The DEU's send and receive data to and from other aircraft systems.
This is the data the DEUs send to the EEC:
Total air temperature
Corrected static pressure
Total pressure
Calculated mach number
Greenwich mean time
Clock date
N1 target
Flight number
Bite data
Landing gear position
Start valve position
Engine run status of other engine
Databus status
Start switch position
Start lever position
Ignition switch position
Control mode switch position Air/ground
Flap position
Engine bleed switch position
ACS pack switch position
Cowl thermal anti-ice switch position
Wing anti-ice switch position Isolation valve position.
The Hard Alternate mode is used when the throttles are almost at idle - if in that phase of flight then an immediate engine response to throttle inputs is deemed necessary for flight safety (Soft Alternate mode makes sure the engine thrust does not have large changes when PT data is not valid.).