eBook: Automating Manufacturing Systems; with PLCs



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, whereas on a personal computer it is uncommon for a user to program their system.
6. This helps detect faulty hardware or software. If an error were to occur, and the PLC continued operating, the controller might behave in an unpredictable way and become dangerous to people and equipment. The self check helps detect these types of faults, and shut the system down safely.
7. Yes, the self check is equivalent to about 1ms in many PLCs, but a single program instruction is about 1 micro second.
8. The normal output Y is repeated twice. In this example the value of Y would always match B, and the earlier rung with A would have no effect on Y.
9. S2:1/14 for micrologix, S2:1/15 for PLC-5, S:FS for ControlLogix processor


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