USBOpen(), USBSend() and USBRecv()

[gehakte_bits]

1) The Interrupt-only reports are the only ones supported by the HP Prime:
0xB1(UART-RX interrupts), 0xC2(I2C dumps), 0xD0-0xDE(I2C R/W) 0xF0~0xFE(UART-RX/TX).

2) Examine your receiving example outputs: they only contain data following the HID Report Structure per 4.2 of User Gude FT260 (AN_394). All other setup packet stuff is hidden/taken care of by the HP Prime USB-HID interface already!

So, you only create UART Write Requests and receive UART Input Reports (and Interrupt Report on an input line).

See the original post with code example, you can create a UART sending routine similar to the I2C, without the iaddr,iflag, and proper UART start opcode. ft260_recv() to receive should work.

If you want to learn about USB-HID, plug in your FT260 in a Windows10/11 and SimpleHIDWRITE utility and Wireshark are now friends to hang out with.

FYI a typial setup packet (which will be provided by the Prime) looks like this:
bmRequestType: 0x21
bRequest: SET_REPORT (0x09)
wValue: 0x02c2
ReportID: 194
ReportType: Output (2)
wIndex: 0 (or 1 depending on Windows that can reports both)
wLength: 5