Honeywell SPZ-8000 Maintenance Manual page 320

10. A. An aircraft engine pressure ratio (EPR) system contains two solid-state,
microprocessor-controlled EPRT, one on the captain's side of the
aircraft and one on the first officer's side. Each EPRT receives fan
duct pressure (FDP) from its associated (on-side) aircraft engine and
two Aeronautical Radio Incorporated (ARINC) 429 data signals. Both
ARINC 429 data signals provide data for total pressure (P~) and
calibrated airspeed (CAS). CAS data is used for built-in test equipment
(BITE)
only and does not affect calculation of EPR. One ARINC 429 data
signal is input from the aircraft on-side air data computer (ADC), and
one is input from the aircraft off-side ADC. During normal operation,
data from the on-side
ADC
is
used. If the data from the on-side
ADC is
not valid, data from the off-side
ADC is used.
B.
The two EPRT calculate the ratio of FDP to PT. Discrete trimplug
signals (TRIMPLUG O through TRIMPLUG 5) adjust this ratio. Two EPRT
transmit the adjusted ratio on the ARINC 429
bus.
c.
The EPRT has three
modes of operation:
Initialization mode.
Normal operation mode.
c Memory access mode.
D.
The EPRT enters the initialization
mode after power-up, or after a power
transient of longer than 50 ms, or after watchdog timer (WDT) time out.
The initialization mode lasts 750 ms maximum. During the initialization
mode, the EPRT
checks internal performance BITE, reads the discrete
input signals (TRIMPLUG O through TRIt4PLUG 5,
SDI 1, and SDI 2),
and
initializes software.
When the initialization
mode is complete, the
EPRT enters normal operation
mode.
(1)
The discrete trimplug signals (TRIMPLUG O through TRIMPLUG 5)
provide aircraft
engine compensation values used during
EPR
calculation.
(2) The
discrete source/destination identifier
(SDI)
signals
(SDI 1
and
SDI 2) identify the location of the EPRT on the aircraft.
E. The EPRT enters the normal operation mode when the initialization mode
is complete. In
the normal operation
mode, the EPRT:
. Receives P~ and CAS data from the aircraft ADC.
. Computes PT.
. Measures FDP from its associated aircraft engine.
. Outputs the calculated FDP on ARINC 429 bus.
22-14-00
Page
198.155
Apr 15/93
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