Automating Manufacturing Systems - Process Control and ...

plc operation - 8.8
8.7 PRACTICE PROBLEMS
1. Does a PLC normally contain RAM, ROM, EPROM and/or batteries.
2. What are the indicator lights on a PLC used for?
3. A PLC can only go through the ladder logic a few times per second. Why?
4. What will happen if the scan time for a PLC is greater than the time for an input pulse? Why?
5. What is the difference between a PLC and a desktop computer?
6. Why do PLCs do a self check every scan?
7. Will the test time for a PLC be long compared to the time required for a simple program.
8. What is wrong with the following ladder logic? What will happen if it is used?
A
B
L
U
X
Y
X
Y
9. What is the address for a memory location that indicates when a PLC has just been turned on?
8.8 PRACTICE PROBLEM SOLUTIONS
1. Every PLC contains RAM and ROM, but they may also contain EPROM or batteries.
2. Diagnostic and maintenance
3. Even if the program was empty the PLC would still need to scan inputs and outputs, and do a
self check.
4. The pulse may be missed if it occurs between the input scans
5. Some key differences include inputs, outputs, and uses. A PLC has been designed for the factory
floor, so it does not have inputs such as keyboards and mice (although some newer types
can). They also do not have outputs such as a screen or sound. Instead they have inputs and
outputs for voltages and current. The PLC runs user designed programs for specialized tasks,