Garmin GPSMAP 720/720s Technical Reference - Page 10

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Garmin GPSMAP 720/720s Technical Reference

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 =

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.

NMEA 2000-

compliant device

LEN =

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 =

Drop cable

Length = 6 m

Backbone cable

Length = 0 m

Voltage Drop = 0.053 × (2 + 20 + 10 + 6) × (4 + 5 + 7) × 0.1 = 3.22 Vdc

Cable

resistance

NMEA 2000-

compliant device

LEN =

Drop cable

Length = 6 m

Backbone cable

Length = 0 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