Function Blocks
Function Block Reference Guide 3/9994
2.33 LTCH Function Block,
continued
Example
Figure 2-30 shows a Function Block Diagram using an LTCH function block
Start
Stop
DO 1
DO 1
DO1
On Timer
1
20 SEC
DO 2
Start/Stop Circuit
The Start/Stop latch circuit is used since no
external confirmation is needed. In this
example, the Operator Panel pushbutton
switches (F1 and F2) are used to substitute
for panel switches. The Push Button function
block is used to assign Start to F1 and Stop
to F2. The latch output turns on the ON
Lamp and starts the timer. After 20 sec.,
Solenoid 4 is activated. Note: the ON and
OFF Delay timers are reset after timeout or if
the logic state to the input goes to logic 0 (or
low).
5 Function Blocks
On Lamp
SOL 4
An ON Delay timer is added to a basic Start/Stop circuit
which activates the ON Lamp. In ladder logic, the DO1
contact status is used to activate the timer and latch in the
start pushbutton action. After 20 sec., SOL4 (DO2) is turned
ON which is held as long as DO1 is ON.
PLC Ladder Logic
UMC 800 Logic
PUMP 1
Figure 2-30 LTCH Function Block Example
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