Echelon Neuron C - Manual (Page 24)

4 Predefined Events
The reference value is the value read the last time the change event evaluated to
TRUE. For the unqualified io_changes event, the event equates to TRUE when
the current value is different from the reference value.
A task can access the input value for the I/O object through the input_value
keyword. The input_value is always a signed long.
For the bit, byte, and nibble I/O objects, changes are not latched. The change
must persist until the io_changes event is processed. The leveldetect input object
can be used to latch changes that might not persist until the io_changes event
can be processed.
Syntax
io_changes ( io-object-name ) [to expr | by expr ]
io-object-name The I/O object name (see the I/O Model Reference ). I/O
objects of the following input object types can be used in
an unqualified change event. The by and to options can
also be used where noted.
bit (to)
byte (by, to)
dualslope (by)
leveldetect (to)
nibble (by, to)
ontime (by)
period (by, to)
pulsecount (by)
quadrature (by)
to expr The to option specifies the value of the I/O state necessary
for the io_changes event to become TRUE. The compiler
accepts an unsigned long value for expr , where expr is a
Neuron C expression. However, each I/O object type has
its own range of meaningful values.
by expr The by option compares the current value with the
reference value. The io_changes event becomes TRUE
when the difference (absolute value) between the current
value and the reference value is greater than or equal to
expr .
The default initial reference value used for comparison purposes is zero. You can
set the initial value by calling the io_change_init( ) function. If an explicit
reference value is passed to io_change_init( ), that value is used as the initial
reference value: io_change_init( io-object-name , value ). If no explicit value is
passed to io_change_init( ), the I/O object’s current value is used as the initial
value: io_change_init(
io-object-name ).
Example 1
IO_0 input bit push_button;
when (io_changes(push_button) to 0)
{
...