Page 189 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

7 70 Chapter Fifteen

is made active and a read operation is initiated; if read-write carries a logic 0,
the internal -wr signal is made active and a write operation is executed

Figure 15-6. RAM with single bi-directional databus

When -rd is active, the tri-state gates are enabled and the data from the
word selected by the address-bus is driven onto the data-bus. When -wr is
active (and therefore -rd is inactive), the tri-state gates are disabled. This
allows external devices to place data onto the data-bus to be written into the

word selected by the address-bus.
If the value on the address-bus changes while -rd is active, the data
associated with the newly selected word will appear on the data-bus. However,

it is not permissible for the value on the address-bus to change while -wr is
active because the contents of multiple locations may be corrupted.

Increasing Width and Depth
Individual memory devices can be connected together to increase the width
and depth of the total memory as seen by the rest of the system. For example,
two 1,024-word devices, each 8 bits wide, can be connected so as to appear to
be a single 1,024-word device with a width of 16 bits. An address-bus
containing 10 bits is required to select between the 1,024 words (Figure 15-7).