THIS IS A DRAFT AND COMPLETELY UNTESTED.

The extension board is meant to connect to a 50 mil ribbon cable,
which in turn connects to a connector for a 7x2 100 mil header.

To reduce crosstalk, signals are always separated by ground. We
cannot use VDD for crosstalk suppression, because the device may
be self-powered and VDD would thus be disabled.

If the connector is reversed, VDD is shorted to GND.