ARMY TM    5-6115-612-34 MARINE CORPS TM 6115-34/8 AIR FORCE TO   35C2-3-471-2 NAVY AG-320B0-MME-000 10-1. PURPOSE  AND  FUNCTION  OF  ENGINE AND  GENERATOR  CONTROLS  AND  INSTRU- MENTS.  (cont) Functional Theory of Operation. (cont) b. These signals are then gated through a switch card exclusive OR gate and then buffered before being applied to the microprocessor. The signal from the EGT thermocouple is an analog voltage signal that varies in proportion to the temperature sensed by the EGT probe. It enters the EECM on connector P5 and is applied to one side of a resistance bridge on the microprocessor card. The bridge is also supplied with a refer- ence  voltage. The output of the bridge is  an analog voltage that varies according  to  the  temperature-induced voltage flowing through the EGT. An amplifier converts the bridge output to a very linear O to +5 Vdc signal that is again proportional to the temper- ature  sensed  by  the  EGT  thermocouple. Because the microprocessor cannot accept an analog input, this signal is changed to an 8-bit digital signal in an analog-to-digital converter. The resulting output is applied to the microprocessor as the digital equiva- lent of the analog temperature sensed by  the  EGT  thermocouple. Reception of this signal initiates an automatic startup sequence that is controlled by the stored software program in the microprocessor. The series of steps in this sequence is described  below. (a) Ignition and engine start. As the fuel pump is activated, the microprocessor checks the speed signal and EGT thermocouple line inputs to ensure that the engine is not already running. If not,   an ignition enable signal is output from  the  micro- processor, buffered and level shifted, and used to turn on a driver transis- tor. When the transistor turns on, it generates the igniter drive level to the engine so that electrical ignition power is supplied in preparation for starting. Next, microprocessor soft- ware generates a starter enable signal which is buffered, sent to the switch card, and modified in a buffer/level shifter circuit. The  resulting  output of this circuit turns on a driver transistor which  then  generates the starter signal. Reception of this signal by the engine causes the starter to engage to start the gas turbine engine. (b) Fuel pump control. The microprocessor checks the speed signal input as described in (2) above. If there are pulses on this input, meaning that the engine is turning, the micro- processor generates a signal on one of its four status/control outputs to the switch card fuel valve enable line. The signal activates a driver circuit, placing a control signal on the fuel 10-3