Medium scale integrated logic symbols and terminology

Medium scale integrated logic symbols and terminology

Medium scale integrated (MSI) logic elements are represented by rectangular blocks with appropriate external AND/OR gates when necessary. A small circle at an external input means that the specific input is active Low i.e. it produces the desired function, in conjunction with other inputs, if its voltage is the lower of the two logic levels in the system. A circle at the output indicates that when the function designated is True, the output is Low. Generally inputs are at the top and left and outputs appear at the bottom and right of the logic symbol. An exception is the asynchronous Master Reset in some sequential circuits which is always at the left-hand bottom comer.

Inputs and outputs are generally labelled with mnemonic letters. Those used in this book and which are commonly used throughout the logic fraternity are listed below. Note that an active Low function labelled outside of the logic symbol is given a bar over the label, while the same function inside the symbol is labelled without the bar.

When several inputs or outputs use the same letter, subscript numbers starting with zero are used in an order natural for device operation.

TTL data

All devices are listed by generic family number e.g. 7400. Certain devices may only be available, however, as members of as few as one series of the 74 family. Readers are referred to relevant data books for further details.

Selection by device number