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Excercise 2:  Checking Limit Switches
 This is the second exercise in determining if the shipping bin model shown on the right has been properly wired. The truth table below has been developed with the assumption that the monitor screen will be used instead of the indicator lights. If the switches have been wired properly, the icons on the monitor screen for the shipping gate and the return gate should match the truth table shown below. An icon shows "Closed" (CL) by being red and inside the duct. It shows "Open" (OP) by being green and outside the duct.
 

To modify a field of the truth table, move the cursor over it, click the mouse, and type the letters you want to insert. When you press the "enter" key or click the mouse over a different part of the screen, the changes will become permanent. To perform the test, begin with the first line of the truth table and make sure the actual positions of the Ship and Return gates match the positions specified by that line. Then note whether each of the two indicator icons shows Closed (CL) or Open (OP). If the condition of the icon does not match the condition specified in the truth table, mark that field with an "X" and enter "X" or "NO" in the last field of the row under the "OK?" heading, as was done in the previous exercise. If both icons are correct, mark the entire line as correct by typing your initials in the field under the "OK?" heading, as before. If you make a mistake, the computer will tell you.

This example of miswired limit switches is detrimental to the elevator. If the Ship Gate sticks partway open, some grain that is supposed to be returned to the elevator will be sent to the ship. While the customer is not likely to complain about getting free grain, the shipping company will want payment for the extra tonnage they carried. Lastly, if the elevator happens to be loading the ship with two different kinds of grain, like corn and soybeans, this wiring error could comingle (mix) the two grains and bring a quality complaint from the customer.


NOTE: If the model does not appear below you do not have a VRML player installed.