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Electronic logic gates can be produced using different production technologies.
In practice we call integrated circuits with logic gates produced using a certain production technology as belonging to a logic family of devices.
The beginning
The list of logic families can be divided into eight main categories (in between brackets is the conventional abbreviation used for the logic family as is often used in the code printed on the device).
here are the main groups of logic families roughly in order of their appearance on the market (oldest first):
Of these eight only the last five are currently still in use.
Emitter-Coupled Logic is only rarely used for special very high speed applications because of it's exorbitant price, and NMOS and PMOS logic families are mainly used in VLSI (Very Large Scale Integrated Circuits) such as CPU's and memory chips which fall outside of the scope of this page.
Simple "building block" logic gate IC's (The "glue" used to interconnect the larger more complex chips) are responsible for the plethora of all the other logic families in use, and these are based on the Transistor-Transistor Logic and the CMOS and BiCMOS logic families.
TTL
Lets start with the oldest family, the Transistor-Transistor Logic family, which uses bipolar transistors.
Here is a list of TTL logic again roughly in order of their appearance on the market (oldest first):
- Transistor-transistor logic (TTL)
- Schottky TTL logic (S)
- Low Power Schottky TTL logic (LS)
- Fast TTL logic (F)
- Advanced Low Power Schottky TTL logic (ALS)
these logic devices normally use the following naming convention:
First a two or three letter prefix which indicated the manufacturer of the device, immediately followed by a two figure secondary prefix, where '74' means that it is a conventional device and '54' means that the device is for military use.
Then up to three letters describing the logic family the device is from (for example LS for 'Low Power Schottky TTL')
Then a number of figures describing the function of the device. There are hundreds of different devices in each family but when this number is the same the function and pin-out of the chip is the same for each of these TTL devices.
For example SN74 ALS 245 means this is a device made by Texas Instruments, it is a conventional (not a military) TTL device, and its is a bi-directional eight bit buffer.
However TTL has a problem, it dissipates a lot of energy and it's speed is slow (compared to current devices).
LS TTL
The German physicist Walter H. Schottky formulated a theory predicting the Schottky effect that lead to the Schottky diode and later the Schottky transistors. Schottky transistors have a much higher switching speed than conventional transistors leading to faster switching gates. The only problem is that gates built with Schottky transistors also used even more power than normal TTL. This was later alleviated somewhat with the low power (LS) version which drew much less power than the original Schottky logic, but still much more than TTL. It was also slightly slower. But finally with the introduction of Advanced Low Power(ALS) these problems were overcome. A faster logic family called 'Fast TTL' (F) was also introduced that was even faster than normal Schottky TTL.
CMOS
Meanwhile, because of the still relatively large power demand of LS-TTL, another technology gained acceptance as a low power alternative. This technology did not use regular bipolar transistor, but used Field effect transistors instead. Because the technology used a complementary pair of FET's (a N-MOS and a P-MOS FET) it was called CMOS (Complementary Metal Oxide Semiconductor logic). In contrast to TTL logic, CMOS uses almost no power, and then only when its output gates switch state.
The first family of CMOS logic carried the prefix 'CD' followed by four figures of which the first was a four it was known as the CD4K logic family of logic gates, also known as the 4000 series.
Unfortunately CMOS logic is a lot slower than LS-TTL, and another practical problem is that the organization of the devices, (which functions were available and which pins on the chips were used for each function) was totally incompatible with the TTL family. For example, A chip containing four 'NAND gates' from the CD4K family was called a CD4011, while the same functionality in the TTL family could be found in the SN7400 device.
So there was no relation between the numbering of these CMOS and TTL devices at all, and also no relation between the functions that were available for each family. Even when a similar device was available the input and output pins of each chip where at completely different pins. This made it impossible to use a CMOS device as a drop-in replacement for a TTL device. Which was a very unfortunate decision of the designers of these first CMOS devices.
Lowering the power supply voltage
One very important feature of CMOS chips is that they work with a broader range of power supply voltages. While the TTL IC's all work only with a power supply voltage of 5 Volt (plus or minus 0.5 Volt), CMOS works with much higher, and also with much lower supply voltages. This fact was also shared with the logic used in VLSI devices (N-MS and P-MOS) such as microprocessors. Because of the fact that these devices used more and more transistors, and used increasingly higher clock speeds they did use more and more energy, creating so much heat that it became a problem. By lowering the supply voltage this trend could be countered. By lowering the power supply from 5 Volt to 3.3 Volt they could cut the dissipated power by almost 60 percent (dissipated power is in proportion to the square of the power supply). Newer CPU's lowered their power supply voltages even further. A popular choice is 1.8 Volt.
HC logic
Because of the incompatibility of the CD4000 series of chips a new standard emerged which combined the best of the TTL family with the advantages of the CD4000 family, it was the 74HC family of devices which used the pin-out of the 74LS family and an improved version of CMOS technology. And it could be used together with other logic devices which used 3.3 Volt power supply's (and thus 3.3 Volt logic levels). In designs that used 5 Volt power supply's and devices that used TTL logic levels.
The logic level problem
There was however a problem when combining CMOS and TTL logic. CMOS inputs inherently recognize other electrical voltages as a valid '0' (low voltage) and a valid '1' (high voltage) than TTL does. Where TTL needs to see a voltage lower than 0.8 Volt to recognize a valid a '0' and voltages above 2.0 Volt to recognize a valid '1' (all voltages in-between 0.8 and 2.0 are 'forbidden' as logic levels) CMOS devices (working with a 5Volt power supply) recognized all voltages below 1.5 Volt as '0' and all voltages above 3.5 Volt as '1'. Additionally a TTL output pin does not rise above about 2.4 Volt for a logic '1' (it drops below 0.4 volt for a logic '0'). This causes a problem when a TTL device tries to drive a CMOS device that works with a 5 volt power supply. The output level of the TTL chip when outputting a logic '1' level does not reach a level that the CMOS device recognize as a valid '1'.
This was solved by the invention of the 74HCT family of devices that uses CMOS technology but TTL logic levels. These devices only work with 5 Volt power supply. They form a perfect drop-in replacement for TTL logic, although HCT is even slower than original TTL (HC logic has about the same speed as original TTL).
Improved versions
With HC and HCT logic and LS-TTL logic competing in the market it became clear that even further improvements were needed to create the 'ideal' logic device that combined high speed, with low power dissipation and compatibility with older logic families. a whole range of newer families emerged that used CMOS technology, a short list of the most important family designators of these newer devises include.
- LV logic (lower supply voltage)
- LVT logic (lower supply voltage while retaining TTL logic levels)
- ALVT logic (and 'advanced' version of LVT logic)
But there are many others including AC/ACT logic, AHC/AHCT logic, ALVC logic, AUC logic, AVC logic, CBT logic, CBTLV logic, FCT logic, LVC logic and LVC logic.
BiCMOS
One mayor improvement was to combine CMOS inputs and TTL drivers to form of a new type of logic devices called BiCMOS logic, of which the LVT and ALVT logic families are the most important. But even the BiCMOS family has many members, The current list includes ABT logic, ALB logic, ALVT logic, BCT logic and LVT logic.
Conclusion
At the moment the most important families still are the LS-TTL and the HC and HCT families, although BCT logic also is starting to become popular. There is no consensus yet which low voltage logic family is 'winning' although AHC logic seems to gain some popularity. But the 'race is still on', especially because not every manufacturer supports every logic family, and some logic families only support a small subset of the complete list of LS-TTL devices available.
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