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Eviation Wiring Guidance
This document provides guidance for the installation of the Eviation Flight Control Computer
(FCC) modules.
1.1 Installation
The FCC_Module_O&I_Drawing (part number TBD) provides the dimensions of the FCC modules
includes the mounting surface and keying of the backshell connectors.
1.2 Shielding, Bonding and Grounding
Proper electrical shielding, bonding, and chassis grounding are key factors in ensuring proper
system operation under normal conditions and under HIRF and lightning environments. This
section and technical specifications referenced here-in must be complied with to satisfy these
requirements.
1.2.1 Bonding and Grounding
All units shall be electrically bonded to the airframe. This is accomplished by ensuring that the
mating surfaces between the LRU mounting feet provides a low impedance ( < 2.5 milliohm)
electrical path.
The mating surfaces must be free of all paint and other non-conductive elements and should be
burnished to ensure a good bond. If the aircraft mating surface is not conductive, a bonding
strap of at least 1/4 inch wide (preferably 1/2 inch wide) tin coated copper braid can be used
between the LRU and the nearest airframe grounding point.
The FCC mounting feet must be electrically bonded to a conductive mounting surface by a low
resistance path of with a maximum resistance of 2.5 milliohms. The 2.5 milliohm value applies to
(1) the initial FCC installation, and (2) anytime the FCC is loosened or removed after the initial
2.5 milliohm buyoff test. Due to the gradual degradation of a new electrical bond over time, the
maximum in-service resistance value can be up to 5 milliohms. In-service readings higher than
this will require a re-bonding with an ‘as new’ bonding limit of 2.5 milliohms.
NOTE: The total resistance from the FCC mounting feet to the central aircraft ground reference
structure must be less than 10 milliohms. Typically this would include the following bonding
points:

FCC mounting feet to aircraft mounting shelf - 2.5 milliohms max new

Conductive mounting surface to aircraft central ground reference - 2.5 milliohms max new
1.2.2 Shield Grounds
Full circumference shielding (also referred to as 360 degree shielding) is recommended for
critical systems or systems that have high susceptibility to EMI.
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1.2.2.1 Multi Point Shield Grounding
The majority of the shielded wires in the FCCs have the shields grounded at both ends. This is
called multi point shield grounding and is specified to minimize the adverse effects of HIRF and
lightning.
Examples of multi point shield grounding methods without bulkhead connectors are shown in
Figure 1.2.2.1-1 through Figure 1.2.2.1-2. Examples of multi point shield grounding methods
with bulkhead connectors are shown in Figure 1.2.2.2-1 through Figure 1.2.2.2-2.
ARINC
429
OUTPUT
J1
H 1
J1
8 H
L 2
9 L
ARINC
429
INPUT
Figure 1.2.2.1-1. Multi-Point Shield Grounding W/O Bulkhead Connector (Example)
ARINC
429
OUTPUT
ARINC
429
INPUT
J1
H 1
J1
5 H
L 2
6 L
ARINC
429
INPUT
J1
H 5
L 6
Figure 1.2.2.1-2. Multi-Point Shield Grounding W/O Bulkhead Connector (Example)
Note: The arrows on the lines mean twisted wires.
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1.2.2.2 Bulkhead Connector Shield Handling
Shielded cables that require multi point shield grounding require that the shields be grounded at
both sides of the bulkhead connectors. Examples of bulkhead connector shield handling with
multi point shield grounding methods are shown in Figure 1.2.2.2-1 and Figure 1.2.2.2-2.
ARINC
429
OUTPUT
J1
H 1
A
A
J1
8 H
L 2
B
B
9 L
ARINC
429
OUTPUT
BULKHEAD
CONNECTOR
Figure 1.2.2.2-1. Multi-Point Shield Grounding with Bulkhead Connector (Example)
ARINC
429
OUTPUT
J1
H 1
A
A
J1
5 H
L 2
B
B
6 L
ARINC
429
INPUT
BULKHEAD
CONNECTOR
ARINC
429
INPUT
J1
H 5
L 6
Figure 1.2.2.2-2. Multi-Point Shield Grounding with Bulkhead Connector (Example)
1.2.2.3 Procedure to attach a pigtail wire to a shielded cable assembly
This procedure describes the technique to attach a pigtail wire to a TSP (twisted shielded pair)
or TST (twisted shielded triple) cable using a solder sleeve. All the steps in this procedure are
referenced in Figure 1.2.2.3-1.
Step 1: Cut the shielded cable outer jacket and shield to provide .25 inches of overall
shield braid and 1.5 inches of the insulated inner conductors.
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Step 2: Cut an approx. 5 inch length of black 22 AWG MIL-W-22759 thin wall (/18,
/32, /33, /44, /45, /46, or equivalent) wire for the pigtail lead. Strip .25 inch of
insulation off one end of the pigtail lead. Position the stripped end of the
pigtail wire adjacent to the exposed cable shield. The pigtail lead must exit
away from the exposed inner cable conductors. Insert a medium size solder
sleeve (Alpha P/N FITSLV 24-100, Newark P/N 66F2461) over the pigtail wire
and shielded cable. Apply heat to shrink the solder sleeve while holding the
pigtail wire in position touching the exposed cable shield. This completes the
assembly procedure.
Figure 1.2.2.3-1 Attach Pigtail Wire to Shield Cable
1.2.2.4 Shield Grounding Recommendation
The recommendation for shield termination is as follows:

Terminate at both ends of the connection

Nearly zero length pigtail as practical

360 degree EMI backshell
1.2.2.5 Shield Grounding Method for the FCC Backshell Mounted Connectors
The shield drain wires are terminated using stainless steel crimp bands.
1.2.2.6 Strain relief
The strain relief can be provided at the connector or close to it, in an appropriate manner, depending on
the installation of the unit in the aircraft.
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1.2.3
Shielding Over-Braid Installation and Grounding
FCC wiring that runs outside of the pressure vessel must be installed within a continuous shielding overbraid material or other enclosed structure that provides similar HIRF, lightning and emissions attenuation
as provided by the pressure vessel enclosure. Typically over-braid is constructed as a wire mesh tube
with 360 degree grounding connectivity to the pressure vessel enclosure and/or other enclosed
structures through which the wiring is routed. 360 degree grounding is required at all over-braid
interfaces. Refer to Figure 1.2.3-1.
Exposed Area
Over-Braid
Cable Bundle
Protective
Enclosure
(or Equipment)
360O Low Impedance Connectivity
between enclosures and over-braid
(example: pressure clamp)
Pressure
Vessel
Enclosure
Figure 1.2.3-1 Over-Braid Connectivity Example
1.2.4
Power/Signal Grounds
1.2.4.1 Multi-point Grounding
Multi-point grounding of the power and signal grounds can be used only if the aircraft has a ground plane
that provides a low impedance path between all the electrical systems and back to the main aircraft
power ground. A good aircraft ground plane requires that special attention be paid to the bonding of all
the aircraft subassemblies that are part of the ground plane. Good bonding will ensure a low impedance
electrical path within the ground plane. Composite structures within a ground plane must be designed to
handle the required ground plane currents, and offer the equivalent low impedance of present day
aluminum structures.
Figure 1.2.4.1-1 shows the multi-point ground plane wiring method.
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Ground Wires :
 DC Grounds
 AC Grounds
 Signal Grounds
Ground
Block
Unit 1
Unit 2
Unit N
Aircraft Ground Plane
Note: Shield grounding requirements are detailed in Section 1.2.2.4
Figure 1.2.4.1-1 Multi-point Ground Plane Wiring
If the aircraft cannot satisfy the multi-point ground plane requirements, central point grounding must be
used. Central point grounding is detailed in Section 1.2.4.2
1.2.4.2 Central Point Grounding
All DC power grounds shall be tied together, all signal grounds shall be tied together, and all lighting
grounds shall be tied together.
DC power, signal, and lighting ground groups are then tied together at a single point and connected to
the airframe. The aircraft grounding diagram below Figure 1.2.4.2-1 illustrates this grounding method.
It is very important that this grounding technique be adhered to. Do not tie the various ground wires to
multiple aircraft frame points and depend on the aircraft structure itself to provide a low impedance path
for the individual grounds. ONLY chassis grounds and shield grounds are grounded at multiple points in
the aircraft.
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Single Aircraft
Grounding Point
(Note 1)
Central
Aircraft
Grounding
Blocks
DC Power
Grounds
Signal
Grounds
Lighting
Grounds
To Various Aircraft Locations
Figure 1.2.4.2-1 Central Aircraft Grounding Blocks
For signal grounds that are low current, multiple signal grounds can be connected to remote
aircraft terminal blocks other than the central grounding blocks as long as these remote terminal
blocks are isolated from ground. The various remote signal ground blocks must all be grounded
only at the aircraft central grounding point. For example, if ten signal grounds are connected to
a remote terminal block, a minimum of one grounding wire must be run from this terminal block
to the aircraft central grounding point.
1.3 Interconnect Requirements
This section provides interconnect guidance/requirements for each interface type. Section 1.3.6
will provide details of the connector types, pin and wire sizes and shielding to be used for each
interface type.
1.3.1 IMB
The Intermodule Data Bus (IMB) are dedicated unidirectional data links among the FCCs. The protocol is
organized as small, fixed length (design time) packets of parameters passed in a datagram (send and
forget) manner. As unidirectional links, a single source module deterministically sends packets at
deterministic points in its operational schedule.
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The aircraft wiring must utilize 100 Ω controlled impedance twisted-shielded pair cable. Cable breaks and
termination points must maintain balanced lead lengths and 360 degree shielding. Individual IMB links
shall be limited to less than 150 feet in length with no more than four production breaks.
Topology Rules and Link Budget Assumptions:

All IMB links have a single transmitter and at most two receivers.
High-quality controlled impedance cable must be used with electrical characteristics as
defined in Table 1.3.1-1.

Connectors deployed typically use pins with approximately 0.254 cm (0.1 inch) spacing. Each
interface is likely a slight deviation (~75 ohms) from the ideal impedance. A conservative
approach is to assume about 0.2 dB attenuation per disconnect.

The transmit source impedance and end terminus receiver impedance are matched to the 100
ohm cable.

It is not recommended to mix cable of different characteristic impedances in any installation.
Table 1.3.1-1 Recommended P2P (IMB) Cable Characteristics
Cable Parameter
Characteristic
Impedance
Nominal Value
+/- 10%
100 Ohm Z 0
Wire Gauge
22 AWG or 24 AWG
Shunt Capacitance
52 pF/m (16 pF/foot)
~6 dB/100m @ 5 Mhz
~2.8 dB/100m @ 1 Mhz
~30 dB/100m @ 100
Mhz
69%
Attenuation
Velocity of Propagation
Tolerance
Consistent through
installation
Approximate, less than
All approximations
minimum
1.3.2 Analog Discrete
Analog discrete signals should be sized as defined in the interconnect tables found in the EICD.
1.3.2.1 Data Load Discrete
There are two discrete inputs used to put the FCC into data load mode. Both inputs need to be
grounded in order to enter data load mode. The data load discrete interfaces should follow the
same guidelines as other analog discrete interfaces. The switch states are shown in Figure
1.3.2.1-1
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FCC 2
FCC 1
DI X
DI Y
GND
Covered Switch
Shown in Data Load Enabled State
Switches NOT located on flight deck
DI X
DI Y
FCC 3
GND
Covered Switch
Shown in Data Load Enabled State
Switches NOT located on flight deck
DI X
DI Y
GND
Covered Switch
Shown in Data Load Enabled State
Switches NOT located on flight deck
Figure 1.3.2.1-1 Data Load Switch Interface
1.3.3 VDT
The FCC uses 5-wire VDTs on flight deck controls. The wiring between FCC and VDT are
Twisted Shielded Pair (TSP) for excitation, and Twisted Shielded Triple (TST) for feedback. A
single VDT will be tied to a single excitation output. Figure 1.3.3-1 shows the interconnect
between the FCC and a VDT. Figure 1.3.3-2 shows the position recovery interface from VDT to
FCC.
This schematic in Figure 1.3.3-2 shows that the VDT to FCC wiring interface has capacitive
characteristics, and that the length of the wiring connection introduces capacitance that can
affect VDT scaling. These need to be considered when connecting the VDT to FCC. The
schematic shows reasonable wiring characteristics that can be achieved when proper cabling is
applied. Eviation and/or the wiring integrator is responsible for identifying and selecting cable
that meets these characteristics, Eviation should work with the sensor supplier to ensure scaling
in the EICD includes potential effects of the wiring (capacitive and resistive) on the signal. It
should be noted that the values shown in Figure 1.3.3-2 are for example only.
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FCE
Excitation
Return
High
Center
Low
Figure 1.3.3-1. Wire VDT Interconnects
FCC
42 pF/ f t
High
42 pF/ ft
1 0 k?
.2 5 W
7 5 k?
.2 5 W
1 0 0p F
8 pF/ f t
M M B Z1 2V A L
8 pF / ft
8 pF/ ft
7 5 k?
.2 5 W
Low
42 pF/ f t
1 0 0p F
1 0 k?
.2 5 W
M M B Z1 2V A L
Figure 1.3.3-2. VDT to FCC Position Recovery Interface
1.3.4 CAN
The maximum allowable length of the CAN bus is 62 meters (203 feet). This length is measured
from one terminating resistor to the other.
The minimum recommended distance between nodes is 31 cm (12 inches). As a recommended
practice this “distance between nodes” also includes the distance between the last node
connection and the terminating resistor on the end.
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The recommended length of stubs (branches) is 3.8 cm (1.5 inches). This length allows the node
“T” connection to be housed in the back shell of the FCC connector or the shell of the aircraft
harness connector to a servo. This length can be increased to 25.4 cm (10 inches) as long as
the stub is shielded and the bus and stub shields are connected together.
Two terminating resistors are required. One terminating resistor must be on each end of the CAN
bus. The recommended value for the terminating resistors is 124 Ohms each.
It is recommended that the wire shielding is connected so as to be electrically continuous. “T’s”,
junctions, bulkhead connectors, etc need to have the shielding connected in some way to satisfy
this. The recommended shielding termination is to ground at each node through the connector
back shell.
The daisy chain sequence may be modified to allow efficient wire routing based on LRU
locations.
1.3.5 Ethernet
The Ethernet used on the Eviation aircraft are used for the flight test interface, but should follow the
same guidelines as would be used for an A664 installation.
Typical aircraft installations utilize star quad cable, which consists of four parallel wires uniformly twisted
around a center filler and encased in an overall shield as shown in Figure 1.3.5-1. One run of this cable
is required for each 10/100BASE-T Ethernet Link and may be used with standard contacts (Size 20 or
22) and quadrax contacts.
Figure 1.3.5-1 Star Quad Cable
The undesired coupling that can occur between the two pairs in the star quad cable is referred to as
Near-End Crosstalk (NEXT). The NEXT performance of the cable should be verified across the full range
of frequencies (1MHz-100MHz) with minimum deviation when subjected to typical aircraft installation
bend radii and clamping conditions. General guidelines for cable NEXT performance criteria are provided
in Appendix I of ARINC 664, Part 2.
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A connector backshell should be used for shield termination. Not more than 0.20 inch (one twist) of the
shield(s) and inner jacket should be removed. These tight dimensions are defined to meet the NEXT and
EMI performance specifications. In those cases where it is desired by the system integrator to increase
these dimensions, it is possible, by testing the desired configuration, to change the dimensions.
Increasing these dimensions may have an impact on the Aircraft Link budget. Connect the shield(s) to
the backshell as a chassis ground using a suitable low impedance method. Backshells which provide 360
degree coverage of the shield may be necessary to meet all DO-160 levels. Example connector layouts
and additional shield termination guidelines are provided in Appendix B of ARINC 664, Part 2.
1.3.6 Sizing and Shielding
This section provides guidance for the FCC interconnects. Table 1.3.6-1 provides suggested
pin/wire size correlation, Table 1.3.6-2 provides shielding information, and Table 1.3.6-3
provides general information for the pin/wire size and shielding information for each interface
type.
Table 1.3.6-1 Pin/Wire Size
Pin Size
Smallest
AWG
Largest
AWG
Size 22D
24
22
Table 1.3.6-2 Shielding Information
Type Acronym
Type Description
TSP
Twisted Shielded Pair
TST
Twisted Shielded Triple
TSQ
Twisted Shielded Quad
BTP
P2P Data Bus Wire, TSP with Table 1.3.1-1
characteristics
BSQ
A664 Data Bus Wire, Star Quad Cable
Table 1.3.6-3 Generic Interconnect Information
IO
BP
Description of Signal
Function
Pin
Size
AWG
Shielding
O
VDT Excitation Return
22D
22
TSP
VDT Excitation
22D
22
TSP
I
VDT High
22D
24
TST
I
VDT Low
22D
24
TST
O
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IO
BP
Description of Signal
Function
AWG
Shielding
VDT Center
Pin
Size
22D
I
24
TST
Analog Discrete
22D
24
O
Analog Discrete
22D
24/22
B
A429 +
(H)
22D
24
TSP
(L)
22D
24
TSP
I
B
A429 -
TST
1
TST
B
FT Ethernet +
(H)
22D
24
BSQ
B
FT Ethernet -
(L)
22D
24
BSQ
B
IMB +
(H)
22D
24
BTP
(L)
22D
24
BTP
B
IMB -
B
CAN +
(H)
22D
24
TSP
B
CAN -
(L)
22D
24
TSP
P
FCC Power Input
22D
22
TST
P
FCC Power Ground
22D
22
TST
I = INPUT
O = OUTPUT
B = BUS
P = POWER
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There are high and low current discrete outputs. Low current discrete outputs can use a 24
AWG wire, while high current discrete outputs can use 24 AWG wire for short wire runs but must
use 22 AWG wire for long wire runs.
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