This application is to demonstrate interfacing to an additional display.
Thd DDM4 from Lascar Electronics is a 4 digit LCD display module controlled
by 3 lines.  Data is sent out in a serial format.  If multiple displays are
required then the DDM4 can be cascaded.  Yhis means that the MDM-1 can control
many displays with different data.  In this example the MDM-1 is displaying
temperature while a voltage is displayed on the DDM4.  The DDM4 is based on the
Microchip AY0438.

Data is passed using write command ddm4_write_p( digit_number, data ).  The
whole display is updated everytime a digit is updated.  While this is wasteful
it does allow for flexibility.  If more compact code is required then modify
the software to only update on the last digit.

During testing I discovered that if the load pin is left high the display works
normally.  The update is so quick that it is not noticeable.  This would mean
that an extra display using 2 pins is possible.

The checksum of the code is 847C.

