Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | 1A | I/O | Input | TTL, 0 V / 5 V | Gate 1, input A. One LS load; an unused input must be tied high rather than left floating. |
| 2 | 1B | I/O | Input | TTL, 0 V / 5 V | Gate 1, input B. |
| 3 | 1Y | I/O | Output | TTL totem-pole | Gate 1 output. Low only when both 1A and 1B are high. |
| 4 | 2A | I/O | Input | TTL, 0 V / 5 V | Gate 2, input A. |
| 5 | 2B | I/O | Input | TTL, 0 V / 5 V | Gate 2, input B. |
| 6 | 2Y | I/O | Output | TTL totem-pole | Gate 2 output. Low only when both 2A and 2B are high. |
| 7 | GND | Ground | Power | 0 V | Ground return for all four gates. |
| 8 | 3Y | I/O | Output | TTL totem-pole | Gate 3 output. Low only when both 3A and 3B are high. |
| 9 | 3A | I/O | Input | TTL, 0 V / 5 V | Gate 3, input A. |
| 10 | 3B | I/O | Input | TTL, 0 V / 5 V | Gate 3, input B. |
| 11 | 4Y | I/O | Output | TTL totem-pole | Gate 4 output. Low only when both 4A and 4B are high. |
| 12 | 4A | I/O | Input | TTL, 0 V / 5 V | Gate 4, input A. |
| 13 | 4B | I/O | Input | TTL, 0 V / 5 V | Gate 4, input B. |
| 14 | VCC | Power | Power | +5 V | Positive supply. Decouple with 100 nF close to the pin. |
Common uses and repair diagnostics
The 74LS00 holds four independent 2-input NAND gates in a 14-pin DIP. It is the single most common piece of glue logic in 1970s and 1980s computers, used for address decoding, reset gating and inverting odd signals, and it is one of the first parts to suspect when a board shows stuck or floating control lines.
The 74LS00 turns up on almost every 8-bit mainboard of the era. Two of its gates are regularly pressed into service as a cheap address decoder: NAND the relevant address lines together so that only one chip-select range pulls low, then feed that into a ROM or I/O chip. The same part gates the reset line on many boards, holding the CPU in reset until the supply has settled, and it inverts the polarity of the odd control signal that some peripheral expects the wrong way round.
On the bench it is also the usual substitute when a board calls for a 74LS01, 74LS03 or 74LS132 — the NAND function is the same and only the output structure differs, so a 74LS00 will confirm whether a gate has died before you order the exact part. The 74HC00 and 74HCT00 are the CMOS and TTL-compatible CMOS equivalents; they swap straight in where lower power matters, but watch the input thresholds if you are mixing them with genuine LS parts.
- With power off, probe pin 14 (VCC) to pin 7 (GND) for a low-ohms supply rail; a dead short or open points to a decoupling capacitor or the chip itself.
- Power up and check pin 14 sits at 4.75 to 5.25 V. A sagging rail pulls the gate outputs out of spec and is often the real fault, not the gate.
- Drive both inputs of a gate high (or use the existing logic) and measure the output: it must pull below 0.5 V. A stuck-high output suggests a failed totem-pole transistor.
- Tie one input of an unused gate to VCC through a 1 kR resistor and leave the other grounded; a floating LS input drifts high and makes the gate oscillate, which couples noise onto the rails.
- If several gates misbehave at once, suspect the supply decoupling first — a 74LS00 with no bypass capacitor can glitch the whole board when its outputs switch.