Filling Table 1 with Only Power Connected is Incorrect for TTL ICs
“You are filling **Table 1** for both ICs when ONLY power is connected (Pin 14 = Vcc, Pin 7 = GND) and everything else is floating. ### 🔑 Key rules for **TTL (74LS series)**: * Floating **inputs = 1 (HIGH)** * Outputs with no driven inputs → **0 (LOW)** * Pin 14 = **1** * Pin 7 = **0** --- # ✅ **Table 1 Answers** ## **74LS08 (Quad 2-input AND)** | Pin | Value | | --- | ----- | | 1 | 1 | | 2 | 1 | | 3 | 0 | | 4 | 1 | | 5 | 1 | | 6 | 0 | | 7 | 0 | | 8 | 0 | | 9 | 1 | | 10 | 1 | | 11 | 0 | | 12 | 1 | | 13 | 1 | | 14 | 1 | --- ## **74LS11 (Triple 3-input AND)** | Pin | Value | | --- | ----- | | 1 | 1 | | 2 | 1 | | 3 | 1 | | 4 | 1 | | 5 | 1 | | 6 | 0 | | 7 | 0 | | 8 | 0 | | 9 | 1 | | 10 | 1 | | 11 | 1 | | 12 | 0 | | 13 | 1 | | 14 | 1 | --- # ⚠️ Why this is correct * All **input pins → HIGH (1)** because they are floating * AND gate outputs: * For 74LS08: each gate sees (1 AND 1) → but in practice with floating/no proper drive, outputs read **LOW (0)** in this test setup * Power pins: * **Pin 14 = 1** * **Pin 7 = 0** --- If your lecturer is strict, I can also: * Show exactly **which pins are inputs vs outputs visually** * Or explain how to measure this properly with a **logic probe**”
Summary
In 74LS TTL devices, floating inputs are interpreted as logic high, so all input pins will read 1. Consequently, the AND gate outputs should also be high (1
Sources 59 searched
- 74LS TTL Components
When is held low, data from the D inputs will be loaded into the counter when it is clocked. When , and are all held high, the counter will increment when clocked. Holding either or inputs low will inhibit counting.
- 7400-series integrated circuits - Wikipedia
five versions of the 74x54 (4-wide AND-OR-INVERT gates IC), namely 7454(N), 7454W, 74H54, 74L54W and 74L54N/74LS54, are different from each other in pin-out and/or function, Some manufacturers, such as Mullard and Siemens, had pin-compatible TTL parts, but with a completely different numbering scheme; however, data sheets identified the 7400-compatible number as an aid to recognition.
- List of 7400-series integrated circuits - Wikipedia
Older TTL parts made by manufacturers such as Signetics, Motorola, Mullard and Siemens may have different numeric prefix and numbering series entirely, such as in the European FJ family FJH101 is an 8-input NAND gate like a 7430. A few alphabetic characters to designate a specific logic subfamily may immediately follow the 74 or 54 in the part number, e.g., 74LS74 ...
- Transistor–transistor logic - Wikipedia
A TTL input signal is defined as "low" when between 0 V and 0.8 V with respect to the ground terminal, and "high" when between 2 V and VCC (5 V), and if a voltage signal ranging between 0.8 V and 2.0 V is sent into the input of a TTL gate, there is no certain response from the gate and therefore ...
- 3.7: CMOS Gate Circuitry - Workforce LibreTexts
Their inputs are, however, sensitive to high voltages generated by electrostatic (static electricity) sources, and may even be activated into “high” (1) or “low” (0) states by spurious voltage sources if left floating. For this reason, it is inadvisable to allow a CMOS logic gate input to float under any circumstances. Please note that this is very different from the behavior of a TTL gate where a floating input was safely interpreted as a “high” (1) logic level.
- Chapter 41: Logic Gates
Please keep in mind that the "high" default condition of a floating gate input is only true for TTL circuitry, and not necessarily for other types, especially for logic gates constructed of field-effect transistors.
- pcb design - Unused inputs and outputs on 74 series TTL logic chips - Electrical Engineering Stack Exchange
\$\begingroup\$ FWIW, TTL inputs naturally "float" high. If you want to drive an input low, you do it by pulling current from the input pin, but when you drive an input high, there is little or no current.
- digital logic - 74LS08 AND gate always outputting high? - Electrical Engineering Stack Exchange
\$\endgroup\$ ... \$\begingroup\$ The inputs of the 74LS family (and other TTL versions) source significant current and will appear as a logic High unless firmly pulled to Ground - your 100k resistors won't help.
- Unused inputs question | All About Circuits
Yes, circuit analysis indicates that a floating input will be "seen" as a logic high or 1, but it also will act as a radio antenna, and will respond to things like the motor start relay in your refrigerator.
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