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Automating Manufacturing Systems - Process Control and ...

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plc operation - 8.8<br />

outputs are not updated.<br />

• PLCs use RAM, <strong>and</strong> sometime EPROMs are used for permanent programs.<br />

8.7 PRACTICE PROBLEMS<br />

1. Does a PLC normally contain RAM, ROM, EPROM <strong>and</strong>/or batteries.<br />

2. What are the indicator lights on a PLC used for?<br />

3. A PLC can only go through the ladder logic a few times per second. Why?<br />

4. What will happen if the scan time for a PLC is greater than the time for an input pulse? Why?<br />

5. What is the difference between a PLC <strong>and</strong> a desktop computer?<br />

6. Why do PLCs do a self check every scan?<br />

7. Will the test time for a PLC be long compared to the time required for a simple program.<br />

8. What is wrong with the following ladder logic? What will happen if it is used?<br />

A<br />

B<br />

L<br />

U<br />

X<br />

Y<br />

X<br />

Y<br />

9. What is the address for a memory location that indicates when a PLC has just been turned on?<br />

8.8 PRACTICE PROBLEM SOLUTIONS<br />

1. Every PLC contains RAM <strong>and</strong> ROM, but they may also contain EPROM or batteries.<br />

2. Diagnostic <strong>and</strong> maintenance<br />

3. Even if the program was empty the PLC would still need to scan inputs <strong>and</strong> outputs, <strong>and</strong> do a<br />

self check.<br />

4. The pulse may be missed if it occurs between the input scans<br />

www.PA<strong>Control</strong>.com

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