• sleeve connects to PIC's ground
• ring and tip each connect to an open pin on the PIC with a 100+Ω resistor in series.

Though this simple circuit has one caveat: when the PIC is unpowered, it defaults to a state that sets both red and white lines to low, which causes the calculator to seem like it's locked up (it's in a tight loop thinking we're trying to start communicating with it). The calculator won't even respond to the off button until the user manually removes the link cable.

Extra circuitry could take care of this by making sure the electrical connection defaults to broken when the PIC is unpowered. But there's a manual workaround (and behavioral workarounds: don't turn on the calculator unless the PIC is powered), so the extra components aren't necessary at least during prototyping.