Rockwell Automation 1756-IH16ISOE ControlLogix Sequence of Events Module - Manual (Page 20)

Publication 1756-UM528A-EN-P - April 2004
2-4 How Does the Sequence of Events Module Operate in a ControlLogix System?
Propagating a Signal From
Field Device to Backplane As shown in Figure 2.1, the Sequence of Events module receives a
signal at the RTB and processes it internally before sending a signal to
the ControlLogix backplane via the Requested Packet Interval (RPI) or
at na Enable CST Capture occurrence.
However, when you operate the Sequence of Events module, you
must account for signal propagation delays that exist during internal
processing. Some of these delays are inherent to the module and
others can be controlled via the user-defined module configuration.
During processing, the following delays exist:
• hardware delay - varies according to module type (i.e., catalog
number) and input transition type (i.e., OFF to ON/ON to OFF)
• input filter delay - user-configurable number from 0 to 50ms
• firmware scan time - up to 25µs, depending on when the input
transitions relative to the sample time
• ASIC delay - 25µs
Typically, the Sequence of Events module can deliver a signal to the
ControlLogix backplane within 275
µs in FIFO mode and 725µs in CST
Per Point mode after it was received at the removable terminal block
(RTB); these 275
µs or 725µs numbers represent a scenario where the
hardware delay is at maximum levels but no input filter is used.
IMPORTANT
The hardware delay is relatively consistent across
inputs on a module and across modules in a chassis.
Therefore, assuming the inputs and modules are
used in similar environmental conditions and with
similar voltages applied, the hardware delay is a
minor consideration affecting timestamping accuracy.
TIP
For fastest propagation of a signal from a field device
to the controller, use the module in FIFO mode in a
local chassis.