RS-485 Modbus RTU link

The code

Embedded C for Cortex-M. It compiles. There are 3 real bugs in it: the kind that pass a bench test and fail in the field.

void rs485_send(const uint8_t *d, size_t n)
{
    gpio_write(DE_PIN, 1);
    for (size_t i = 0; i < n; i++) {
        while (!(USART1->ISR & USART_ISR_TXE)) {}
        USART1->TDR = d[i];
    }
    gpio_write(DE_PIN, 0);
}

static uint8_t rx[64];
static size_t  rx_len;

void on_byte(uint8_t b)
{
    rx[rx_len++] = b;
    if (rx_len >= 8) {
        modbus_handle(rx, rx_len);
        rx_len = 0;
    }
}

What an interviewer expects

• Point to each line you'd stop in a code review.
• Say what actually breaks: the failure mode, not just the rule it violates.
• Fix it, then defend the fix when asked the follow-up.

The bugs, reference fixes and the follow-up question are in the practice app, after you've tried it.

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