
    if (USART3 -> SR & USART_SR_TC){
	USART3 -> SR = ~(USART_SR_TC);
        //Переводим PHY в режим приемника 
	DIR_LOW;
        //Включаем таймер (таймаут)
	TIM4 -> CR1 |= TIM_CR1_CEN;
        //Настраиваем ДМА для приема пакета
	DMA1_Channel3 -> CCR &= ~(DMA_CCR1_EN);
	DMA1_Channel3 -> CMAR = (uint32_t)answ_buf;
	DMA1_Channel3 -> CNDTR = answer_len;
	DMA1_Channel3 -> CCR |= DMA_CCR1_EN;
    }
}

void DMA1_Channel3_IRQHandler(){
    uint16_t crc;

    DMA1 -> IFCR |= DMA_IFCR_CTCIF3;
    TIM4 -> CNT = 0;
    TIM4 -> CR1 &= ~(TIM_CR1_CEN);
    DMA1_Channel3 -> CCR &= ~(DMA_CCR1_EN);

    crc = crc_chk(answ_buf, answer_len - 2);
    if (crc == (answ_buf[answer_len - 2] | (answ_buf[answer_len - 1] << 8))){
	stat.modbus_error = 0;
    } else {
	stat.modbus_error = 1;
    }
    stat.modbus_busy = 0;
}

void TIM4_IRQHandler(){
    TIM4 -> SR = ~(TIM_SR_UIF);
    stat.modbus_busy = 0;
    stat.modbus_error = 1;
    DMA1_Channel3 -> CCR &= ~(DMA_CCR1_EN);
}

typedef struct {
    uint8_t id;
    uint8_t func;
    uint16_t addr;
    uint16_t length_data;
    uint8_t *write_data;
    uint8_t *answer;
} _modbus_request;

_modbus_request modbus
uint8_t modbus_answer[100];

void main(void) {
    MODBUS_Master_Init();

    SEGGER_RTT_Init();

    modbus.id = 1;
    modbus.func = 3;
    modbus.addr = 0;
    modbus.length_data = 2; 
    modbus.answer = modbus_answer;

    while(1){
	MODBUS_Master_Send(modbus);
	modbus_stat = MODBUS_Master_GetStatus();
	if (modbus_stat != 0xFF){
	    if (modbus_stat == 0){
                 LED1_ON;
	    } else {
                 LED1_OFF;
                 SEGGER_RTT_WriteString(0, "RECEIVE ERROR\r\n");
	    }
	}
    }
}