118 Programmable Logic Controllers: Hardware and Programming
6.7 Programming Ladder Logic Diagrams
with Latch and Unlatch Instructions
Frequently, industrial control systems dictate that an output
device must turn on when a pushbutton is pressed. The same output
device must turn off when another pushbutton is pressed. This is pos-
sible if PLC latch and unlatch instructions are used. Figure 6-23 dis-
plays a relay logic diagram with two latch and unlatch instructions.
Switch one and pushbutton one control the green light. Switch two
and pushbutton two control the red light.
Figure 6-24 displays the input/output connections of an Allen-
Bradley fixed SLC 500 PLC device. Table 6-4 illustrates the input/out-
put ports’ assignments.
A fixed SLC 500 processor is used to create the PLC ladder logic
displayed in Figure 6-25. Figure 6-25 shows the PLC ladder diagram
for the relay logic diagram displayed in Figure 6-23. Close switch one
to latch the address B3:0/0 and turn on the green light. Then, press
pushbutton one to unlatch the address B3:0/0 and turn off the green
light. Press switch two to latch the address B3:0/1 and turn on the red
light. Then, press the pushbutton two to unlatch the address B3:0/1
and turn off the red light.
A
B
Figure 6-22. Integer files. A. RSLogix 500 software from Rockwell
International. B. LogixPro software from The Learning Pit.