Pinout diagram
Summary specifications
Full pin reference
| Pin | Name | Group | Direction | Levels | Description |
|---|---|---|---|---|---|
| 1 | 1A | I/O | Input | TTL, 0 V / 5 V | Inverter 1 input. An unused input must be tied to a rail, not left floating. |
| 2 | 1Y | I/O | Output | TTL totem-pole | Inverter 1 output. Low when 1A is high. |
| 3 | 2A | I/O | Input | TTL, 0 V / 5 V | Inverter 2 input. |
| 4 | 2Y | I/O | Output | TTL totem-pole | Inverter 2 output. Low when 2A is high. |
| 5 | 3A | I/O | Input | TTL, 0 V / 5 V | Inverter 3 input. |
| 6 | 3Y | I/O | Output | TTL totem-pole | Inverter 3 output. Low when 3A is high. |
| 7 | GND | Ground | Power | 0 V | Ground return for all six inverters. |
| 8 | 4Y | I/O | Output | TTL totem-pole | Inverter 4 output. Low when 4A is high. |
| 9 | 4A | I/O | Input | TTL, 0 V / 5 V | Inverter 4 input. |
| 10 | 5Y | I/O | Output | TTL totem-pole | Inverter 5 output. Low when 5A is high. |
| 11 | 5A | I/O | Input | TTL, 0 V / 5 V | Inverter 5 input. |
| 12 | 6Y | I/O | Output | TTL totem-pole | Inverter 6 output. Low when 6A is high. |
| 13 | 6A | I/O | Input | TTL, 0 V / 5 V | Inverter 6 input. |
| 14 | VCC | Power | Power | +5 V | Positive supply. Decouple with 100 nF close to the pin. |
Common uses and repair diagnostics
The 74LS04 is the hex inverter — six independent NOT gates in a 14-pin DIP. It is the part you reach for whenever a signal needs its polarity flipped: an active-high chip-select becomes active-low, a reset line that the CPU drives high gets inverted to drive a peripheral that expects low, and a clock that arrives positive-edge gets turned over for the one device on the board that wants the negative edge. The six gates share only the supply rails, so any of them can be pressed into service independently.
The 74LS04 is the most common inverter package on 8-bit and 16-bit boards. It inverts chip-select and reset lines, flips the polarity of interrupt and wait signals, and builds the simple oscillator that some boards use for their clock. On the bench it is also the usual substitute when a board calls for a 74LS05 or 74LS06 — the inverter function is the same and only the output drive differs, so a 74LS04 will confirm whether a gate has died before you order the exact part. The 74HC04 and 74HCT04 are the CMOS equivalents and drop straight in where lower power matters.
- 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 inverter outputs out of spec and is often the real fault, not the gate.
- Drive an input high and measure the output: it must pull below 0.5 V. Drive the input low and it must rise above 2.7 V. A stuck level suggests a failed totem-pole transistor.
- Tie any unused inverter input to VCC through a 1 kR resistor; a floating LS input drifts high and makes the gate oscillate, coupling noise onto the rails.
- If several inverters misbehave at once, suspect the supply decoupling first — a 74LS04 with no bypass capacitor can glitch the whole board when its outputs switch.