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HONEYWELL Aerospace Electronic Systems Page 12
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2.2.3 LRU Interfaces
In general, the Mark III CMU can support any LRU that uses the ARINC 429 File Data Transfer Protocol or
Williamsburg Protocol to communicate as an end system, as defined in ARINC 619 (ACARS file transfer
protocols for other devices). ARINC 758 identifies a number of specific LRUs and the Mark III CMU supports
interfaces to these LRUs:
Flight Management Computer (FMC)
The Mark III CMU can be configured to interface via low speed ARINC 429 with up to three ARINC 702
compliant FMCs operating as end systems. The Mark III CMU can alternatively be configured to interface
with up to three ARINC 702A FMCs over high speed ARINC 429. The Mark III CMU can also accept ARINC
429 broadcast data from the FMCs. This broadcast data provides the CMU with data that is captured in the
FIDB and available for use by the HGI and AMI.
Performance Computer (PZ)
In some aircraft installations, there is a separate performance computer (PZ) in addition to an FMC. The
performance computer communicates with a CDU through the CMU (See Figure 5) via RS-422. The Mark III
CMU can support a single interface to a performance computer with a software upgrade.
Figure 5 CMU/PZ Pass Through Interface
Maintenance Computer
The maintenance computer name varies depending upon the Original Equipment Manufacturer (OEM) but is
usually referred to as the Central Maintenance Computer (CMC), Centralized Fault Display System (CFDS),
or Central Fault Display Interface Unit (CFDIU). The Mark III CMU can be configured to interface with one
maintenance computer over low speed ARINC 429. This interface supports both end system
communications as well as the maintenance computer protocol.
The maintenance computer protocol varies based upon airframe. The Boeing 747 uses broadcast fault bits
while most other air transport aircraft (e.g., MD-10, MD-11, B-717, A-320, A-330, A-340) use an interactive
data block transfer utilizing MCDU screens. Other aircraft types do not currently support maintenance
computers (e.g., MD-80, MD-90, B-737, B-757, B767, regional and business jets). The baseline Mark III
CMU supports the broadcast fault bits. Interactive data block transfer protocol will be available as a planned
upgrade.
Aircraft Condition Monitoring System (ACMS)
The Mark III CMU can be configured to interface with a single Aircraft Condition Monitoring System (ACMS)
or Digital Fault Data Acquisition Unit (DFDAU) via low speed ARINC 429. This interface supports end
system communications using the standard ARINC 619 protocol in addition to providing the CMU access to
Performance
Computer
Mark III
CMU
CDU
RS-422 RS-422
NZ-2000