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by the other half of CIO, and the other half of C9 controls memory
block 3.
The individual status of these four flip/flops is indica
ted by the desigated red light-emitting diodes located in the upper
left hand corner of the board.
minated then that block is protected and writing into that block
cannot
occur.
NOTE:
memory.
If a 1024 word block of memory is selected by its chip enable being
decoded by the 74LS156 at location D8, and its respective write
protect flip/flop at locations CIO or C9 are not in the protect
state, then the section or the 74LS02 at location C8 associated with
this block will have a high output.
input of the 7425 at location C 7 , will cause the output of C7 to
go low and this will assert one of the chip enable pins (pin 151
of the 74LS156 at location D I O .
asserted on the PWR bus line; the second is an assertion on the
MWRITE bus line.
at D8) and issue a write pulse only to the selected 1024 word block.
The four write protect flip/flops at locations CIO and C9, as des
cribed earlier, are set and reset under the control of two sets of
decoders whose outputs are wired ORed.
74LS156 at location DIO that is used to set the flip/flops, and a
section of the 74LS156 at location D8 that is used to clear (or re
set) the flip/flops, is utilized when the protect/unprotect switch
controls the assertion of the protect and the unprotect bus lines
whose assertion is utilized via the chip enable input (pin 1) of DIO
and D 8 .
The other chip enable (pin 2) of both DIO and D8 is'con
nected to the
B D E N A
74LS20 at location C4.
be decoded to one of four output assertions are the address bus lines
All and A10.
The other set of decoders are both sections of the 74LS156 at loca
tion D 9 .
These are utilized when the four write protect flip/flops
are going to have their status changed by programmed commands in
the software.
The command used is an output command, one of 256
available.
The board is created to use output command FE, and only
this one command is used for all (a maximum of 16) RAM 4A memory
boards on a bus.
is done by putting board address (the same one as is used for board
selections from the address bus-this feature is provided by the 74LS157
data selector at location D5), the two bits used to select one-of-
four blocks of memory, and the two bits that are decoded to perform
one-of-three actions, out on the system data bus at the time an FE
output command bus is used.
at location D4 are the setting or the clearing (resetting) of -the
write protect flip/flop of the memory block as decoded from DO 3 and
DO 2 by the 74LS156 one-of-four decoder at location D9.
The third action decoded from DO 1 and DO 0 by the 74LS139 one-of-
Xf the red LED for a block is illu
A system reset will unprotect all blocks of
The second chip enable of DIO is
DIO will decode address bus bits All and A10 (as
signal generated by the output (pin 8) of the
The two input lines to DIO and D8 that will
The necessary board selection, and block selection,
Two of the actions decodecDby the 74LS139
RAM 4A
Theory of Operation
This high output, seen at the
One set, a section of the
6 - 3
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