Garmin StreetPilot III Owner's Manual & Reference page 52

Reference
System Setup
The Interface tab provides eight possible Input and
Output settings (including 'None').
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To select an input/output interface format:
1. With the System Setup on the screen, select the fi le tab for 'Interface' and press ENTER.
2. Use the ROCKER KEYPAD to select the 'Format' fi eld and press ENTER. A pop-up window will
show the available settings.
3. Use the ROCKER KEYPAD to select the desired setting and press ENTER.
4. If required for the setting (for RTCM In and RTCM In/Text Out), highlight the 'BAUD RATE' fi eld, press
ENTER, select the desired rate and press ENTER again.
5. For Garmin DGPS and RTCM In/NMEA Out, the 'Beacon' fi eld may be set to 'Scan' or 'Manual'. If
'Manual' is selected, enter the beacon frequency and bit rate in the appropriate fi elds.