Hi I am trying to build a circuit with 2 common cathode 7 segment display that will count from 0-14 (even numbers only) and we are not allowed to use a decoder ic. only logic gates. My problem is that 7400 series ic requires ALL open inputs to be tied to a 1 k resistor then grounded to drive it at logic low, other wise the outputs are always on.(plus it will get really messy ) Are there any IC that has inputs that behaves at logic low when they are open? Preferrably NAND,AND, OR gates? thanks! I am using a 9 v battery and dip switch
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1\$\begingroup\$ Given the \$9V\$, may you use the CD4xxx series devices? They work directly off of a \$9V\$ battery. Also, their inputs can be tied down or up, I think, without resistors. (They use mosfet gates for inputs.) \$\endgroup\$– jonkSep 4, 2016 at 5:49
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\$\begingroup\$ The Absolute Maximum power supply voltage for 7400 series ICs is 5 volts - if you apply 9 volts, you may destroy the ICs. TTL inputs can be connected directly to Ground if you want a permanent Low input - no resistors required. \$\endgroup\$– Peter BennettSep 4, 2016 at 16:17
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2 Answers
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This because of internal circuit structure , something like this picture.(this is not Gate)
Logic IC have 2 type :
1-CMOS
2-TTL
You're choose is 74XX.Most of them is TTL.
In some CMOS IC this Undefined input is fixed the only input declare "1" that is connected to 5v or IC Voltage . Choose similar CMOS.
but you're "0" logic doesn't need 1k resistor it can be direct connected to ground.
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\$\begingroup\$ This picture is not a correct description of a TTL input. \$\endgroup\$– CL.Sep 4, 2016 at 9:19
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The behaviour of a TTL input is determined by its construction. The only way to have a TTL input default to low would be to integrate the pull-down resistor into the chip, but this would increase the power usage when the input is driven high, so nobody makes such a chip.
Please note that in Designing with TTL (AN-363), Fairchild says:
Unused inputs on TTL devices float at threshold, anywhere from 1.1V to 1.5V, depending upon the device and its family. While this usually simulates a “high”, many application problems can be traced to open inputs. Inputs floating at threshold are very susceptible to induced noise (transmitted from other lines) and can easily switch the state of the device. A good design rule is to tie unused inputs to a solid logic level.
So you should never have any open input, whether high or low.
To reduce the amount of components (but not cables) you need, you can share a single, appropriately-sized pull-down resistor for multiple fixed-low inputs. Alternatively, use CMOS logic ICs, whose inputs can be tied directly to VCC/GND.
