Positive And Negative Transition Detectors - Siemens SIMATIC S7 System Manual

7.1.6

Positive and negative transition detectors

LAD FBD
Input / output
IN (FBD)
OUT (FBD)
Note
Because the Positive Transition and Negative Transition instructions require an on-to-off or
an off-to-on transition, you cannot detect an edge-up or edge-down transition on the first
scan. During the first scan, the CPU saves the initial input state in a memory bit. On
subsequent scans, these instructions compare the current state and the state of the memory
bit, to detect a transition.
See also
Bit logic input examples (Page 182)
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
STL Description
EU
The positive transition contact instruction (Edge Up) allows power to
ED
flow for one scan for each OFF-to-ON transition.
The negative transition contact instruction (Edge Down) allows power
to flow for one scan for each ON-to-OFF transition.
S7-200 SMART CPUs support a combined total (positive and nega-
tive) of 1024 edge detector instructions in your program.
LAD: Positive and negative transition instructions are represented by
contacts.
FBD: The transition instructions are represented by the P and N box-
es.
STL: The positive transition is detected by the EU (Edge Up) instruc-
tion. Upon detection of a 0-to-1 transition in the value on the top of the
stack, the top of the stack value is set to 1; otherwise, it is set to 0.
The negative transition is detected by the ED (Edge Down) instruc-
tion. Upon detection of a 1-to-0 transition in the value on the top of the
stack, the top of the stack value is set to 1; otherwise, it is set to 0.
Data type Operand
BOOL I, Q, V, M, SM, S, T, C, L, Logic flow
BOOL I, Q, V, M, SM, S, T, C, L, Logic flow
Program instructions
7.1 Bit logic
177