Internals of RCX Output Ports and Actuators

H-Bridge

For each of the three output ports of the RCX there is an H-Bridge that controls the current flow through the actuator connected to the output port. With transistor ONE and transistor THREE of Figure 1 enabled the current will flow in one direction through the motor and make it turn in one direction. The oppositite direction is obtained by enabling TWO and FOUR. If none of the transistors are enabled the motor is allowed to run freely (float) and enabling TWO and THREE will brake the motor. This is the principle behind the control of the current state through the bitfields of the output port device register: Each of the four bitcombinations is decoded into enabling signals to transistors in a H-bridge.

Figure 1 John K. Bennett, LegoRobot Hardware,
Spring 2002, (18.9.2007), contains this drawing of the general form of an H-bridge circuit.

Jim Brown's Brief H-Bridge Theory of Operation is a tutorial on H-Bridges that also contains references to circuits that implement H-Bridges. A brief description can also be found in Fred Martin, Robotics Explorations, Chapter 4, Motors, Gears, and Mechanism.

Motors

There are several types of LEGO motors. Each with there own characteristics with respect to power consumption, rotations per minut etc. Philippe Hurbaine has made very detailed investigations of LEGO motors.

Figure X Philippe Hurbain, LegoŽ 9V Technic Motors compared characteristics, 6.2.2004, contains photos, circuit
diagrams and detailed operation of the different motors.

Figure X Philippe Hurbain, LegoŽ Technic Motor (43362) internals, 6.2.2004, contains detailed LegoŽ Technic Motor
photos and lots of information about different versions of this motor.