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A Analog-to-Digital Converter-A device for converting a volt age level that is the measure of some physical condition into a digital word. ( An 8-bit ADC generates an 8-bit word; a 12-bit ADC generates a 12-bit word. The encoding is more precise as more bits are used.) Asynchronous-Clearing a circuit to all zeros or presetting to some condition independently of the clock. B Bit-A bit is one character of a digital word. It can be either a 1 or a 0. The position of the bit in the word usually determines its significance. The first bit is often a sign bit, with 0 representing a plus sign and a 1 representing a minus sign. Blanlang Input-A control input for decoding circuits. Boolean Equation--Mathematical expression of logic relation ships. The Boolean equation F = A • B means function " F" is true when "A" is true AND "B" is true. ( Thus, F will be a 1 if both A and B = 1; for all other combinations of values for A and B, F will be a 0.) Buffer-A device which has sufficient fan-out to drive all the internal gates while presenting only a unit load ( or less) to the driving source. Sometimes, one or more of the inputs--for example, the clock signal of a shift register or counter-is required to drive a number of gates. To prevent these multiple tasks from putting too large a load on the driving source, the input signal is fed to a buffer stage-often an inverter. Byte-A byte is a group of bits ( usually 8 bits) . Clear-A circuit is said to be cleared when it is set to all 0s. C Clear Input--The Clear Input generates the Clear function. Clock--Also called Clock Pulse and Clock Signal. It can be derived from a crystal oscillator, a multivibrator, or other type of oscillator. Most digital systems use a clock pulse as a pacing signal. When the clock goes high, the gates and other elements respond as the circuit requires. When the clock goes low, the gates may go to a new state. On the next high clock pulse, the gates respond again, going to a new state if required. The clock signal must repeat often enough to allow the system to complete its task in a reasonable time, but it must not occur so fast that circuits cannot respond properly before the next transition occurs. D Digital-to-Analog Converter--A device for converting a digital word into an analog voltage level. DIP-Abbreviation for dual-in-line package, it is usually mentioned as a DIP package. Its connection pins extend down from the body of the IC package in two parallel rows. Duty Cycle-The ratio of on-time to total cycle time is the duty cycle: On time Duty cycle - On time + Off time E Enable-An input which, when true, allows the circuit to function. Fan-Out-The number of additional gates one gate can drive. A typical TTL gate has a fan-out of 10 unit loads. If a gate is required to drive more than its maximum fan-out, a buffer gate can be used as one or more of the loads to increase the fan-out. Flat pack-An IC package. Its connection pins come straight out from the sides of the IC package, giving a flat, pancake structure. G Gate-The simplest logic circuit is called a gate. Its output voltage will be high or low depending on the state of the inputs and the type of gate. I IC-Abbreviation for integrated circuit. It is usually a monolithic circuit. Inhibit-An input which, when true, prevents the circuit from functioning. Input Buffer-See Buffer. Input Clamping-Use of input clamping diodes to prevent ringing. They allow the input signal to go positive but act as a low-impedance short circuit when the signal goes negative; thus, they effectively clamp the input at 0 voltage or above. Input Gating-Input gates keep a circuit from responding un less a special input condition has been satisfied. A clock, strobe, or other control signal may be used to activate the input gate. Inverter-A gate in which the output signal is the inversion of the input signal. Least Significant Bit--The bit in a number that is the least important or having the least weight. LSI-Abbreviation for large-scale integration. A chip containing more than 100 gates. M Monolithic-In electronics, this refers to a circuit built on one chip of silicon. Most Significant Bit-The bit in a number that is the most important or that has the most weight. MS/-Abbreviation for medium-scale integration. A chip having from 10 to 100 gates. Multiplexing-The process of combining the data from a number of sources into one stream of data. The reverse process of sorting out multiplexed data is called demultiplexing. N Negative-Edge Gating-The circuit responds as the control signal goes from high to low. Negative Logic-See Positive Logic. O Open-Collector Output-A TTL gate with only one output transistor instead of the traditional 2-transistor totem pole output. P Parallel Data-All the bits in a word are available simultaneously on a parallel front. Positive-Edge Gating-The circuit responds as the control signal goes from low to high. Thereafter, the circuit is quiescent until the clock first goes low, then high again. Positive Logic-A positive voltage ( 2.0 to 3.6 volts in TTL) is a true signal, and a zero or negative voltage is a false signal (a not-true signal). In negative logic, the more negative of two voltages is true, and the more positive is false ( not-true), even if both voltages are negative. Preset-A procedure for loading a circuit, such as a counter, with certain data prior to going into operation. The circuit will then start from the preset condition. Propagation Delay-The time required for a circuit to go to the state demanded by the inputs. Alternatively, the time required for a signal to travel from one point in a circuit ( or signal channel) to another point. Pull Up/Pull Down-When an open-collector output stage is in its high-impedance state, it is generating the equivalent of a high logic level, except that it cannot supply a positive voltage to a load. By connecting a resistor between the output collector and the usual +5-volt supply, the collector voltage is pulled up to a positive voltage when the transistor is off or in its high impedance state. When the transistor is on, the collector volt age falls to the saturation voltage of the transistor ( nominally about 0.4 volt). A pull-down resistor, on the other hand, is used to pull a circuit to ground in the absence of a driving signal. Q Quiescent Dissipation-The power consumed by a device when it is at rest. R Reset-Setting a flip-flop to the Q = 0 state. Also applies when any circuit is placed or driven to its normal starting condition. Ringing-When a circuit is driven from low to high or high to low, stored energy may cause the signal level to oscillate briefly between the high and low levels, much as a bell rings for a short time when it is struck. Ripple Through-When two binary numbers are added, the two least significant bits can produce a carry bit that must be added to the next two least significant bits, which in turn may produce a carry that will affect the next two bits, etc. The correct output cannot be obtained until all the carrys have rippled through. S Serial Data-The data is available as a series of bits occurring one after the other in a single file. Set-Placing a flip-flop in the Q = 1 state. S/-Abbreviation for small-scale integration. A chip containing up to about 10 or 12 gates. Substrate-The silicon chip on which transistors and resistors are formed or deposited. Synchronous-Running a circuit that is normally driven by a clock at a constant frequency synchronous with the clock. T Tri-State Gate--A special gating circuit that can be driven to one of three states: 0, 1, or high impedance, which is the equivalent of OFF. Truth Table-A table showing the various combinations of input conditions and the logically possible response of a circuit. U Unit L out--Each input of a standard TTL gate is called a unit load. When an input circuit requires more current than a standard unit load, it is rated at 2 or 3 unit loads. If the input circuit requires less current than a standard unit load, it may be rated as a fractional unit load. Data sheets for every TTL device in all families will show the loads they present to driving circuits. A gate should not be asked to drive more unit loads than its maximum rating. V VLSI-Abbreviation for very large-scale integration. A chip containing more than 1000 gates. W Word-A digital word is composed of a group of bits-and often a group of bytes. Digital words can be any length, but they are usually multiples of 2: 8, 12, 16, 24, 36, 64 bits, etc. Z Zero Suppression-Suppressing unnecessary zeros in a number register. The zeros may lead or trail the significant figures of the number. Prev: GUIDELINES FOR SYSTEM DESIGN Next: (none) Guide Index : Transistor-Transistor Logic (early 1970s)
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