SC4x0 Technical manual
Page 16 of 34
SeaCom
Rev
2.00
6. Protective measures
6.1 Reverse voltage
The device is protected against wrong polarization of the supply voltage. In the supply line a protective
diode is connected across in non-conducting direct when connected right. When connected in reverse,
this diode will short through a PTC fuse and interrupt the supply current.
Supply must be disconnected for a few seconds before the fuse resets.
6.2 Excessive voltage
The device is protected against high voltage from e.g. battery charger. If the supply voltage exceeds
approx 33.5 volt a crow bar circuit will short the supply through a PTC fuse. This will either blow the
external fuse or lower the internal supply to below 0.5 volt until supply is disconnected.
Supply must be disconnected for a few seconds before the fuse resets.
6.3 Excessive temperature
If the board temperature exceeds 70°C a warning is given to the internal alarm log and error condition
is indicated by the ‘Call’ indicator.
If the temperature exceeds 85°C an alarm is given, backlight supply is disconnected if the unit is active
volume is lowered to step 7 if set higher for reducing the power dissipation.
When temperature of the power amplifier reaches approx 140°C it will shut down due to thermal
protection.
6.4 Program deadlock protection
The processors internal watchdog that is enabled at a rate of 106ms protects the device software.
It is only serviced by the main loop and a few time-consuming inline functions. In case software
accidental or by electrical noise injection enters a deadlock a timeout will occur and reset the
processor.
In parallel to that, an optional external watchdog can be enabled by means of a jumper in J7. If a
jumper is located in pos no. 3 from left to right, the watchdog is enabled. If the jumper is omitted, it is
disabled.
The external watchdog is for future implementing of PA and other options requiring an independent
monitor source and is have a service timeout of 1,6 sec.
6.5 PCB coating
The PCB is coated to protect it from the harsh environment at sea.
