The ABCU sends the braking orders , combined or not with antiskid control orders, to the alternate brake DDVs. The ABCU inhibits such braking orders according to a logic based on alternate pressure transducer and park on signals.Such inhibition prevents the alternate brake DDVs from operating in low hydraulic load conditions.
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 ABCU
 Figure 016
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 Braking System Control - Schematic
 Figure 017

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 (3)
 Emergency Braking

 When Emergency braking is activated the ABCU inhibits the antiskid protection and maintains a predeterminated level of braking even when the brake pedals are fully released.

 (4)
 Monitoring of inputs

 The ABCU continuously monitors the specific inputs, coming from the alternate brake pedal transmitter unit and the park accumulator pressure transducer and sends the indications to the BSCU or SDACs in case of failure detection with outputs discretes.

 (5)
 Functional Test

 At each flight before landing , the BSCU activates a functional test of the braking system.In order to test the alternate braking capability, it sends functional test orders to the ABCU in predeterminated test sequences. When the ABCU receives these orders , it modifies chosen inputs with predeterminated stimuli, in order to place itself in various operational configurations. The BSCU monitors the proper performances of the functional test with specific pressure sensors located in the Alternate system.

 (6)
 Services

 The ABCU provides electrical power supply to the alternate brake DDVs and to the normal brake selector valve pressure transmitter (96GG). Electrical power supply of the DDVs is cut by the ABCU in case of load short circuit.

 (7)
 Unit architecture

 (Ref. Fig. 018)
 The ABCU is composed of two functional modules:
 -
the power supply selection module (PSSM)

 -
the main functional module (MFM)
 The unit is supplied with two different DC power supplies:

 -
DC Essential power supply (101PP).

 -
DC Battery power supply (701PP)

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 ABCU - Architecture
 Figure 018
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 7. Operation/Control and Indicating________________________________
 A. Control
 To provide anti skid protection, the BSCU needs to know that the Yellow Main Hydraulic Power supply (Ref. 29-13-00) is available. It reads the input from the pressure switch (3151GN) to get this data. If the BSCU cannot give or control the anti skid function, the caption ANTI SKID shows in amber on the system display (WHEEL page). The braking system then operates in the Alternate Braking without Anti Skid mode (Ref. 32-44-00). The regulators used for the Normal system are also used for anti skid regulation.
 A pressure transducer 3068GN (on the Alternate HP Brake Manifold Assembly) continuously measures the (Yellow) pressure available in the Alternate braking system. The top gage of the triple pressure indicator (Ref. 32-44-00) shows this pressure.
 When the ABCU gets an input from the brake pedal transmitter 103GG it:
 -energizes the alternate brake selector valve 99GG
 -sends the necessary control current to the direct drive valves (DDVs) in the system to get the correct pressure (set at the brake pedal/s) at the related brakes.
 The operation of the selector valve causes the Yellow hydraulic pressure to go to the Alternate Brake Manifold Assembly at each landing gear.
 At the same time that the selector valve is energized, the BSCU sends a control current through the ABCU to the applicable DDVs.The control current is set for the necessary pressure at the related brakes. The pressure output from each DDV goes through its related shuttle valve to isolate the supply from the parking brake system. The safety valves let the pressure go through to the second set of pistons in the brakes. The pistons extend and the brakes operate to decrease the speed of the wheels.
 A pressure transducer in the supply line to each pair of brakes measures the hydraulic pressure in the brake supply line. The bottom two gages of the triple pressure indicator show the pressure in the brake supply lines (Ref. 32-44-00).
　
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