Automating Manufacturing Systems - Process Control and ...

plc states - 12.23
Each of the alternate transitions out of a state should be give a priority, from highest to
lowest. The state equations can then be written to suppress transitions of lower priority
when one or more occur simultaneously. The state diagram in Figure 12.25 has two transitions
A and C that could occur simultaneously. The equations have been written to give A
a higher priority. When A occurs, it will block C in the equation for STC. These equations
have been converted to ladder logic in Figure 12.26.
STA
B
D
STC
A
C
STB
first scan
STA = ( STA + STB ⋅ A) ⋅ STA ⋅ B
STB = ( STB + STA ⋅ B + STC ⋅ D) ⋅STB ⋅ A ⋅ STB ⋅ C + FS
STC = ( STC + STB ⋅ C ⋅ A) ⋅ STC ⋅ D
Figure 12.25
State Equations with Prioritization