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74LS属于TTL类型的集成电路，而74HC属于CMOS集成电路。
LS、HC 二者高电平低电平定义不同，HC高电平规定为0.7倍电源电压，低电平规定为0.3倍电源电压。LS规定高电平为2.0V，低电平为0.8V。 带负载特性不同。HC上拉下拉能力相同，LS上拉弱而下拉强。 输入特性不同。HC输入电阻很高，输入开路时电平不定。LS输入内部有上拉，输入开路时为高电平。
（1）74LS系列是“低功耗肖特基TTL”，统称74LS系列。其改进型为“先进低功耗肖特基TTL”，既74ALS系列，它 的性能比74LS更好。
（2）74HC系列，它具有CMOS的低功耗和相当于74LS高速度的性能，属于一种高速低功耗产品。
（3）上述两者的工作频率都在30mHz以下，74ALS略高，可达50mHz。
（4）但它们的工作电压却大不相同：74LS系列为5V，74HC系列为2~6V。
（5）扇出能力：74LS系列为20，而74HC系列在直流时则高达1000以上，但在交流时很低，由工作频率决定。
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LinkTo：http://forums.parallax.com/showthread.php/79113-74LS595-verses-74HC595
Question：
I have been using 74LS595 as LED Display drivers at 15ma per 595 output.· These 595's work with the SX28.· I previously noticed in the Jameco Catalog a vast price difference in the HC595 $.62 and the LS595 $4.55 .· The pinouts are the same so I ordered a few to see if they work, and they did.· However I lack the expertise to determine from the data sheets what the difference is and if there are any drawbacks to using the HC verses the LS for projects.
Answer：
Short answer to your question; it is fine to use the HC (high Speed CMOS) series logic with the SX.· This is one of the few times in life you actually do get more for less.· A quick perusal of the data sheet shows that the devices are roughly equivilent, but the HC version has a much higher allowable clock frequency (100MHz vs 20MHz), this is good as far as the super fast SX is concerned.
As others have stated, the LS series is based on TTL (total transistor logic) which means that·thay are made with·bipolar transisitors (BJT).· BJT's were·one of the first transistors used in ICs.
The HC series use CMOS (complementary metal oxide semiconductor) devices·that are a later generation transistor that use, as the name denotes, metal over an oxide as the gate in the transistor (nowdays it's polysilicon over an oxide).· The advantage to these devices·is that the input impedance is much higher than the BJT.· This translates into much lower current and power usage.
As a simple rule (which has exceptions), CMOS is used in logic devices where the low power·needs are good, and BJTs are used in higher power devices (switching, power management apps., etc).· There are also BiCMOS devices that put both CMOS and BJTs on the same die and use the best atributes·of·each device.
The major reason for the price difference is as Paul said, supply and demand.· Well actually, supply.· The HC series has about 8 bazillion made per year, while there are only a piddling of the LS series produced per year.· So use the HC series and laugh at the fact that you are getting more for less.