To use a power supply line also as a signal line so as to surely transmit a multiplex signal with a large pulse duration by adding a function of rapidly charging a capacitor of a constant-voltage circuit of a sub-control device simultaneously with the end of transmitting the header of a request signal to a main control device.
When an IG switch 2 is turned on, and a microcomputer 25 of a driver's seat air bag system 35 starts operating to perform an electrostatic capacity diagnosis, immediately after turning off a switch circuit 32 through a signal line Y, the microcomputer 25 turns on a switching transistor 31 for designated time to discharge a backup capacitor 23 through a resistance 30. At this time, the microcomputer reads the terminal voltage to obtain the voltage change amount, and decides whether the electrostatic capacity is a regulated value or not, and if abnormal, a warning device is operated. After that, the switch circuit 32 is turned on to start feeding to the driver's seat air bag system through a communication circuit 33 and a power supply line 36.
HAYASHI HARUKA
KISHI TAKAYUKI
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