Testing Limit Switch Wiring With Truth Tables

Sensors such as limit switches are the only way an electronic monitoring system has of knowing what is going on in the world. If the sensors are not wired properly, the rest of the system is helpless. In an actual case of miswired limit switches, a malfunction was missed that caused a ship to be short by over one million pounds of grain when it was loaded.

To verify that a set of limit switches is properly wired, we first construct a "truth table" as shown. A truth table is a table that shows all possible combinations of input conditions for a logic circuit on the left, and the states of the outputs on the right. The 19th-century English mathematician George Boole called such tables "truth tables" because he was analyzing logic, and the elements of his tables could be either "true" or "false". Boole's techniques, known as "Boolean algebra", were first applied to electrical circuits by the American electrical engineer Claude Shannon in his 1937 master's thesis.

The truth table is constructed to show how the outputs SHOULD behave. If the limit switches are wired correctly, the ACTUAL behavior of the outputs will match the truth table. We document an incorrectly wired gate by marking any output cells of the truth table that do not match the observed states of the outputs.

The following truth table is for a basket valve. This gate has only one moving element, with an indicator arm which moves between two limit switches. Therefore the gate's truth table has 3 possible input combinations:
(1) The gate's indicator arm is touching the "Ship Duct Closed" limit switch: the indicator lights should show the Ship Duct closed and the Return Duct open.
(2) The gate is between the limit switches, not touching either one: the indicator lights should show both the Ship Duct open and the Return Duct open.
(3) The gate's indicator arm is touching the "Return Duct Closed" limit switch: the indicator lights should show the Return Duct closed and the Ship Duct open.

The left side of the table (the input side) shows all possible positions of the basket valve. The right side of the table (the output side) shows the proper states of the indicator lights for each position. A light which is supposed to be lit is marked "ON", while a light which is supposed to be off is marked "-".

Truth Table for Basket Valve Indicators
Basket Valve
Positions
Indicators
Open to
Ship
Open to
House
Ship Duct Closed-ON
Both Open
(gate moving)
ONON
Return Duct ClosedON-

To test the wiring, you put the gate in each of its possible positions and verify that the indicators are ON or OFF as specified in the truth table. Elevators often do not have a way to stop the gates in an intermediate position, so in that case you have to verify the states of the indicators for the "both open" position while the gate is in motion.

The next truth table, which is shown next to the 3-D model, is for a pair of independent slide gates. Again, the left side of the table (the input side) shows all possible positions of the gates, as seen by the limit switches. Since each slide has two limit switches, it has three possible positions:
(1) Fully Closed (FC), touching the fully closed limit switch;
(2) Fully Open (FO), touching the fully open limit switch;
(3) Open (OP), when it does not touch either limit switch.
Since there are two slides forming the diversion point, there are 3X3=9 possible input positions. The right side of the table (the output side) again shows the proper states of the indicator lights for each input position. An indicator light which is supposed to be on is marked "ON", while a light which is supposed to be off is marked "-". The first cell of each row of the table is active. Clicking on it operates the model to put the gates in the specified positions. Then you can compare the indicators to the way they should display according to the truth table. Since this example is properly wired, each indicator light is ON or OFF every time the table says it should be.

NEXT: An Example of Miswired Limit Switches.
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