Page 131 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

112 Chapter Eleven

The use of a 2:4 decoder with active-low outputs ensures that only one of
the tri-state buffers is enabled at any time. The enabled buffer will propagate
the data on its input to the common output, while the remaining buffers will
be forced to their tri-state condition.
With hindsight it now becomes obvious that the standard primitive gates
(AND, OR, NAND, NOR, etc.) depend on internal Z states to function
(when any transistor is turned OFF, its output effectively goes to a Z state).
However, the standard primitive gates are constructed in such a way that at
least one of the transistors connected to the output is turned ON, which means
that the output of a standard gate is always driving either O or I.

Combinational versus Sequential Functions
Logic functions are categorized as being either combinational (sometimes
referred to as combinatoyial) or sequential. In the case of a combinational function,
the logic values on that function’s outputs are directly related to the current
combination of values on its inputs. All of the previous example functions have
been of this type.
In the case of a sequential function, the logic values on that function’s
outputs depend not only on its current input values, but also on previous input
values. That is, the output values depend on a sequence of input values. Because
sequential functions remember previous input values, they are also referred to
as memory elements.

RS Latches
One of the simpler sequential functions is that of an RS latch, which can be
implemented using two NOR gates connected in a back-to-back configuration
(Figure 11-8). In this NOR implementation, both reset and set inputs are
active-high as indicated by the lack of bobbles associated with these inputs on
the symbol. The names of these inputs reflect the effect they have on the q,
output; when reset is active q, is reset to 0, and when set is active q is set to 1.
The q, and -q outputs are known as the true and complementdry outputs,
In
re~pectively.~ the latch’s normal mode of operation, the value on -q, is the

4 In this case, the tilde ‘-’ character prefixing the output name -9 is used to indicate that this
signal is a complementary output. Once again, the use of tilde characters is discussed in detail
in Appendix A.