Most boards have the serial port in a header. It is the same pinout that I/O cards in old PCs in the 1980s used for extra ports. Careful that they are at RS232

Most boards have the serial port in a header. It is the same pinout that I/O cards in old PCs in the 1980s used for extra ports. Careful that they are at RS232 levels, not TTL. It is a shame that it isn't directly exposed in the backplate. The control lines can be abused as GPIOs, but I would not recommend doing so. Many USB to TTL UART chips do also expose GPIO functionality, if you want somewhat direct access to GPIOs.