PRACTICE PROBLEM SOLUTIONS

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4. In MCR blocks the outputs will all be forced off. This is not a problem for outputs such as retentive timers and latches, but it will force off normal outputs. JMP statements will skip over logic and not examine it or force it off.

5. Timed interrupts are useful for processes that must happen at regular time intervals. Polled interrupts are useful to monitor inputs that must be checked more frequently than the ladder scan time will permit. Fault interrupts are important for processes where the complete failure of the PLC could be dangerous.

6. These can be used to update inputs and outputs more frequently than the normal scan time permits.

7. The main differences are: Shift registers focus on bits, stacks and sequencers on words Shift registers and sequencers are fixed length, stacks are variable lengths

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10. a) Timed, polled and fault, b) They remove the need to check for times or scan for memory changes, and they allow events to occur more often than the ladder logic is scanned. c) A few rungs of ladder logic might count on a value remaining constant, but an interrupt might change the memory, thereby corrupting the logic. d) The UID and UIE

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13. The first element of the array is loaded if the input to the SQO is true on the first scan, but after that it is never used again. So in this example the array[0] value will be used the first time, and the array[1] to array[5] values will be used for the normal sequence.

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