Page 187 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

168 Chapter Fifteen

(The -read control is sometimes called -output-enable. Alternatively, some
devices have both -read and -output-enable controls.) These control signals
are commonly actiue-low; that is, they are considered to be ON when a logic 0 is
a~p1ied.l~ The -chip-select signal indicates to the ROM that its attention is
required, and it is combined with -read to form an internal signal -rd which is
used to control the tri-state gates (Figure 15-4).

memory
‘. . memory
device
.
.
external .*. . device ,/
-’
-
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system , external
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address-bus[Z:O]

data-bu 5 [3:O]

Figure 15-4. ROM control decoding and data bus

When -rd is active, the tri-state gates are enabled and the data stored in
the word selected by the address bus is driven out onto the data bus. When -rd
is inactive, the tri-state gates are disabled and the outputs from the device are
placed into high-impedance Z states.
In addition to the control signals used by ROMs, RAMs require a mechanism
to control the writing of data into the device. Some RAMs employ separate
buses for reading and writing called data-out and data-in, respectively. These
components usually have an additional control signal called -write which is
also active-low. Once again, the -chip-select signal indicates to the RAM that
its attention is required, and it is combined with -read to form an internal -rd

15 Tilde ‘-’ characters prefixing signal names are used to indicate that these signals are active-low.
The use of tilde characters is discussed in detail in Appendix A.