Orion-I Hardware Revisions

Revision A (mid-2016): This is the first PCB created to obtain a reasonable
PLL with built-in frequency control. It made provisions for an RS-232
transciever (off-board) and switchable reference input (internal or external).

Revision B (March, 2017): This re-spin sought to include several refinements
over the initial design, including an on-board RS-232 transceiver,
an improved reference switch, an on-board 5V regulator, and a prototype area.

Revision C (April, 2017): This revision corrected the noise problems introduced by
the RS-232 transceiver's charge-pump circuits. Provision to disable the
digital circuits was also provided along with some minor layout tweaks.
Note: This version is NOT software compatible with the previous revisions.
The source code features #defines to identify the target hardware.
Be sure to set this properly if you modify the source code for a given Orion-I.