Garmin GPSMAP 721xs Guide Referenz - Seite 10

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Garmin GPSMAP 721xs Guide Referenz
NMEA 2000 Fundamentals
End-powered NMEA 2000 network, incorrectly designed:
-
+
Power cable
Length = 2 m
When the voltage drop formula is applied to this example, we see
that the voltage drop is greater than 3.0 Vdc, so this NMEA 2000
network will not function correctly when powered at the end.
This NMEA 2000 network must be redesigned with the power connected to the center of the network in order to function correctly.
Middle-powered NMEA 2000 network, correctly designed:
NMEA 2000-
compliant device
LEN = 4
Drop cable
Length = 2 m
When the NMEA 2000 network is redesigned with the power source in the center, you calculate the voltage drop in both directions. If the
T-connector to which you connect the power source is connected directly to another T-connector (as shown in this example), use the LEN from
that device as part of the calculation for both directions.
When the voltage drop formula is applied to both the left and right
sides of the power source in this example, we see that the voltage drop
is less than 3.0 Vdc on each side, so the NMEA 2000 network will
function correctly.
Note: The equation and examples provide conservative estimates for calculating voltage drop.
6
NMEA 2000-
compliant device
LEN = 4
Drop cable
Length = 2 m
Backbone cable
Length = 20 m
-
+
Power cable
Length = 2 m
Backbone cable
Length = 20 m
NMEA 2000-
compliant device
LEN = 5
Drop cable
Length = 6 m
Backbone cable
Length = 10 m
Voltage Drop = 0.053 × (2 + 20 + 10 + 6) × (4 + 5 + 7) × 0.1 = 3.22 Vdc
Cable
resistance
NMEA 2000-
compliant device
LEN = 5
Drop cable
Length = 6 m
Backbone cable
Length = 10 m
Voltage Drop Left = 0.053 × (2 + 20 + 2) × (4 + 5) × 0.1 = 1.145 Vdc
Cable
resistance
Voltage Drop Right = 0.053 × (2 + 10 + 6) × (5 + 7) × 0.1 = 1.145 Vdc
Technical Reference for Garmin NMEA 2000 Products
NMEA 2000-
compliant device
LEN = 7
Drop cable
Length = 6 m
Distance
Network load
NMEA 2000-
compliant device
LEN = 7
Drop cable
Length = 6 m
Distance
Network load