hexadecimal keyboard

There are various ways of producing a hexadecimal keyboard. Normally, it is based on a number of key contacts in a matrix,• but here a rather simpler method is used: 16 key contacts (0... F) that are commoned to the positive supply line. Such keyboards are commercially available. Code conversion is carried out by two priority encoders, IC3and IC4. If one of the inputs 10...17of these ICs is connected to the positive supply line via one of the contacts S... S16, i.e., made logic high, the relevant binary code appears at the associated out- put, Q0. . .Q2, of which O. is the least significant bit (LS8). As the encoders are cascaded, there is a total of 16 inputs. Corresponding outputs of the encoders are combined in OR gates N6. . . N8to form the lowest three out- put bits Do. . D2. the fourth data bit is taken from the GS (group select) output of IC4. This output is logic high when one of key contacts So. . S16(8... F) is closed. As the GS outputs of the two ICs are combined in O...