Hardware (BOM):

Specs

The GPIOs of the STM32 board are connected similar to the Unocart-2600 except for the SD card. And for Data IN and OUT the PlusCart uses the GPIOs PC0-PC7. The ESP8266 is connected to USART1 (PA9 TX and PA10 RX ) of the STM32 Board.

Atari 2600 STM32F407VGT6 ESP8266 Note
A0 to A12 PD0 to PD12 nc Adressbus
D0 to D7 PC0 to PC7 nc Databus
5v VCC nc 5 V
GND GND GND Ground
nc V33 VCC 3.3 V
nc PA9 RxD Tx -> RxD
nc PA10 TxD Rx -> TxD
nc V33 CH_PD CS