Figure 6-4. Buffer Unit Functional Block Diagram

6-14 ARMY TM 5-6675-238-14 MARINE CORPS TM 08839A-14/1 Figure 6-4. Buffer Unit Functional Block Diagram transistor turns on, which causes the output transistor to be turned off, producing an output voltage level greater than +3.5V. For darkened inputs (no hole), the input transistor is turned off causing the output transis- tor to produce a voltage output of less than 0.8 VDE, The light level at which the photocell amplifier switches from off to on is determined by the setting of the input bias potentiometer for each amplifier input. The amplifier outputs are parallel transferred via con- nector J2 to external using equipment. Amplifier out- puts are also made available at test points located on terminal board TB1. c. Drive System and Control. The tape drive con- trol system consists of pulse rate generator and logic plug-in circuit card, forward/reverse control plug-in circuit card, and two motor driver control plug-in circuit cards used for drive and control of step motor Bl. Also included is servo amplifier plug-in circuit card which accepts constantly varying DC inputs for bidirec- tional tape spooling control of spooling motors B2 and B3. Spooling disable is provided by a switch mounted on the front panel, This switch removes the +28V from all internal circuits, disabling the spooling and stopping functions. This allows removal and installation of the tape while external power is applied to the tape reader. d. Step/Run Control. When a step command is applied to the pulse rate generator and logic plug-in circuit card, a negative-going output step trigger is generated. This action continues at the external step rate, provided the run input is at 0V. The step trigger is applied to both motor driver control plug-in circuit cards. In addition, a reverse signal is developed and imported to the forward/reverse control plug-in circuit card. When the external forward input control signal is at 0V, the output signal levels of the forward/reverse control logic are the same as the input, and the forward sequencing condition exists, When the input control signal is at + 5V, the output signal levels are inverted and the reverse sequencing condition exists. The alter- nating output levels are applied to the set and clear inputs of the flip-flops on each motor driver card, The flip-flops are constrained by the input gating action and provide the correct sequence of output pulses to step the stepping motor in the forward direction (counter- clockwise sprocket rotation) or in the reverse direction. The flip-flop output from each motor driver card and the step trigger pulse provide control and drive to motor driver output circuits, The motor driver outputs are connected to the windings of step motor Bl. e. Forward/Reverse Control. In the forward mode of operation, power is applied to servo feedback poten- tiometers R2 and R3 in a direction to supply a servo drive input voltage to an associated servo amplifier circuit on servo amplifier plug-in circuit card. The input is proportional to the amount of tape on the reels as controlled by the movement of each tape sensing