The holy grail of the BEEP(1400, 0.08) command: USB-HID

[gehakte_bits]

The UART in the FT260 is very well done (see its specs). But you cannot use feature reports on the Prime to configure the UART, since the Prime only supports get/set reports. You are now limited to the default UART settings of N81, 19200 baud (not supported by the hp48 I think). These limitations make the I2C interface a no brainer to use.

The FT260 UART TX/RX levels are 3.3V. If you need true RS232 levels you will need a converter (i.e. MAX232). It can be powered from the Prime. You might see degraded performance once the Prime is below 25% battery level.

My latest rendition has most wires covered, with access to the I2C, UARTs and USB port on the outside of the breadboard.