RS-485 half-duplex extra byte received.

[sramberg2]

IDF4.3
Sending 7 bytes out UART2 configured as RS485 Half-duplex every 80ms at 115200 baudrate.
While debugging this issue, https://www.esp32.com/viewtopic.php?f=13&t=21835
Occasionally I would see a data byte in the RX FIFO following a transmission. The byte was always 0x00.
I modified the TX DONE interrupt, in UART.c, to disable the RX FIFO flushing and I received the extra byte close to 100% of the time. It appears that the RX FIFO flush would remove this extra byte most of the time.

Code: Untitled.c Select all

        } else if(uart_intr_status & UART_INTR_TX_DONE) {

            if (UART_IS_MODE_SET(uart_num, UART_MODE_RS485_HALF_DUPLEX) && uart_hal_is_tx_idle(&(uart_context[uart_num].hal)) != true) {

                // The TX_DONE interrupt is triggered but transmit is active

                // then postpone interrupt processing for next interrupt

                uart_event.type = UART_EVENT_MAX;

            } else {

                // Workaround for RS485: If the RS485 half duplex mode is active

                // and transmitter is in idle state then reset received buffer and reset RTS pin

                // skip this behavior for other UART modes

                UART_ENTER_CRITICAL_ISR(&(uart_context[uart_num].spinlock));

                uart_hal_disable_intr_mask(&(uart_context[uart_num].hal), UART_INTR_TX_DONE);

                if (UART_IS_MODE_SET(uart_num, UART_MODE_RS485_HALF_DUPLEX)) {

//SRR 7/23/21                    uart_hal_rxfifo_rst(&(uart_context[uart_num].hal));

                    uart_hal_set_rts(&(uart_context[uart_num].hal), 1);

                }

                UART_EXIT_CRITICAL_ISR(&(uart_context[uart_num].spinlock));

                uart_hal_clr_intsts_mask(&(uart_context[uart_num].hal), UART_INTR_TX_DONE);

                xSemaphoreGiveFromISR(p_uart_obj[uart_num]->tx_done_sem, &HPTaskAwoken);

            }

        } else {

I do not see the extra byte on the output of the RS-485 driver.

Any idea on how to prevent this extra byte?

[WiFive]

Did you try moving it after uart_hal_set_rts

[HitecSmartHome]

We experience the same problem. We got `0x00` byte frames on every uart. In our case these are occasional and we observe them sometimes multiple times / minute but there are cases where it isn't there for several minutes. Increasing the RX symbol timeout seems to help but it slows down the communication.