Furuno CSH-83 Operator's Manual page 92

Gray test
This test checks for screen distortion. To escape from
the test reset the power.
SIO test
This test checks for proper communication between
the display unit and transmitter unit, and requires a
jumper plug. To escape from the test reset the power.
13-4
GRAY TEST
Q Q
Figure 13-6 Gray test
SIO TEST
DRAW SIO = OK
IF SIO = OK
PISO SIO1 = OK
SIO2 = OK
PIF SIO1 = OK
Figure 13-7 SIO test