You are correct that all EM machines with a score motor will have one or more switches that open while the motor is running. During the time that these switches are open (in other words, while the motor is running), a variety of scoring and other functions are disabled.

Many game functions require the use of the score motor because they need a sequence of pulses to complete an operation - for example, 30 points for a target (3 pulses to the 10 point score unit) or 500 points for a kickout hole (5 pulses to the 100 point score unit, followed by a pulse to the coil that ejects the ball from the kickout). The score motor provides these timed sequences of pulses during each score motor cycle.

All of these types of functions depend on being initiated only when the score motor is at the home position, for two main reasons.

First, if the function is initiated when the score motor is, say, half-way through some other operation, then some of the pulses needed for the newly activated function may not occur. This will happen if these pulses are generated during the first half of the score motor cycle, which has already passed.

Second, most game functions that use the score motor are designed to operate one at a time. Unintended behavior (some of it bad) could occur if two or more arbitrary game functions are active at the same time.

Since most EM score motors take about 3/4 of a second to complete one cycle, it's possible for the ball on the playfield to hit multiple switches, each of which could activate some game function that requires the use of the score motor. The locking out of these game functions while the score motor is busy ensures that these functions will be activated one at a time.

- TimMe