STATIC CONTROL OF MACHINES 247

STATIC CONTROL OF MACHINES 247

STATIC CONTROL OF MACHINES 247

static control of machines 247

static control of machines 247

STATIC CONTROL OF MACHINES 247 In the AND gate circuitry when any of inputs A , B or C is low the voltage at the base of transistor Q1 is low (0) as most of the + 5 volts drops across resistance R1 . Transistor Q1 is therefore in cut off state . Thus voltage at collector of transistor Q1 i . e . , at point X is high and this voltage is coupled to the base of transistor Q2 . Thus base of transistor Q2 is at higher voltage with respect to its emitter . Transistor Q2 therefore conducts and is in saturation state . Voltage at point Y on the collector side of transistor Q2 is therefore low . When all the inputs A , B and C are + 5 V high i . e . , + 5 volts , the voltage at the base of transistor Q1 is also high and therefore transistor Q1 conducts , and the voltage at point X or at the base of transistor Q2 is low , thus it does not conduct . If Q2 is cut off then voltage at point Y is high (+ 5 V). Thus it is clear that output Y will be high (+ 5 V) only when all the inputs are also high . Now , referring to the OR gate circuitry , when any of the input is high , the base of transistor Q1 is also high with respect to its emitter , as a result the transistor conducts . This leads to low voltage at point X and thus transistor Q2 remains cut off . As Q2 is cut off , the voltage level at point Y is high . If all the inputs a e low then Q1 will be cut off and Q2 will conduct . This gives low voltage at Y . Thus we see that this circuit fulfills the conditions described for an OR gate . In practice the high and low values for static devices are not rigidly fixed at + 5 V or 0 volts . For example with 7400 series TTL devices any input voltage between 0 and 0 . 8 V is considered low and any input voltage between 2 and 5 V is considered a high input . This is so ,

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