Furuno FSV-84 Operator's Manual page 141

How to set the fish alarm zone
Note 1: There must be at least three degrees difference between the starting
and ending points to get a fan-shaped alarm zone as shown in (a) and (b) below.
Otherwise, the unit paints a 360-degree alarm zone as in (c) where there is less
than 3 degrees between the starting and ending points and (d) where the
bearing marker has been rotated one full turn.
(a)
How to create desired alarm zone
Note 2: The echo strength which triggers the alarm can be set with ALARM
LEVEL on the ALARM & AUDIO menu. The setting range is 0-30.
This value corresponds with the number of 32-echo colors.
Note 3: To disable the fish alarm, choose OFF at step 5 in the procedure on the
previous page.
<<ALARM ZONE SETTING>>
PLACE CURSOR AT STARTING POINT
AND PRESS MENU KEY TO SET.
PRESS R/B KEY TO CANCEL.
<<ALARM ZONE SETTING>>
PLACE CURSOR AT ENDING POINT
AND PRESS MENU KEY TO
COMPLETE ALARM ZONE SETTING.
More than 3˚
Within 3˚
(b) (c)
4. SLANT MODE
(d)
4-31