The Russian Tupolev Tu-204 and variants are reported to be digital fly-by-wire. Are they quadruplex redundant? Do they have manual or analog back-up systems?
The Tu-204 does not have mechanical back-up, but it does have analog back-up. Its FBW is three-channel (same hardware, but different software).
(...) being the only Soviet airliner to have flown with a fly-by-wire control system, lacking any mechanical backup (...)
The Tu-204's digital fly-by-wire control system is probably the most advanced feature of any Soviet airliner. It works on all three axes—pitch, roll, and yaw—and is broadly similar in function to that found in the A320.
The same hardware, but dissimilar software, is used in the three channels of the digital fly-by-wire system, says Lanovski. A "quorum filter" monitors their outputs, and if two channels fail a three-channel analogue system automatically takes over, giving a catastrophic failure probability for the complete system of less than 10-9 per flight-hour, says the Soviet official. Pilots have felt no difference when the analogue system has been switched in during flight tests, says Lanovski.
Source: Tupolev's new twin, Flight International, 20 May 1989