Rockwell Automation 1756-IH16ISOE ControlLogix Sequence of Events Module - Manual (Page 89)

Publication 1756-UM528A-EN-P - April 2004
Using The Sequence of Events Module in FIFO Mode 7-3
Once data is stored on the module, the controller must retrieve it.
Typically, the controller retrieves data from the first slot in the
on-board buffer; the data in the first slot is also known as the current
event.
After the controller retrieves the current event data, it acknowledges
the data and clears it from the Sequence of Events module’s on-board
buffers, and the data from the next slot in the buffer becomes the
current event (i.e., the module produces this data for the controller).
In FIFO mode, generally the following occurs:
1. You configure the Sequence of Events module to operate in
FIFO mode via the Communication Format selection.
2. The Sequence of Events module timestamps each transition and
stores the data in its on-board buffer. The module can timestamp
each transition with a unique CST as long as the transitions
occur 25
µs apart.
3. The controller retrieves data from the Sequence of Events
module as described in the following steps:
a. Immediately after the Sequence of Events module timestamps
an input transition, it records data in the first slot of its
on-board buffer and produces the data for the controller; the
data is the current event. The module produces the data from
the current event at every subsequent RPI until the controller
clears it (as described in step c).
b. The controller copies the data from the controller tags to a
separate data structure for later use.
c. The controller acknowledges the current event in the
Sequence of Events module’s buffer by I.EventNumber to
O.EventNumber.
d. Once the current event is cleared from the Sequence of
Events module’s buffer, data for the next transition stored in
the buffer becomes the current event, and the module begins
producing this data for the controller as described in step a.
4. The Sequence of Events module timestamps input transitions
and records the data in its on-board buffer as long as the buffer
is not full (i.e., less than 160 input transitions worth of data is
stored in the buffer).
IMPORTANT
The current event is the event for which the
Sequence of Events module is currently producing
data. The current event is NOT the most
recently-timestamped input transition.