chpt_8_review
describe two common applications for counters.
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what determines the maximum speed of transitions that a PLC counter can count? why?
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what kind of counter would you choose to keep track of the number of parts remaining in a hopper that originally contained 10 parts in the full hopper?
A count down counter
what type of counter would you use to count the total number of parts made during each shift?
A count up counter
compare the operation of a standard examine on contact instruction with that of an off-2-on transitional contact.
AN off-2-on transitional contact stays on for 1 scan and goes off. it then stays off until the trigger goes off, and then comes on again.
what kind of counter would you use to keep track of the current number of parts in a stage of a process as they enter and exit?
an up down counter
state 4 pieces of information usually associated with A PLC counter instruction.
counter type counter address counter preset value accumulated count
when is the output of a PLC counter energized?
each time the counted event occurs
explain how an OSR 1 shot rising instruction can be used to freeze rapidly changing data.
the 1 shot allows the rapidly changing accumulated time from the timer to be frozen to insure a readable, stable display.
what is the normal function of a transitional contact used in conjunction with a counter?
the 1 shot is perfect for resetting both counters and timers since it stays on for only 1 scan.
the counter instructions of a PLC are normally retentive. Explain what this means.
the counter will either incriment or decrement whenever the count input transfers from an off state to an on state.
describe the basic programming process involved in the cascading of 2 counters.
the output of the first counter is programmed into the input of the second counter. the status bits of both counters are programmed in series to produce an output. these 2 counters allow twice as many counts to be measured.
when is the overflow bit of an up counter set?
the overflow bit is true whenever the counter Counts past its maximum value.
when is the under flow bit of a Down counter set?
the underflow bit will go true when the counter counts below the preset.
name the 3 forms of PLC counter instructions, and explain the basic operation of each.
the up counter is an output instruction who's function is to increment it's accumulated value on false to true transitions of its instructions. the down counter instruction will countdown or decrement by 1 each time the counted event occurs.
in a PLC counter instruction, what rule applies to the addressing of the counter and reset instructions?
when programmed, the counter reset coil is given the same reference address as the counter that it is to reset.
