- #1
timeforplanb
- 17
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Why do TTL integrated circuits assume unconnected inputs to be at logic level HIGH? Does the answer lie in the circuitry itself or some other factor?
TTL Gate Circuits: Unconnected Inputs Logic Level HIGH?
In summary: If the input is grounded, then the output will be low because the output transistor doesn't get the current it needs to turn on. If the input is not grounded, then the output will be high because there is enough current going through the transistor to turn it on.
- #2
vk6kro
Science Advisor
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In the diagram, the inputs are only low if current flows out of the device at A or B. Otherwise, they are high.
If either of the inputs is grounded, then the output transistor will not get base current so the output will be high.
Only if both inputs are not grounded, ie high, then the output will be low.
This is a NAND gate, but the input logic is similar for other TTL devices
- #3
jedishrfu
Mentor
- 14,785
- 9,123
Not sure you'll get a definitive answer but it makes sense for multiple input gates to leave unconnected gates as high. As an example, a 4 input AND gate where you only need 3inputs you'd set the fourth one high.
Last edited:
- #4
timeforplanb
- 17
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vk6kro said:
In the diagram, the inputs are only low if current flows out of the device at A or B. Otherwise, they are high.
If either of the inputs is grounded, then the output transistor will not get base current so the output will be high.
Only if both inputs are not grounded, ie high, then the output will be low.
I'm not that familiar with the internal circuitry of TTL gates yet. In the diagram, Vcc is one of the inputs right? Where is the other one? Do A and B represent the output terminals?
So, if the input is grounded we know that it goes through a certain circuit so we expect its output voltage to be low? Otherwise, it is high?
- #5
vk6kro
Science Advisor
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timeforplanb said:
No, the inputs are A and B, but the output transistor gets its base current from Vcc via the base-collector diode of the odd-looking transistor at the left and the series resistor at left. The output is "Q".
Grounding either emitter robs the output transistor of its base current by providing a single diode path to ground for the base current of the output transistor, while the path through the output transistor's base involves two diodes and two diode drops.
So, all the base current goes via the left path.
1. What is a TTL gate circuit?
A TTL (transistor-transistor logic) gate circuit is an electronic circuit that uses transistors to process and transmit digital signals. It is commonly used in digital devices such as computers and microcontrollers.
2. How does a TTL gate circuit work?
A TTL gate circuit consists of multiple transistors connected in a specific arrangement to perform logical operations on input signals. The output signal is determined based on the state of the input signals and the logic function of the specific gate (AND, OR, NOT, etc.).
3. What are unconnected inputs in a TTL gate circuit?
Unconnected inputs refer to input pins on a TTL gate circuit that are left floating, meaning they are not connected to either a logic HIGH (1) or logic LOW (0) signal. These unconnected inputs can cause unpredictable behavior in the circuit and should be avoided.
4. What happens when unconnected inputs in a TTL gate circuit have a logic level HIGH?
When unconnected inputs in a TTL gate circuit are left floating and have a logic level HIGH (1), they can act as antennas and pick up stray electrical signals, causing false inputs and output errors. Therefore, it is important to connect all inputs to a known logic level (either HIGH or LOW) to ensure proper operation of the circuit.
5. How can I prevent issues with unconnected inputs in a TTL gate circuit?
To prevent issues with unconnected inputs in a TTL gate circuit, it is important to properly design and test the circuit before use. This includes connecting all inputs to a known logic level, minimizing the length of the input wires, and using decoupling capacitors to reduce noise. It is also important to follow the manufacturer's guidelines and datasheets for the specific TTL gate circuit being used.
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