Digital logic devices are the circuits that electronically perform logic operations on binary variables. The binary information is represented by high and low voltage levels, which the device processes electronically. The devices that perform the simplest of the logic operations (such as AND, OR, NAND, etc.) are called gates. For example, an AND gate electronically computes the AND of the voltage encoded binary signals appearing at its inputs and presents the voltage encoded result at its output.
The digital logic circuits used in this laboratory are contained in integrated circuit (IC) packages, with generally 14 or 16 pins for electrical connections. Each IC is labeled (usually with an SN74xx or SN54xx number) to identify the logic it performs. The logic diagrams, pin connections, and electrical characteristics for these IC’s are described in the TTL Data Book by Texas Instruments.
The transistor-transistor logic(TTL) IC’s used in this laboratory require a 5.0 volt power supply for operation. TTL inputs require a voltage greater than 2 volts to represent a binary 1 and a voltage less than 0.8 volts to represent a binary 0.
Pin numbering is standard on IC’s. Figure 1 illustrates the pin numbering for a 14-pin dual in-line package (DIP). With the IC oriented as shown, the numbering starts at the top left and proceeds counter-clockwise around the chip:
FIGURE 1
To construct circuits with IC’s, a circuit board that allows easy connections to IC pins should be used. The circuit board contains rows of solderless tie points, a 5-volt power supply, a common circuit point (ground), toggle switches for input, and LEDs (light emitting diodes) for output.
