Quoting KC135TopBoom (Reply 3): I believe the VC-25As were both delivered with the Mil. Spec. 1553 data bus already installed. That allows for future digital avonics (and outher new electronic equipment) to be easier to install.
I bet these puppies receive regular upgrades...and such upgrades will not be publicized.
When they call the roll in the Senate, the Senators do not know whether to answer 'Present' or 'Not guilty.' -Theodore Roosevelt