E.  
The PSEU has ten circuit cards. They are primary and alternate proximity circuit cards, primary and alternate logic driver cards, BITE monitor microprocessor cards, and four thrust reverser/auto restow cards. There is one thrust reverser/auto restow card for each engine.

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 4. Operation
_________
 A. Functional Description - Air/Ground Relays
 (1)  
In ground mode, the PSEU supplies a ground signal from its ground mode output to three master relays. These relays are R778 and R7073 of the primary relay system and R988 of the alternate relay system. The ground signal makes the master relays energize. This allows only the control relays connected to the energized master relays to operate. The control relays supply a path to operate airplane systems.

 (a)  
For the primary relay system, both master relays (R778 and R7073) energize control relays.

 (b)  
For the alternate relays system, master relay R988 energizes control relays. It also supplies a connection to the lower cargo compartment fire extinquishing system.

 (2)  
In the air mode, the PSEU supplies a ground signal from its air mode output to three master relays. This allows R910, R952 of the primary relay system and R1128 of the alternate system to energize.

 (a)  
For the primary relay system, master relay R910 energizes one control relay. It supplies a connection to the Bus Control Unit (BCU) 2 and the left upper deck door flight lock actuator. Master relay R952 operates control relays only.

 (b)  
For the alternate relay system, master relay R1128 allows several control relays to energize. It also supplies a connection to the right upper deck door flight lock actuator.

 B.
Functional Description - PSEU (Fig 2)

 (1)  
The PSEU collects input signals from proximity sensors and discrete mechanical input switches. Then the PSEU does a logic driver operation and sends out digital and discrete information.

 (a)  
Digital outputs go to three EFIS/EICAS Interface Units (EIU). The EIU's move the signals to other systems to use.

 (b)  
Discrete outputs supply signals to relays for indication.

 (2)  
The PSEU gets digital inputs from the CMC. These inputs supply test commands and general data for use by the PSEU.

 (3)  
The PSEU collects proximity sensor signals from these subsystems:

 (a)  
Primary landing gear

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 (b)
Alternate landing gear

 (c)
Thrust reversers

 (d)
Auto restow

 C.  
The proximity switch sensors are a variable reluctance core-coil combination. They monitor mechanical target positions. The sensor signals cause the PSEU to be a switch with no contact necessary. The proximity circuit card supplies a periodic pulse to each sensor. The switch channel on the card uses the periodic pulse to measure sensor inductance. A large inductance occurs when a target is near and a small inductance occurs when the target is far (Ref 32-61-00/001).

 (1)
Subsystems and sensors:

 (a)
The primary landing gear subsystem has these sensors:

 1)  The primary uplock sensors:
 -S398 LH Wing Gear
 -S400 LH Body Gear
 -S402 RH Body Gear
　
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