HP Calculator Serial Programming Reference
New Mini USB Pinout
The pinout is (from memory) +6V, TX, RX, GND
115200, 1 stop, no parity.
There is no more info than that available, the only things you can do really are: open the port, read data, send data, and get information on how much data is available in the buffer for you to read.
The following system RPL commands should be available (they are similar to the normal RPL ones):
PUTSERIAL (Xmit, but does not perform the openio and can generate an error)
GETSERIAL (SRECV, takes a # as an input, but does not perform the openio and can generate an error)
DOSRECV (SRECV, takes a real as input)
UARTBUFLEN (BUFLEN, returns ``#s`)