Perform Your Own Wiring Check

This is the first of two exercises in which you determine if the limit switches on a pair of slide gates have been wired correctly. In this exercise, the system uses indicator lights to show the positions of the gates. In the following exercise, the system uses a monitor display.

The shipping bin model shown on the right has been re-wired. If it has been wired properly, the behavior of the indicator lights should match the truth table shown below. Your job is to determine if it has been correctly wired.

The truth table is composed of text entry fields. To modify a field, 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 four indicator lights is lit (on) or not lit (off). If the on/off condition of the light does not match the condition specified in the truth table, mark that field with an "X" and mark that line of the table as incorrect by typing an "X" or a "NO" in the last field of the line (under the "Ok?" heading). If all four indicator lights are correct, mark the entire line as correct by typing your initials in the field under the "Ok?" heading. If your initials are "NO," use different initials. If you make a mistake, the computer will tell you.

The previous version of miswired limit switches probably represents a simple mistake. Although it can allow the elevator to take back good grain that has been charged to the customer, it can just as easily allow the elevator to unintentionally give away grain (although this grain would probably be substandard). However, the gates shown here have been wired so that the return gate's "fully closed" and "fully open" limit switches are powered from the "normally closed" contacts of the ship gate's "fully open" limit switch. This means those indicators are disabled whenever the ship gate is fully open. This setup can only divert grain back to the elevator. The elevator would have a hard time convincing anybody that this was a simple mistake.

With this arrangement, the ship gate cuts the power to the return gate limit switches when it is fully open. If these gates were electric, this would also cut the power to the motor that moves the return gate, and the gate could not be operated. However, pneumatic gates can still be operated because the pneumatic cylinders do not use the limit switches to limit their motion.

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