The symbols used thus far in the chapter for representing different types of gate are the ones that are better known to all of us and have been in use for many years. Each logic gate has a symbol with a distinct shape. However, for more complex logic devices, e.g., sequential logic devices like flip-flops, counters, registers or arithmetic circuits, such as adders, subtractors, etc., these symbols do not carry any useful information. A new set of standard symbols was introduced in 1984 under IEEE/ANSI Standard 91–1984. The logic symbols given under this standard are being increasingly used now and have even started appearing in the literature published by manufacturers of digital integrated circuits. The utility of this new standard will be more evident in the following paragraphs as we go through its salient features and illustrate them with practical examples.

This standard uses a rectangular symbol for all devices instead of a different symbol shape for each device. For instance, all logic gates (OR, AND, NAND, NOR) will be represented by a rectangular block. A right triangle is used instead of a bubble to indicate inversion of a logic level. Also, the right triangle is used to indicate whether a given input or output is active LOW. The absence of a triangle indicates an active HIGH input or output. As far as logic gates are concerned, a special notation inside the rectangular block describes the logic relationship between output and inputs. A ‘1’ inside the block indicates that the device has only one input. An AND operation is expressed by ‘&’, and an OR operation is expressed by the symbol ‘≥1’.

A special dependency notation system is used to indicate how the outputs depend upon the input. This notation contains almost the entire functional information of the logic device in question. This will be made clearer as we illustrate this new standard with the help of ANSI symbols for some of the actual devices belonging to the category of flip-flops, counters, etc., in the following chapters. All those control inputs that control the timing of change in output states as per logic status of inputs are designated by the letter ‘C’. These are ENABLE inputs in latches or CLOCK inputs in flip-flops.

Most of the digital ICs contain more than one similar function on one chip such as IC7400 (quad two-input NAND), IC7404 (hex inverter), IC74112 (dual-edge triggered JK flip-flop), IC7474 (dual D-type latch), IC 7475 (quad D-type latch) and so on. Those inputs to such ICs that are common to all the functional blocks or in other words similarly affect various individual but similar functions are represented by a separate notched rectangle on the top of the main rectangle.

Figure 4.44 shows the ANSI symbol for IC7400, which is a quad two-input NAND gate. The figure is self-explanatory with the background given in the preceding paragraphs. Any other similar device, i.e., another quad two-input NAND gate belonging to another logic family, would also be represented by the same ANSI symbol. As another illustration, Fig. 4.45 shows the ANSI symbol for IC 7420, which is a dual four-input NAND gate.