Different circuit configurations and production technologies are used during the production of digital integrated circuits. Each of these approaches is called a specific Logic Families. Now the idea of having different approaches or different logic families is that each ICs of same family when fabricated will have identical electrical characteristics. The characteristics which are bound to be identical are supply voltage range, speed of response, dissipation of power, input and output logic levels, current sinking capability, current sourcing capability, noise margin, fan-out etc.
Significance of Logic Families
When we talk about digital systems actually the digital ICs are the ones which make up the whole system. And if all the ICs are of same logic family then they are compatible to each other and the intended logic functions are performed and the goal is achieved.
But in case ICs belonging to different logic families are used in a digital system then to ensure compatibility interfacing techniques must be used. And that is the reason why we must understand different logic families and use the best combination of ICs during the design of a digital system. Now the question arises what might be the consequence of choosing wrong combinations of ICs. The answer is that it may not match the necessary capability needed.
Types of Logic Family
The digital ICs are designed using any of either bipolar devices or MOS or a combination of both. The logic families which fall under the first kind are called bipolar families, this include diode logic (DL), emitted coupled logic (ECL), resistor transistor logic (RTL), diode transistor logic (DTL), transistor transistor logic (TTL). The members of other logic family i.e. MOS family are PMOS, NMOS family, CMOS family. Now the Bi-MOS logic family is the one that uses both bipolar and MOS devices.
Of the above mentioned families DL, RTL and DTL are not used these days they have become obsolete. TTL, CMOS, ECL, NMOS and Bi-CMOS are the families which are still used. We will discuss about few of them in this article.
The TTL family consists of various subfamilies such as standard TTL, low-power TTL, high power TTL, low power Schottky TTL, Schottky TTL, advanced low-power Schottky TTL, advanced Schottky TTL and fast TTL. The ICs which belong to TTL family are designated as follows – 74 or 54 for standard TTL, 74L or 54L for low-power TTL, 74H or 54H for high power TTL, 74LS or 54LS for Low power schottky TTL and so on.
This is a popular logic family which includes 4000A, 4000B, 4000UB, 54/74C, 54/74HC, 54/74HCT, 54/74AC and 54/74ACT families. The subfamilies are divided on the basis of voltage difference and other parameters.
ECL stands for Emitter Coupled Logic family and it was introduced by ON semiconductor in 1962. The first product launched of this family was MECL-1 series. Later MECL-II, MECL-III, MECL-10K, MECL-10H series came into existence.