(3)  
When the mode annunciator test is actuated either the upper or lower mode legends appear depending on the test switch position selected.

5.  Fuel Summation Unit
A.  Voltages, proportional to the weight of fuel in each fuel tank, are combined and amplified in the fuel summation unit. An ac voltage proportional to total fuel on board the airplane is routed to the PDC (10v ac for full tanks). The unit also develops an ac reference voltage (7-5 - 12.5 v RMS) used in the computer to improve accuracy of the total fuel quantity signal by modifying the level in accordance with any line voltage variations (Ref 28-41-01).
6.  Auto Throttle/Automatic Flight Control Systems (GJ B-2509 and B-2510)
A.  The PDC provides the Auto Throttle and Flight Control computers with data via a digital bus. The data includes EPR, EPR limits, target air speed, engaged flight mode, outside air temperature, total air temperature valid signals, auto throttle arm, and BITE signals. The A/P PDC SPD mode commands the aircraft to fly the target airspeed from the PDCS values. In A/T EPR modes, the A/T flies to PDC EPR values.
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B.  The PDC has a digital receiver that receives data from the AFCS mode control panel (MCP). The PDC signal receives an auto engage signal from the MCP, engaging the PDC automatically for GA or TO modes. This includes driving the EPR indicator command bugs to appropriate EPR command values. The other signal from the MCP panel is the PDC ON signal, which causes the PDC to perform in the same manner as if the ENGAGE switch were pushed.
7.  Mach/Airspeed Indicator (Fig. 1)
A.  A target airspeed command bug installed in each pilot's electric mach/airspeed indicator (Ref 34-12) is driven by the performance data computer when the command bug is engaged by positioning the CDU mode selector to the desired flight mode and pressing the ENGAGE key or on airplanes with AFCS the PDC switch on the AFCS control panel to the ON position. The command bug then drives to the speed displayed on the CDU. The command bug can be disconnected from the performance data computer and manually set by pulling out the manual bug setting knob and rotating the knob.
B.  The command bug stows at 440 knots when not engaged with the performance data computer and is not manually set. An airspeed cursor INOP flag is displayed if a PDC failure or invalid signal is present or a servo failure occurs.
8.  Engine Pressure Ratio (EPR) Indicator (Fig. 1)
A.  An EPR command bug in each EPR indicator (Ref 77-11-0) indicates target thrust settings when engaged to the performance data computer by positioning the CDU mode selector to the desired flight mode and pressing the ENGAGE key. When the EPR set knob is pushed in, the lower digital readout and command bug are set by signal from the performance data computer. Pulling out a set knob to manually set the command bug, disconnects the PDC input and causes an "M" flag to appear on the dial face. A signal proportional to EPR is routed from each indicator to the PDCS computer for use in computation.
B.  Failure of an EPR transmitter or indicator results in a failure flag appearing over the upper digital readout. Failure of the PDC or indicator servo drive circuits results in a failure flag appearing over the lower digital readout. The command bug stows at 1.00 if the EPR indicator is not engaged and is not manually set.
CAUTION:  WHEN THE PDCS IS INOPERATIVE OR TURNED OFF, SET THE EPR COMMAND BUG SET KNOB TO THE MANUAL MODE (PULLED OUT). IF THE COMMAND BUG SET KNOB IS IMPROPERLY SET TO THE SERVOED MODE WITH THE PDC INOPERATIVE OR TURNED OFF, EXCESSIVE WEAR TO THE COMMAND INDEX CLUTCH WILL OCCUR.
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9.  Functional Description - Input
A.  The analog input signals are applied to the PDC discrete signal conditioners and the isolation input transformers (Fig. 3).
(1)  
The ac synchro analog signals, such as airspeed and altitude, are applied to the input isolation transformers and converted to X-Z, Y-Z components or X to common, Y to common components. These component signals are applied to two pairs of synchronized FET switches that allow only the ac peak values of each signal to pass to a holding capacitor. In this way the ac synchro input is converted to do. These do signals are multiplexed under control of the digital I/O circuits and sent to the analog-to-digital (A/D) converter. In the A/D converter, the do voltages are converted into timing pulses through the use of a comparator and ramp generator. The comparator generates one timing pulse when the ramp input voltage equals a reference voltage and one timing pulse when it equals the do input voltage. These timing pulses control a digital counter in the digital I/O circuits, where a digital word is generated corresponding to the length of time between timing pulses and, therefore, the do input voltages.
　
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