PPE_byte() returns a 'parallel poll enable'
command byte corresponding
to the dio_line
and sense
arguments. The dio_line
(valid values are 1 through 8)
specifies which dio line the device being configured should use to send
back its parallel poll response. The sense
argument
specifies the polarity of the response. If sense
is nonzero, then the specified dio line will be asserted to indicate
that the 'individual status bit' (or 'ist') is 1. If sense
is zero, then the specified dio line will be asserted when ist is zero.