Joe McCauley writes:

>Does anyone know what the test input on a 17C42 does?

  It puts the 17C42 into programming mode by forcing the PC to begin at FFE0
instead of 0000.  There is a ROM located at this address called the TEST
ROM.  The 17C42 programs itself under software control.


>What would the effect be of leaving it unconnected?

  The datasheet doesn't say, but it is not recommended.  Tie it to ground
and be done with it.

  -- Hank