GMI 20 INSTALLATION INSTRUCTIONS

INSTALLATION INSTRUCTIONS
GMI 20 Installation Instructions
To obtain the best possible performance, install this marine instrument according to these instructions. If
you experience difficulty during the installation, contact Garmin Support, or seek the advice of a professional
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installer.
This instrument communicates with NMEA 2000 sensors and devices, and shows information such as speed,
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heading, and water depth when connected to the appropriate sensors. The instrument can also communicate
with a NMEA 0183 device using an optional data cable.
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Important Safety Information

WARNING
See the Important Safety and Product Information guide in the product box for product warnings and other
important information.
Failure to install this device according to these instructions could result in personal injury, damage to the vessel
or device, or poor product performance.

CAUTION
To avoid possible personal injury, always wear safety goggles, ear protection, and a dust mask when drilling,
cutting, or sanding.

NOTICE
When drilling or cutting, always check what is on the opposite side of the surface to avoid damaging the vessel.

Mounting Considerations
NOTICE
This device should be mounted in a location that is not exposed to extreme temperatures or conditions. The
temperature range for this device is listed in the product specifications. Extended exposure to temperatures
exceeding the specified temperature range, in storage or operating conditions, may cause device failure.
Extreme-temperature-induced damage and related consequences are not covered by the warranty.
The mounting surface must be flat to avoid damaging the device when it is mounted.
Using the included hardware and template, you can flush mount the device in the dashboard. If you want to
mount the device using an alternative method where it appears flat with the front of the dashboard, you must
purchase a flat-mount kit (professional installation recommended) from your Garmin dealer.
When selecting a mounting location, observe these considerations.
• The mounting location should be at or below eye level to provide optimal viewing as you operate your vessel.
• The mounting location should allow easy access to the keys on the device.
• The mounting surface must be strong enough to support the weight of the device and protect it from
excessive vibration or shock.
• To avoid interference with a magnetic compass, the device should not be installed closer to a compass than
the compass-safe distance value listed in the product specifications.
• The area behind the mounting surface must allow room for the routing and connection of the cables.

March 2025 GUID-D193C1DF-4085-4AE2-9B92-78318B159AFD v6

Mounting the Device
NOTICE
If you are mounting the device in fiberglass, when drilling the pilot holes, use a countersink bit to drill a
clearance counterbore through only the top gel-coat layer. This will help to avoid cracking in the gel-coat layer
when the screws are tightened.
The included template and hardware can be used to flush mount the device in your dashboard.
1 Trim the flush-mount template and ensure it fits in the location where you plan to mount the marine
instrument.
The flush-mount template is included in the product box.
2 Remove the liner from the adhesive on the back of the template and adhere it to the location where you plan
to mount the marine instrument.
3 If you plan to cut the hole using a rotary tool instead of a 90 mm (3.5 in.) hole saw, use a 10 mm (3/8 in.) drill
bit to drill a pilot hole to begin cutting the mounting surface.
4 Using the 90 mm (3.5 in.) hole saw or rotary tool, cut the mounting surface along the inside of the dashed
line indicated on the flush-mount template.
5 If necessary, use a file and sandpaper to refine the size of the hole.
6 Place the marine instrument into the cutout to confirm that the mounting holes on the template are in the
correct locations.
7 If the mounting holes are not correct, mark the correct locations of the mounting holes.
8 Remove the marine instrument from the cutout.
9 Drill the 2.8 mm (7/64 in.) pilot holes.
If you are mounting the marine instrument in fiberglass, use a countersink bit as advised in the notice.
10 Remove the remainder of the template.
11 Place the included gasket on the back of the device and apply marine sealant around the gasket to prevent
leakage behind the dashboard.
12 If you will not have access to the back of the device after you mount it, connect all necessary cables to the
device before placing it into the cutout.
NOTE: To prevent corrosion of the metal contacts, cover unused connectors with the attached weather caps.
13 Place the marine instrument into the cutout.
14 Securely fasten the marine instrument to the mounting surface using the supplied screws.
If you are mounting the marine instrument in fiberglass, use a anti-galling lubricant as advised in the notice.
15 Snap the bezel into place.

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Connection Considerations
The marine instrument connects to power and to data sources through a NMEA 2000 network.
Although the instrument cannot directly receive NMEA 0183 data, it can display NMEA 0183 data from sources
connected to a GNX™ 20 or GNX 21 device (sold separately) on the same NMEA 2000 network.
The instrument can also receive data from Nexus instruments and sensors using a GND™ 10 device (sold
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separately).

NMEA 2000 Connection Considerations

NOTICE
If you are connecting to an existing NMEA 2000 network, identify the NMEA 2000 power cable. Only one NMEA
2000 power cable is required for the NMEA 2000 network to operate properly.
A NMEA 2000 Power Isolator (010-11580-00) should be used in installations where the existing NMEA 2000
network manufacturer is unknown.
If you are installing a NMEA 2000 power cable, you must connect it to the boat ignition switch or through
another in-line switch. NMEA 2000 devices will drain your battery if the NMEA 2000 power cable is connected to
the battery directly.
The marine instrument connects to a NMEA 2000 network on your boat. The NMEA 2000 network provides
power to the marine instrument and data from NMEA 2000 devices such as a wind sensor. The included NMEA
2000 cables and connectors allow you to either connect the device to your existing NMEA 2000 network or
create a basic NMEA 2000 network if needed.
If you are unfamiliar with NMEA 2000, you should read the Technical Reference for NMEA 2000 Products at
garmin.com/manuals/nmea_2000.

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 Wind transducer

GND 10 black box bridge

Marine instrument

Ignition or in-line switch

NMEA 2000 power cable

NMEA 2000 drop cable

Power source

NMEA 2000 terminator or backbone cable

NMEA 2000 T-connector

NMEA 2000 terminator or backbone cable

NMEA 0183 Connection Considerations

• The marine instrument can receive NMEA 0183 data from one device using a NMEA data cable (not
included), but it cannot transmit data from the NMEA 0183 device to the NMEA 2000 network.
• If you are replacing a legacy Garmin marine instrument that currently uses a NMEA data cable, you do
not need to purchase a new data cable, but you might need to replace the quarter-turn locking ring with a
threaded locking ring. See your local Garmin dealer or www.garmin.com for more information.
• The installation instructions provided with your NMEA 0183 compatible device should contain the
information you need to identify the transmitting Tx/A (Out +) and Tx/B (Out -) wires.
• When connecting NMEA 0183 devices with two transmitting wires, it is not necessary for the NMEA 2000 bus
and the NMEA 0183 device to connect to a common ground.
• When connecting a NMEA 0183 device with only one transmitting Tx (Out) wire, the NMEA 2000 bus and the
NMEA 0183 device must connect to a common ground.
• For extended runs, you should use at least 0.33 mm2 (22 AWG) wire.
• You must solder and seal all connections with heat-shrink tubing.

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Wire Color Wire Function

Red A red wire is present only on some variations of the data cable, and should not be connected.

Accessory (-). This wire is used only when connecting the marine instrument to a Garmin HVS GPS
Black
antenna.

Accessory (+). This wire is used only when connecting the marine instrument to a Garmin HVS
Yellow
GPS antenna.

Tx/A (Out +). This wire is used only when connecting the marine instrument to a Garmin HVS GPS
Blue
antenna.

Tx/B (Out -). This wire is used only when connecting the marine instrument to a Garmin HVS GPS
White
antenna.

Brown Rx/A (In +)

Green Rx/B (In -)

NMEA 0183 Connection Diagrams

This diagram is an example of a connection to a standard NMEA 0183 device with two Tx wires.

Item Description

Marine instrument with a NMEA data cable (not included)

Power source

NMEA 0183 compliant device

Item Garmin Wire Function Garmin Wire Color NMEA 0183 Device Wire Function

N/A N/A Power

N/A N/A Data ground

Rx/A (In +) Brown Tx/A (Out +)

Rx/B (In -) Green Tx/B (Out -)

This diagram is an example of a connection to a standard NMEA 0183 device with one Tx wire.

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Item Description

Marine instrument with a NMEA data cable (not included)

NMEA 2000 network (must connect to the same ground as the NMEA data cable)

Power source

NMEA 0183 compliant device

Item Garmin Wire Function Garmin Wire Color NMEA 0183 Device Wire Function

N/A N/A Power

N/A N/A Power ground

Rx/B (In -) Green Data ground

Rx/A (In +) Brown Tx

This diagram is an example of a connection to a Garmin HVS GPS antenna.

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Item Description

Marine instrument with a NMEA data cable (not included)

Power source

Garmin HVS GPS antenna

Item Marine Instrument Wire Color Antenna Wire Color

N/A Red

Black Black

Yellow Orange

Blue White

White White/orange

Brown Gray

Green White/red

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Specifications
Specification Measurement

Dimensions without sun cover (H×W×D) 110 x 115 x 30 mm (4.33 x 4.53 x 1.18 in.)

Dimensions with sun cover (H×W×D) 115 x 120 x 35.5 mm (4.53 x 4.72 x 1.40 in.)

Weight without sun cover 247 g (8.71 oz.)

Weight with sun cover 283 g (9.98 oz.)

Temperature range From 5° to 158°F (from -15° to 70°C)

Compass-safe distance 209 mm (8.25 in.)

Case: fully-gasketed polycarbonate

Material
Lens: glass with an anti-glare finish

Water Resistance IEC 60529 IPX7 1

Power usage 1.35 W max

Unit max. voltage 32 Vdc

NMEA 2000 input voltage From 9 to 16 Vdc

NMEA 2000 load equivalency number (LEN) 3 (150 mA at 9 Vdc)

1 The device withstands incidental exposure to water of up to 1 m for up to 30 min. For more information, go to www.garmin.com/waterrating.

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NMEA 2000 PGN Information
Transmit and Receive

PGN Description

059392 ISO acknowledgment

059904 ISO request

060928 ISO address claim

61184 Product information

126208 NMEA: Command, request, and acknowledge group function

126996 Product information

Transmit

PGN Description

126464 Transmit PGN list group function

Receive

PGN Description

126992 System time

127245 Rudder

127250 Vessel heading

127488 Engine parameters: Rapid update

127489 Engine parameters: Dynamic

127508 Battery status

128259 Speed: Water referenced

128267 Water depth

129025 Position: Rapid update

129026 COG and SOG: Rapid update

129029 GNSS position data

129283 Cross track error

129284 Navigation data

129285 Navigation route and waypoint info

129539 GNSS dilution of precision (DOP)

130306 Wind data

130310 Environmental parameters

130311 Environmental parameters

130312 Temperature

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 PGN Description

130313 Humidity

130314 Actual pressure

NMEA 0183 Information

When connected to an optional NMEA 0183 compatible device, the instrument can receive these NMEA 0183
sentences.

Sentence Description

DBT Depth below transducer

DPT Depth

GRMB GPS data

GRME GPS position error data

GSV GNSS satellites in view

HDG Heading, deviation, and variation

HDM Heading, magnetic

HDT Heading, true

MWV Wind speed and angle

RMB Recommended minimum navigation information

RMC Recommended minimum specific GNSS data

VHW Water speed and heading

XTE Cross track error

You can purchase complete information about National Marine Electronics Association (NMEA) format and
sentences from www.nmea.org.
© 2014 Garmin Ltd. or its subsidiaries
Garmin and the Garmin logo are trademarks of Garmin Ltd. or its subsidiaries, registered in the USA and other countries. GMI 20 is a trademark of Garmin Ltd. or its
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subsidiaries. These trademarks may not be used without the express permission of Garmin.
NMEA , NMEA 2000 , and the NMEA 2000 logo are registered trademarks of the National Marine Electronics Association.
® ®

© 2014 Garmin Ltd. or its subsidiaries support.garmin.com