<<-- Back Repairing the Dec/RA Drive Next -->>
Return to Bruce Johnston's homepage
Beginning at the Beginning -- part 2
Remember that because of the slight offsetting of the mask slots, the light will be detected by each light detector just a bit out of step from each other.
If we graph the light as a slot of the disk passes each sensor and mask, it would look the following:
As the slots in the disk begin to align with the slots in one of the masks, the light intensity would begin to increase, as in the black curve. At that time, the slots in the disk haven't begun to align with the slots in the mask for the other sensor.
Time passes (very little time!) and the light represented by the black line continues to increase. Just as the light in the first sensor reaches a peak, the slots in the disk begin to align with the slots in the mask for the second sensor, so the amount of light represented by the red curve, begins to increase.
After a period of time, the light in the first sensor has just become fully blocked again, and the light in the second sensor reaches its peak at that same time.
If the light falling onto the sensors is converted into a voltage, then this plot of light intensities would also represent the voltages created. That's exactly what DOES happen.
The next step that takes place, is that the voltage from each of the light sensors is fed into a circuit called a 'comparator', and if the voltage is below a certain level called a 'trigger level' ..... which we'll assume to be about half way between zero and maximum possible....... then the circuit will give us a 'low' voltage out. (Zero volts, for all practical purposes.) If the voltage is above that level, it will give us a 'high' or plus voltage out. When set ideally, the output will be 'low' for about as long as it is 'high', as shown above. This shape is traditionally called a 'square wave' for obvious reasons (I hope!).
The drawing above shows both of the signals from the light detectors after having been converted to a square wave by their respective comparators. In this image, it's evident that the two pulses are shifted in time from each other. The rising edge of the red image is happening right in the middle of the center of time of the black one. Since in electronic terms, one full signal is considered to be 360 degrees before it repeats itself, we say that the two signals are 90 degrees out-of-phase.
If the motor is running in the opposite direction, then the pulses will be generated with the opposite relationship, where the red square wave now leads the black one, as shown above. Using this relationship of the square waves ... which square wave leads which ... the electronics on the main logic board can detect in which direction the motor is turning. The frequency of the square waves indicates the speed at which the motor is turning.
Loose ends ........
Before we leave the mechanical area of the gear train, we'll show the two differences between the R.A. gear train and the DEC Gear train.
On the R.A. gear train only, there is a small magnet mounted inside a hole on the worm shaft. You can just barely see the tip of it protruding from it's mounting hole. Also, glued to the housing, right next to the magnet, is a magnetic sensor, called a "Hall Effect Sensor" that will detect when the tip of the magnet passes it.
During the power on sequence, after the proper direction and speed of the motor is detected, this magnet is then detected. It's used primarily for knowing when to begin a "Periodic Error Correction" training session, and when to begin to play it back, so that the two places are always at the same point.
The second difference has to do with the cable coming from the main logic board to the assembly. The far end of the cable on the R.A. assembly has a seven pin connector that attaches to the mother board directly, shown above. On the DEC assembly, the far end is a standard 8 pin RJ45 type of male connector that plugs into a socket on the right fork arm.