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TTL-series 7485 comparator demonstration (24 bit)DescriptionThis circuit demonstrates how to interconnect multiple 7485 chips
to build an n-bit comparator.
In the example, six chips are uses to create a 24-bit magnitude
comparators for positive binary numbers (note that the comparison
also works for binary-coded decimal numbers).
Obviously, the clean way to build the 24-bit comparator would have
been to use six chips (with four A/B inputs each) for the first level,
and then two more chips for the second level.
The design presented here, taken from the original
TTL series datasheet from Texas Instruments,
uses a simple trick to reduce the number of required chips.
It uses the A<B_in and A>B_in inputs of the circuits to
effectively provide five bits of input.
However, this can lead to the funny situation that a first
level chips asserts both its A<B_out and A>B_out outputs
at the same time - if A equals B and the cascade inputs are both low.
Luckily, the second level chip interprets this situation correctly
and the circuit works as it should.
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Impressum | 24.11.06