Chapter 16 PLC Function ApplicationsMS300
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16-4-4 Common basic program design examples
Start, stop, and protection circuits
Some applications may require a brief close or brief break using a button 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 N.O. contact X1 = ON, and the stop N.C. contact X2 = OFF, Y1 = ON. If
X2 switches to ON, coil Y1 is no longer electrified, and this is therefore referred to as
priority stop.
Figure 16-36
Example 2: Priority start protective circuit
When the start N.O. contact X1 = ON, and the stop N.C. contact X2 = OFF, Y1=ON, and
coil Y1 is electrified and protected. If X2 switches to ON, coil Y1 still protects the contact
and continues to be electrified, and this is therefore referred to as priority start.
Figure 16-37
Example 3: Setting (SET) and reset (RST) command protective circuit
The following diagram shows a protective circuit composed of RST and SET commands.
A priority stop occurs when you place the RST command after the SET command.
Because the PLC executes programs from the top down, at the end of the program,
the state of Y1 indicates whether coil Y1 is electrified. When X1 and X2 both
actuate, Y1 loses power, and this is therefore referred to as priority stop.
A priority start occurs when you place the SET command after the RST command.
When X1 and X2 both actuate, Y1 electrifies, and this is therefore referred to as
priority start.
Figure 16-38
SET
Y1
RST
Y1
X1
X2
Top priority of stop
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