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THEORY OF OPERATION
The memory circuits used on the IMSAI RAM 4A memory board are 2102-
style integrated circuits housed in sixteen pin DIP packages.
organization is 1024 words, each of which is one bit wide.
address inputs are used to select the desired word and there is a
chip enable to select the chip.
input is provided for data in, and one output is provided for data
out.
To implement the storage o f data words that are eight bits
wide, eight of the above described chips are used to store 1024
words.
Three more of these eight chip groups can be used to give
the IMSAI RAM 4A memory board a maximum storage capacity of 4096
eight bit words.
Bits A9, A8, A 7 , A6-, A5, A4, A3, A2, Al, and AO of the address bus
come onto the memory
address pins on each memory chips.
a section of the 74LS156 at location D8 to select the desired 1024
word block by assertion of the chip enable signal for only those
eight memory chips comprising the desired 1024 word block.
Bits A15, A14, A13, and A12 of the address bus are used to give
each memory board o n the bus a unique address.
through (if Idle memory board is involved in the utilization of its
memory function through a memory-read operation, or memory write
operation) the 74LS157 data selector at location D 5 .
output, and the complement of the direct output (obtained through
the 74LS04 inverters at location C6) of the four output pins of
the 74LS157 a t ,location D5 go to DIP jumper provision at location
C 5 .
Provision is made so that either the aquivalant polarity, or
its complement, of the above mentioned four address bits can be
implemented through the correct use of jumpers at location C5.
When the polarity of the above-mentioned four address bits are in
such an arrangement that they satisfy the address requirements of
a particular memory board the four input pins of a section of the
74LS20 at location C4 will be high.
an individual memory board.
in this manner for each of the sixteen different polarity arrange
ments of these four address bits.
Each 1024 word block of memory has its own circuitry to implement
the write-protect feature.
ways.
One is from the "PROTECT/UNPROTECT" switch on the front panel.
The other is from program commands contained in software.
There are four flip/flops whose two states enable or prevent the
changing of the contents of their respective 1024 word blocks when
a memory write is received.
section of a 74LS74 at location CIO and at location C9.
block 0 is controlled by half of C9, memory block 2 is controlled
2
6
RAM 4A BOARD
There is a read/write input.
board and go directly to the appropriate
Bits All and A10 are decoded by
This effects the selection of
Thus, only one board should respond
This feature is manipulated in two
Each of these four flip/flops is a
xneory or operation
Ten
These bits first go
The direct
Memory
Their
One
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