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Testing 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 can construct a "truth table" as shown below. The first truth table is for a basket valve. This valve has two limit switches, and it can touch only one at a time. Therefore the limit switches can see three possible positions:

(1) Ship Duct Closed, when the valve touches the limit switch on the ship duct side;

(2) Return Duct Closed, when the valve touches the limit switch on the return duct side;

(3) Both Ducts Open, when it does not touch either limit switch.


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 states of the indicator lights for each input position. A light which is supposed to be lit is marked "ON", while a light which is supposed to be off is marked with a "-".

 

Truth Table for Basket Valve Indicators
Valve
Positions
Indicators
Ship
Closed
Return
Closed
Ship Duct
Closed
ON -
Both Open - -
Return Duct
Closed
- ON

 

To test the wiring, you put the valve in each of its possible positions and verify that the indicator lights are turned ON or OFF as specified in the truth table. For a three-row truth table with one valve and two indicators, this is simple.

The next truth table, which is much more complicated, 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 states of the indicator lights for each input position. A light which is supposed to be lit is marked "ON", while a light which is supposed to be off is marked with a "-". Clicking on the left side of any row of the truth table will display the positions of the gates and the states of the indicator lights which correspond to that row. Since this example is properly wired, each indicator light is ON every time the table says it should be.