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transistor-to-transistor logic (TTL)

By Stephen J. Bigelow

What is transistor-to-transistor logic (TTL)?

Transistor-to-transistor logic -- also known as simply transistor-transistor logic or TTL -- is a family of digital logic design built from a family of bipolar junction transistors that act on direct-current pulses. Many TTL logic gates are fabricated onto a single integrated circuit (IC). TTL ICs usually have four-digit numbers beginning with 74 or 54.

TTL technology was invented in 1961 by James L. Buie, and Sylvania released the first commercial TTL ICs in 1963. Three years later, Texas Instruments introduced its 7400 logic family of TTL IC devices, providing electronics designers with a set of building blocks for creating the complex digital logic circuit used in a range of devices, including early computers. TTL devices from the 7400 family served as glue logic between complex ICs as components evolved and diversified over the next several decades.

TTL technology and the 7400 family have been largely obsolete since the 1990s, replaced by complementary metal oxide silicon and other low-power, high-density and high-speed IC technologies. However, the fundamental logic gates -- such as AND, OR, NAND and NOR gates -- embodied in TTL ICs remain essential logical constructs and are incorporated into digital logic circuitry fabricated onto very large-scale integration (VLSI) IC microprocessors to this day. For example, flash memory devices such as thumb drives and solid-state drives are built on the concepts of NAND gates and NOR gates common in classic TTL ICs.

How does TTL work?

TTL gates are designed using at least two transistors and supporting components, including resistors and diodes. Each component serves specific purposes:

Figure 1 illustrates a basic two-input TTL NAND gate schematic. Transistor Q1 is the input transistor. Inputs, such as A and B, feed the emitter of Q1. Transistor Q2 serves as a phase splitter, and transistors Q3 and Q4 create a totem pole output that provides high stability and a high fan-out capability for the output.

When inputs A and B are on -- logic 1 or high -- transistors Q2 and Q3 turn on and act as amplifiers, while transistor Q4 turns on to create a logic 0 or low logic output. When either or both inputs A and B are off -- logic 0 or low -- transistors Q2 and Q4 turn off to create a logic 1 or high logic output.

The logical representation of this NAND gate circuit is shown in Figure 2.

Other logic gates will use slightly different circuit configurations to achieve each respective logical behavior -- such as AND, OR and NOR -- but the overall concept is similar.

Characteristics and considerations of TTL

TTL gate circuits and the IC packages that hold the gates involve important characteristics that designers consider in their digital circuit designs. While TTL components are no longer widely used in commercial digital circuit design, the fundamental characteristics remain important for VLSI components, such as an application-specific integrated circuit (ASIC), processor and other complex digital components used in modern electronic devices.

These characteristics include the following:

Types of TTL

TTL circuit designs evolved and diversified over decades of use to optimize certain characteristics such as speed, power consumption and output power to drive other components. The most popular types of TTL include the following:

Applications of TTL

TTL was the standard for digital electronic circuit design from its inception in the 1960s through the 1980s. At that time, the broad adoption of highly integrated, custom-fabricated digital components such as VLSI and ASIC chips displaced the chips with individual gates.

TTL was embraced for its advantages, such as low cost, solid noise margin for stable and reliable logic signal levels, ample fan-out to drive subsequent TTL gates, and modest power dissipation to keep circuits cooler and more energy-efficient.

However, TTL was relatively slow, and circuits composed of many gates were power-hungry. Even though individual gate delays measured just a few nanoseconds, the need to construct large, complex circuits from many gates and chips made the propagation delays cumulative, limiting the overall circuit's top speed. The printed wiring needed to interconnect the gates in each chip added to the latency and made large circuits vulnerable to electrical signal noise. As a result, TTL wasn't a good choice for high-performance circuits, such as early processors.

TTL saw most of its service in controller-type digital circuits, including simple controllers, basic computer interface designs such as early storage interface controllers for floppy disks and early magnetic hard drives, and dedicated circuits for industrial electronic systems. It wasn't until the logic functions of TTL gates were integrated into high-density chips such as VLSI and ASIC devices that digital electronics exploded into commercial and consumer applications.

TTL vs. emitter-coupled logic

Emitter-coupled logic (ECL) is an alternative electronic circuit design used in the construction of digital logic gates. Compared with TTL construction, ECL construction uses an overdriven bipolar transistor differential amplifier and limited emitter current to prevent saturating the transistors and turning them fully on. ECL is often referred to as current-steering logic, because current is steered between the emitter-coupled transistor pair.

ECL became a notable alternative gate design; its unsaturated transistors can be switched much faster, and with far lower propagation delay, than TTL gate designs. ECL gates demonstrate a typical propagation delay of 1 to 2 ns compared with about 10 ns for a standard TTL gate. ECL fan-out is also much higher at about 25 gate loads compared with a TTL fan-out of about 10, allowing ECL gates to drive more digital devices.

The main disadvantage of ECL is its much higher power dissipation than TTL. In addition, the low voltage difference between logic 0 and logic 1 leaves ECL gates with poor noise immunity. These two factors limit ECL circuit complexity and use cases.

TTL provided the foundation for today's logic gates and digital circuitry. Find out more about various kinds of logic gates and how they work.

21 Jul 2023

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