LOGIC FAMILIES

Logic families are groups of logic circuits that are based on particular types of elements (resistors, transistors, and so forth). Families are identified by the manner in which the elements are connected, and, in some cases, by the types of elements used.

Logic circuits of a particular family can be interconnected without having to use additional circuitry. In other words, the output of one logic circuit can be used as the input to another logic circuit. This feature is known as compatibility. All circuits within a logic family will be compatible with the other circuits within that family.

As a technician, your responsibility will be to identify defective parts and repair or replace them as required. It will be beneficial for you to have a basic knowledge of the types of logic that are used in digital equipment.

Logic circuits are usually manufactured as integrated circuits and packaged in dual-inline packages (DIP), modified transistor outlines (TO), or flat packs. These packaging techniques are described in NEETS, Module 7, Introduction to Solid-State Devices and Power Supplies.

Circuitry in a package is normally shown using standard logic symbols instead of individual components such as transistors, diodes, and so forth. Figure 3-34 shows four examples of this type of packaging. The numbered blocks (1-14 and 1-16) are the pins on the package. Circuit packages are also identified by a manufacturer's part number. Similar circuits produced by different manufacturers will not carry the same identification numbers in all cases.

Figure 3-34. - Logic packages.

As mentioned before, logic families are identified by the elements used and the manner in which the elements are used. A brief description of some of the more common logic families follows.

RTL (RESISTOR-TRANSISTOR LOGIC)

In this type of logic, inputs are applied to resistors, and the output is produced by a transistor. RTL is normally constructed from discrete components (individual resistors and transistors). Some circuits are manufactured as integrated circuits and packaged in modified transistor outline (TO) packages, as shown in figure 3-35. An in-depth coverage of circuit packaging can be found in NEETS, Module 14, Introduction to Microelectronics.

Figure 3-35. - RTL integrated circuits.

DTL (DIODE TRANSISTOR LOGIC)

Input signals are applied to diodes in this logic family. The diodes either conduct or cut off and produce the desired output from the transistor. DTL is normally found in dual-inline packages (DIP) as well as older discrete component logic.

TTL (TRANSISTOR-TRANSISTOR LOGIC)

In TTL, transistors with multiple emitters are used for the logic inputs. Additional transistors are used to produce the desired output. TTL is normally packed in DIPs and is quite common in military equipment.

CMOS (COMPLEMENTARY METAL OXIDE SEMICONDUCTORS)

The CMOS logic circuits use metal oxide semiconductors similar to field-effect transistors (FETs).

LOGIC FAMILY USE

The logic family used in a piece of equipment is determined by the design engineers. The type of logic used will be based on the requirements of the equipment and on what family best fulfills the requirements.

The use of integrated circuits enables designers to produce equipment that is very small and highly efficient when compared to other methods of construction. The block diagram shown in figure 3-36, view A, represents an 8-bit, serial-input and parallel-output shift register. This circuit is contained in a standard 14-pin DIP measuring about 0.75 inch long and 0.25 inch wide. View B shows this circuit package.

Figure 3-36. - Integrated logic circuits: A. Shift register; B. Logic package.

Q.61 What are RTL, DTL, and TTL examples of?
Q.62 What type of logic family uses diodes in the input?
Q.63 What is the most common type of integrated circuit packaging found in military equipment?
Q.64 Circuits that can be interconnected without additional circuitry are known as ____________ circuits.