Rockwell Automation D64046.5.1 U MNL WIN DDMC - Manual (Page 40)

Configuring SDS Instructions
Chapter 4
4-4
Graphic Example of Combinatorial Equations
LS3 OFF AND LS1 OFF
LS2 ON AND LS1 ON
LS2 OFF AND LS3 ON
Step
1
Step
2
Step
3
With traditional state-based programming, you need to take into account all of
the probable states for the implementation. (The number of possible states is 2
to the nth power, where n is the number of inputs.)
The SDS instruction that uses combinatorial equations eliminates this need to
account for all possible input states, and provides a means to enter Boolean
expressions similar to ladder logic. In this case, the SDS instruction will not
advance to another step until all conditions defined in an equation are fulfilled,
regardless of their order or timing relationship with each other.
Using the SDS instruction in combinatorial mode also allows you to replace
external ladder logic. By using combinatorial equations in the SDS instruction,
you can get diagnostic information on the logical conditions not yet fulfilled.
This feature can be useful in generating operator guidance messages, as well as
diagnostic messages.