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Digital Electronics
Figure 7.36 Four-bit magnitude comparator.
(7.27) respectively represent A > B and A < B conditions. Figure 7.36 shows the logic diagram of a
four-bit magnitude comparator.
Magnitude comparators are available in IC form. For example, 7485 is a four-bit magnitude
comparator of the TTL logic family. IC 4585 is a similar device in the CMOS family. 7485 and 4585
have the same pin connection diagram and functional table. The logic circuit inside these devices
determines whether one four-bit number, binary or BCD, is less than, equal to or greater than a
second four-bit number. It can perform comparison of straight binary and straight BCD (8-4-2-1)
codes. These devices can be cascaded together to perform operations on larger bit numbers without the
help of any external gates. This is facilitated by three additional inputs called cascading or expansion
inputs available on the IC. These cascading inputs are also designated as A = B, A > B and A < B
inputs. Cascading of individual magnitude comparators of the type 7485 or 4585 is discussed in the
following paragraphs. IC 74AS885 is another common magnitude comparator. The device is an eightbit magnitude comparator belonging to the advanced Schottky TTL family. It can perform high-speed
arithmetic or logic comparisons on two eight-bit binary or 2’s complement numbers and produces two
fully decoded decisions at the output about one number being either greater than or less than the other.
More than one of these devices can also be connected in a cascade arrangement to perform comparison
of numbers of longer lengths.
Digital Electronics
Figure 7.36 Four-bit magnitude comparator.
(7.27) respectively represent A > B and A < B conditions. Figure 7.36 shows the logic diagram of a
four-bit magnitude comparator.
Magnitude comparators are available in IC form. For example, 7485 is a four-bit magnitude
comparator of the TTL logic family. IC 4585 is a similar device in the CMOS family. 7485 and 4585
have the same pin connection diagram and functional table. The logic circuit inside these devices
determines whether one four-bit number, binary or BCD, is less than, equal to or greater than a
second four-bit number. It can perform comparison of straight binary and straight BCD (8-4-2-1)
codes. These devices can be cascaded together to perform operations on larger bit numbers without the
help of any external gates. This is facilitated by three additional inputs called cascading or expansion
inputs available on the IC. These cascading inputs are also designated as A = B, A > B and A < B
inputs. Cascading of individual magnitude comparators of the type 7485 or 4585 is discussed in the
following paragraphs. IC 74AS885 is another common magnitude comparator. The device is an eightbit magnitude comparator belonging to the advanced Schottky TTL family. It can perform high-speed
arithmetic or logic comparisons on two eight-bit binary or 2’s complement numbers and produces two
fully decoded decisions at the output about one number being either greater than or less than the other.
More than one of these devices can also be connected in a cascade arrangement to perform comparison
of numbers of longer lengths.