circuitry". The function of this pulse stretcher is to "stretch" the small differential pulse generated by the comparitor long enough for the next pulse to arrive.
   
Servo evolution - Originally our servos were quite large, due mostly to the fact that the electronics portion was constructed of discrete components (individual transistor, resistor, & capacitors) and the unavailability of high performance small motors.

With the increasing popularity of RC, manufacturers began developing integrated circuits and motors especially for our hobby. What we now have is extremely small, yet reliable, servos (and radios) at a lower cost.       

4 wire servos - Although many of you may never have seen one, early servos used 4 wires instead of 3. These servos are not really usable on todays receivers.

These early servos used a 2.4 V  motor versus the current 4.8 V. The battery packs was center taped and the motor driver circuitry did not reverse both motor leads. The center tap of the battery was attached to one terminal of the motor and the motor driver circuitry then switched the other motor lead to one end or the other of the battery (achieving the same end results).

Although these servos could be retrofitted to one of todays radios by center taping the battery pack, it is

hardly worth the effort as their output torque was low.
 
Negative pulse servos - All of todays major manufacturers use similar integrated circuits in their servos which  use a "Positive" input pulse. This means that the input pulse goes positive for the duration of the input signal.

In the early days, some manufacturers used negative input pulse servos. Negative input servos cannot be used on a positive pulse system or vice versa.

Interchangeability - To my knowledge, all of todays servos are interchangeable between manufacturers  if you alter the plug. If you alter the plug does not mean just changing it to fit in the hole, but also ensuring that the three wires are correctly oriented. Specifically, Airtronics puts the + power lead (red) on the outside, while JR, Futaba, & Hitec place the + power lead in the center. To use an Airtronics servo on JR, Futaba, or Hitec you must alter the position of the power leads in the plug. Likewise to use JR, Futaba, or Hitec on an Airtronics they must be altered to the Airtronics configuration.

Although there is a slight design difference in the connectors (See Figure 2), all of them are now using the .1 inch pin spacing, and with a little rework the JR, Hitec, and Futaba plug can be made to cross fit between radios. Looking at the Futaba plug from the end you will find a tab coming out the side which is used to "key" it into the socket. If this tab is removed, and the edges are beveled, then the plug will fit into JR or Hitec. Watch out because you  no longer have the keying tab to prevent you from plugging this in backwards in the Futaba. The JR or Hitec plug, with the above precaution, will fit into a Futaba without change.

Reversing Servo Rotation - With todays servo reversing transmitters, you rarely have a need to reverse the direction of a servo. In early transmitters there were no servo reverse switches, and you had to be careful installing the servos and push rods to ensure that the control surfaces moved in the right direction.