Understanding the Basic Elements of Ladder Logic When you write a program in ladder, you create and arrange the graphical componen...
Understanding the Basic Elements of Ladder Logic
When you write a program in ladder, you create and arrange the graphical components to
form a network of logic. As shown in Figure, the following types of elements are available
for creating your program:
· Contacts: each of these elements represents a switch through which power can flow
· Coils: each of these elements represents a relay that is energized by power flowing to
that relay.
· Boxes: each of these elements represents a function that is executed when power flows
to the box.
· Networks: each of these elements forms a complete circuit. Power flows from the left
power rail through the closed contacts to energize the coils or boxes.
Contacts
Standard Contacts
The Normally Open contact instructions (LD, A, and O) and Normally Closed contact instructions (LDN, AN, ON) obtain the referenced value from the memory or from the process-image register. The standard contact instructions obtain the referenced value from the memory (or process-image register if the data type is I or Q). The Normally Open contact is closed (on) when the bit is equal to 1, and the Normally Closed contact is closed (on) when the bit is equal to 0. In FBD, inputs to both the And and Or boxes can be expanded to a maximum of 32 inputs. In STL, the Normally Open instructions Load, AND, or OR
the bit value of the address bit to the top of the stack, and the Normally Closed instructions Load, AND, or OR the logical NOT of the bit value to the top of the stack.
Immediate Contacts
An immediate contact does not rely on the S7-200 scan cycle to update; it updates immediately. The Normally Open Immediate contact instructions (LDI, AI, and OI) and Normally Closed Immediate contact instructions (LDNI, ANI, and ONI) obtain the physical input value when the instruction is executed, but the process-image register is not updated.
The Normally Open Immediate contact is closed (on) when the physical input point (bit) is 1, and the Normally Closed Immediate contact is closed (on) when the physical input
point (bit) is 0. The Normally Open instructions immediately Load, AND, or OR the physical input value to the top of the stack, and the Normally Closed instructions immediately Load, AND, or OR the logical NOT of the value of the physical input point to the top of the stack.
NOT Instruction
The Not instruction (NOT) changes the state of power flow input (that is, it changes the value on the top of the stack from0 to 1 or from1 to 0).
Set and Reset Dominant Bistable Instructions
The Set Dominant Bistable is a latch where the set dominates. If the set (S1) and reset (R) signals are both true, the output (OUT) is true.
The Reset Dominant Bistable is a latch where the reset dominates. If the set (S) and reset (R1) signals are both true, the output (OUT) is false. The Bit parameter specifies the Boolean parameter that is set or reset. The optional output reflects the signal state of the Bit parameter.
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