Allen-Bradley A-B QUALITY DL20 Series User Manual page 83

DC Sampling
The parallel port is always sampled more than once and all readings must
agree. This provides a variable degree of insensitivity to noise on the data
lines.
For example, assume you are sending the binary data 010 and change it to
100. For a very brief time the data lines may assume the intermediate state
of 000 or 110 before settling at 100, the desired number. If the parallel port
was sampled only once, the erroneous intermediate state could be read
instead of correct data.
DC Sampling Debounce
The debounce process works like this: during each scan, the parallel port is
read, and the debounce sampling is started. Every 1/512 second until the
debounce count is satisfied, the parallel port is read again. All readings must
agree, or the data is ignored. For example, if the debounce value is set to 2,
the parallel port will be read 3 times (having one initial reading) and the two
readings must match the initial reading for data to be accepted.
Data Hold Time
For DC data input to be recognized on the parallel port, it must be present
and unchanging for a minimum of 15 milliseconds when using the default
scan rate and debounce.
Note: The Parallel Input Converter Series C or later uses a DC sampling
method. For the data input to be recognized on the parallel port, it must be
present and unchanging for 100 ms.
Chapter 7
The Parallel Port
7–11