Relay Enable & SPI Interface - Block F


        This relay enable circuits is needed to assure that all the relay drivers and stepper motor driver are disabled at power up.  One annoying quirk of the MPC555 CPU is the power on state of the external I/O ports.  Some are pulled high, some low, some open.  The initial state of DATA6 line is pulled high at power up.  This state, along with the inverted condition driven by U15F, will disable all relay drivers, stepper motor drivers, and some general usage I/O to the outside world until after the processor has booted up.  One of the first orders of business after initial boot and code loading process will be to configure the default state of all the I/O lines of the Microcontroller.  Once configured the interfaces to the outside world can be brought up in a known state.
       The three pull down resistors on the SPI lines are there for the same type of reason.  Open collector initial state.  These resistors assure that the internal serial devices are not fed with a floating signal level.