Thanks all, I'm now totally baffled.....
I thought ISIS stood for Independent Standby Instrument System...
If it still relies on ADIRU inputs, it's NOT "independent"......
As an ancient, I come from the era of the SFENA standby horizon, which was a little 3" electromechanical gyro-driven horizon (most pilots here will be familiar with the "SFENA", or its American license-manufactured equivalent). No ARINC 429, nothing... just an electrically-powered self-contained high-inertia gyro, that ran off the DC emergency bus (via an inverter), and that would keep running (inertia) for several minutes, even after the DC went....
Without access to the AMM, can somebody point me to the relevant ISIS documentation (if it exists on the 'net, or has been referenced earlier on one of the AF447 threads?).
CJ