In general, the TTL circuit’s PCB’s +5 V and 0 V supply rail tracks must be as wide as possible (ideally, the 0 V track should take the form of a ground plane). Consequently, practical TTL circuits should always be powered from a low-impedance, well-regulated supply such as one of those shown in Figures 1 to 3, and must be used with a PCB (printed circuit board) that is very carefully designed to give excellent high-frequency supply decoupling to each TTL IC.
The 74-series TTL ICs are designed to be used over a very limited supply voltage range (4.75-5.25 V), and - because they generate very fast pulse edges and have relatively low noise-margin values - must be used with supplies with very low output impedance values (typically less than 0.1 Ω). Assuming that the matter of fan-in and fan-out has already been taken care of (as described in Part 2 of this mini-series), four other basic usage themes remain, and these are described under the headings Power Supplies, Input Signals, Unused Inputs, and Interfacing. It is usually a fairly simple matter to design logic circuitry using “74-series” TTL ICs, provided that a set of TTL basic usage rules are observed.
