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1. PLC Concepts
1-17
Example 2 - Start First latched circuit
When X1 (START) = ON and X2 (STOP) = OFF, Y1 will be ON
and latched. If X2 is turned ON, Y1 remains ON. This is a Start
First circuit because START button has the control priority than
STOP
X2
Y1
X1
Y1
Example 3 - Latched circuit of SET and RST
X2
Y1
X1
SET
Y1
RST
Stop first
The diagram opposite are latched circuits consist of RST and
SET instructions.
In PLC processing principle, the instruction close to the end of
the program determines the final output status of Y1. Therefore,
if both X1 and X2 are ON, RST which is lower than SET forms a
Stop First circuit while SET which is lower than RST forms a
Start First circuit.
X2
Y1
X1
SET
Y1
RST
Start first
Example 4 - Power down latched circuit
The auxiliary relay M512 is a latched relay. Once X1 is ON, Y1
retains its status before power down and resumes after power
up.
X2
M512
X1
SET
RST M512
Y1
M512
Example 5 - Conditional Control
X3
Y1
X1
Y1
X4
Y2
X2
Y2
Y1
X1
X3
X2
X4
Y1
Y2
Because NO contact Y1 is connected to the circuit of Y2 output, Y1 becomes one of the conditions
for enabling Y2, i.e. for turning on Y2, Y1 has to be ON