Chapter 16 PLC Function ApplicationsC2000-HS
16-21
16-4-4 Commonly used basic program design examples
Start, stop, and protection
Some applications may require a brief close or brief break using the buttons to start and stop
equipment. A protective circuit must therefore be designed to maintain continued operation in these
situations; this protective circuit may employ one of the following methods:
Example 1: Priority stop protective circuit
When the start NO contact X1=On, and the stop NC contact X2＝Off, Y1=On; if X2=On at
this time, coil Y1 will no longer be electrified, and this is therefore referred to as priority stop.
Y1
X2
X1
START
STOP
Y1
Example 2: Priority start protective circuit
When start NO contact X1=On, and the stop NC contact X2＝Off, Y1=On, and coil Y1 will be
electrified and protected. At this time, if X2=On, coil Y1 will still protect the contact and
continue to be electrified, and this is therefore priority start.
Y1
X2
X1
START
STOP
Y1
Example 3: Setting (SET) and reset (RST) command protective circuit
The following figure shows a protective circuit composed of RST and SET commands.
Priority stop occurs when the RST command is placed after the SET command. Because the
PLC executes programs from the top down, at the end of the program, the state of Y1 will
indicate whether coil Y1 is electrified. When X1 and X2 are both actuated, Y1 will lose power,
and this is therefore priority stop.
Priority start occurs when the SET command is placed after the RST command. When X1
and X2 are both actuated, Y1 will be electrified, and this is therefore priority start.
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