Allen-Bradley A-B QUALITY DL20 Series User Manual page 79

Edge Triggered Strobe
Note: Refer to event driven sampling in this chapter for a description of how
to use the ETS inputs.
The edge triggered strobe (ETS) is typically used with non Allen-Bradley
programmable controllers. The DL20 can be set up to either look at the data
and strobe lines periodically (time-driven sampling); or based on a signal
present on edge triggered strobe (event-driven sampling).
The edge triggered strobe is especially useful with non Allen-Bradley
programmable controllers which support word I/O modules. These modules
typically multiplex two or more channels onto one set of data lines. A strobe
is brought out for each channel; a channel's strobe becomes active when data
for that channel is present on the data lines.
Edge triggered strobe does not alter the values present on the data lines, nor
does it affect their interpretation.
Unchanged Data Rejection
When data is read at the parallel port, a copy is saved. If the next reading is
the same, the reading is ignored. This prevents a single message from being
read multiple times.
For example, if the sampling time is 4 and the debounce time is 1, the
parallel port is scanned about 100 times per second. If the control system
changed the data on the parallel port from null to trigger message 12 and held
the trigger for 100 milliseconds before returning to null, the message would
be displayed once, not ten times, even though the parallel port will have
been sampled ten times.
Chapter 7
The Parallel Port
7–7