Could somebody explain why exactly was the uart_wait_tx_done() call replaced here with the uart_ll_is_tx_idle() loop?

The ModbusMaster library is now broken in ESP32S2 v2.0.3, and it was working correctly in v2.0.2 (checked, I switch between the two versions at will). Our project, like many others, uses RS485 and flips the data-enable and read-enable pins, and uses flush() to wait until TX done before switching to RX mode. I still have to do more tests, but this change in particular seems the most likely culprit.

@avillacis

Have you read the issues related to this PR? They are related to MODBUS and flush() issues.

In summary:
uart_wait_tx_done() returns as soon as data is stored in TXRingBuffer, before the TX FIFO is really empty.

uart_ll_is_tx_idle() returns true just when TX FIFO is completely empty.
The code for it is here - but it is the same for all other SoC, thus, the issue you see should happen for any ESP32-xx
https://github.com/espressif/esp-idf/blob/master/components/hal/esp32s2/include/hal/uart_ll.h#L714-L717

I tested flush() and I'm sure it works fine.
Maybe the issue you see isn't related to this PR.

Objection withdrawn. With access to the actual hardware, I found out that our RS485 card was echoing back request commands, and this echo was previously hidden by the uart_set_mode() call. Now I have manually added the call in our own code, and the issue was fixed.

However, is there any upstream bug report documenting this issue with uart_wait_tx_done()? Is it a true software bug, or a documentation error?

Apparently uart_wait_tx_done() works correctly when used with uart_set_mode(uart_num, UART_MODE_RS485_HALF_DUPLEX), considering an IDF environment.

But for Arduino, it isn't useful because of the its generic UART usage and APIs.
Thus we had to change it for the LL API.

By default, IDF starts UART with uart_set_mode(uart_num, UART_MODE_UART) and in this case, it returns as soon as data is stored in TXRingBuffer, before the TX FIFO is really empty.

could anyone recommend any solution to be using with Arduino as ModbusMaster is not working any more?

could anyone recommend any solution to be using with Arduino as ModbusMaster is not working any more?

The solution I adopted in my project is as follows:

#include "driver/uart.h"
/* ... */
Serial1.begin(9600, SERIAL_8N1, GPIO_RX, GPIO_TX);
while (Serial1) {  /* Wait for serial init */ }
uart_set_mode(UART_NUM_1, UART_MODE_RS485_HALF_DUPLEX);

Where UART_NUM_{N} must be chosen to match Serial{N} as used in the project. The #include "driver/uart.h" is to have access to the function uart_set_mode().

After this, initialize ModbusMaster as before with the Serial{N}.

@avillacis Thank you. It works fine. I really appreciate your support