Page 188 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Memory ICs 169

signal which is used to control the tri-state gates. Additionally, -chip-select is
combined with -write to form an internal -wr signal (Figure 15-5).

external ,
system ,.

Figure 15-5. RAM with separate data in and data out busses

When -wr is active, the data present on the data-in bus is written into the
word selected by the address-bus. The contents of the word pointed to by the
address-bus are always available at the output of the array, irrespective of the
value on -wr. Therefore, devices of this type may be written to and read from
simultaneously.
In contrast to those devices with separate data-in and data-out buses, the
majority of RAMS use a common bus for both writing and reading. In this case,
the -read and -write signals are usually combined into a single control input
called something like read-write. The name read-write indicates that a logic 1
on this signal is associated with a read operation, while a logic 0 is associated
with a write. When the -chip-select signal indicates to the RAM that its
attention is required, the value on read-write is used to determine the type of
operation to be performed. If read-write carries a logic 1, the internal -rd signal