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We’ve already spent time looking at the TTL logic family, even though I only introduced the first part in the family.
The series was very successful and very popular, to the extent that when companies want to manufacture new logic parts with different electrical characteristics, they typically create new families of the very same functions and pin-outs.
There are some other family offshoots; this list is those flavors of currently in production from one manufacturerTexas Instruments.
For a long time, the original series was the best choice as a general-purpose logic family, and the family had more members than any of the offshoot families. However, if you were picking a single “best” logic family today it would probably have to be the 74LS00 family.
This family arguably has the widest manufacturer support and range of available line of components. The 74LS00 family improves technically on the family in two key areas: The only limitation of the 74LS00 family compared to the family is that the 74LS00 family can only source half as much current.
74LS – Digiware Store
However part of the current reduction in the 74LS00 family was on inputs, such that a typical 74LS00 family part can drive twenty LS loads that is, one output can fan out to twenty inputs within the 74LS00 family; compare this to the family, which could only drive ten dstasheet in the same family. The five standard two-input logic gates in the 74LS00 family are datahseet in the table below.
Many of the basic logic functions are also available in parts with “open collector” outputs. This is a modified output circuit within the chip which sinks current to indicate a logic “0,” but turns off to indicate a logic “1. These can be used in cases where the final output of a logic function only needs to sink current; a later section will give examples where this can be used.
Open collector gates have an interesting property, which is that they can be ANDed together just by wiring several satasheet together. Ordinarily this is a Bad Thing because outputs could drive into each other, but for open collector it’s fine: If all outputs are off, the combination of signals is also off. Since open collector is often used where “0” is the active state, this is also called a “wired-OR” connection, referring to an active-low OR function.
Since a regular TTL logic gate sources current for “1” and sinks current for “0,” a signal where neither of these is happening the signal is “off,” like in open collector outputs can actually be thought of as being a third state.
Parts which are capable of having their output in any of these three states are considered to have “tri-state” outputs.
Tri-state outputs allow multiple gates to “take turns” driving an output signal. You might signal a logic function to drive its output high or low as the function may determineand then later tell that function to turn off its output so that a different function may drive the signal instead. As long as other logic determines that no more than one output is driving the dagasheet signal at the same time, all is well.
This method of combining multiple outputs on the same signal path is used heavily in microprocessor circuits, where several different memory or input circuits may be sharing a common set of data signals.
A tri-state buffer or inverting buffer looks like a regular buffer or inverter, except there is an additional “enable” control signal entering the gate. When the enable is “1” the buffer is driving the output; when the ratasheet is “0,” the output is turned off “tri-stated”. Because dattasheet buffers have many applications in microprocessor circuits, there are several useful chips available which are designed to function on eight data signals bits at one time.
The truth tables for the 74LS and 74LS introduce a new symbol, “Z,” which represents the 74le540 state.
The correct ratasheet for this is “high-impedance,” often shortened to “high-Z. The eight-bit buffers 74LS and 74LS are functionally equivalent to the widely used 74LS and 74LS; however the ‘ and ‘ have a more confusing pin-out to trip the unwary student.
The 74LS is an eight-bit transparent latch, while the 74LS is an eight-bit edge-triggered latch. Each has an output enable pin that can tri-state all the outputs.
74LS540 数据手册 ( 数据表 ) – Motorola => Freescale
These chips 74lls540 convenient pin-outs separating the input side from the output side. These four-bit counter parts can count up or down. Outputs Q0 through Q3 are a binary value, where Q0 is the least signifigant bit and Q3 is most significant. There is a CLR input to reset the counter to Although less common, there are decimal versions of these chips. The 74LS is identical to the 74LS except it only counts from 0 datashert 9 through binary ; the 74LS is similarly identical to the 74LS except for the limited range.
You might use these variants if you want to count in decimal digits rather than pure binary.
There are plenty of other useful chips in the 74LS00 logic family, but these are some of the most useful building blocks. At the very least, they provide you with a lot of different options for creating complex logic functions and circuits. Along the 74ls450 I’ll also cover some additional chip characteristics that can be useful in complex designs. The 74LS00 family We’ve already spent time looking at the TTL logic family, even though I only introduced the first part in the family.
Two-input logic gates The five standard two-input logic gates in the 74LS00 family are listed in the table below. There are six of these on a single chip. Latches The 74LS is an eight-bit transparent latch, while the 74LS is an eight-bit edge-triggered latch.
Schematics and Components Previous: This section introduces a number of useful digital logic chips for use in a variety of projects.