One important consideration when building when building adders is the direction each input rotates. Coupled motors are usually powered from the same source, resulting in an identical directions match, the two motors can work together or against each other. The latter case is obviously undesirable, as it results in decreased torque and speed.
All the examples shown above have motors running in the same direction; however, in some cases, it's convenient to have the motors oriented so that they run in opposite directions. For the adds to work properly in such a case, we need to reverse the direction of one motor, either by powering it from a power source with the opposite polarity or by connecting it to a shared power source through a can reverse the polarity of a motor by simply rotating its wire connector 90 degrees.
Figure 18-8 shows two examples of adders that need one of their motors reversed