how to enter a interlock in FBD
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in the FBD using the interlock is not much different from and other programming language
since it is in series with the interlocked loop
in ladder you normally use an open contact inn you rung , while the interlock signal is normally close input , or vice versa
in the FBD using the AND gate will do the same effect
also you can use set of logic gates(conditions) to act (as group)interlock
Didn't get you exactly on text "program in which 3 will be interlocked with a Swtich bool" in question description. But in general interlock with another bool variable use AND function block. Or if any analog value use Equal block from comparators and if both the value matches output goes 1. that you can connect to bool input to Switch block. There are several possibilities as freelance is having rich default function blocks.
Pls describe in details if this does not answer your question. will resolve with your logic requirement.