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Structured lab text problems
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Latching In ladder logic it is very easy to latch.
Consider the following implementation of the seal circuit on the next slide
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To implement this program in structured text we need to implement a latch
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Structured text program
IF (NO_PB=TRUE AND NC_PB=TRUE AND Int_relay=FALSE THEN (*conditions to latch the output on - not latch must be off and the latch is set when the output goes on*) Int_relay:=TRUE; (*latch is set*) Output:=TRUE; (*turns on output*) END_IF IF (NC_PB=FALSE AND Int_relay=TRUE) THEN (*conditions to latch the output off - note latch must be set to enter into this routinet*) Int_relay:=FALSE;(*latch is reset*) Output:=FALSE;(*turns off output*)
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Program definitions PROGRAM PLC_PRG VAR Int_relay: BOOL :=FALSE;
END_VAR
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Global variables that interact with the operator screen
VAR_GLOBAL NC_PB: BOOL:=1; (*static declaration*) NO_PB: BOOL; Output: BOOL; END_VAR
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Implementation of a timer using structured text
To implement a time one calls up the timer function. Consider the following problem A Normally open pushbutton (NO_PB) causes a delayed start to a motor of 10s. If the stop push button has been pressed again the motor switches off and the timer is reset.
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Structured text code IF NO_PB=TRUE THEN motor_start:=TRUE; END_IF
timer(IN:= motor_start, PT:= t#10s , Q=>motor_on , ET=> ); IF NC_PB=FALSE THEN motor_start:=FALSE;
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Local variables PROGRAM PLC_PRG VAR motor_start: BOOL; timer:TON; END_VAR (*note that the timer has been assigned the type timer_on ‘TON’ which is created by the insertion of a function block TON)
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Global Variables VAR_GLOBAL motor_on: BOOL; NO_PB: BOOL;
NC_PB: BOOL:=1; END_VAR (*note these are to interface with the operator screen*)
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