TODO
====

 * Support for "I2C bus monitor board"
   http://www.i2cchip.com/pdfs/busmon_sch.pdf

   _SDA is pin 13 select
   _SCL is pin 12 paper empty
   _IRQ is pin 11 busy
   unused is pin 10
   
   The data pins D7,6,5 must be set high to power the isolator circuit.

