74LS125 Quad buffer, three-state
The 74LS125 holds four non-inverting buffers, each with its own active-low enable. With the enable low the output follows the input; with the enable high the output is disconnected (high impedance), so several outputs can share one wire.
74LS125 pinout
Top view of the 14-pin DIP package. Pin 1 is at the bottom left, beside the notch, and the numbers run anticlockwise. Power is on pin 14 (VCC, +5 V) and pin 7 (GND).
| Pin | Name | Type | What it is |
|---|---|---|---|
| 1 | 1G′ | Input | Input, active-low |
| 2 | 1A | Input | Input of gate 1 |
| 3 | 1Y | Output | Three-state output |
| 4 | 2G′ | Input | Input, active-low |
| 5 | 2A | Input | Input of gate 2 |
| 6 | 2Y | Output | Three-state output |
| 7 | GND | Ground | 0 V |
| 8 | 3Y | Output | Three-state output |
| 9 | 3A | Input | Input of gate 3 |
| 10 | 3G′ | Input | Input, active-low |
| 11 | 4Y | Output | Three-state output |
| 12 | 4A | Input | Input of gate 4 |
| 13 | 4G′ | Input | Input, active-low |
| 14 | VCC | Power | +5 V supply |
74LS125 function table
| G′ | A | Y |
|---|---|---|
| 0 | 0 | 0 |
| 0 | 1 | 1 |
| 1 | X | Z |
X is either value. Z is high impedance: the output is disconnected.
What the 74LS125 is used for
- Putting several sources on a shared bus, one enabled at a time
- Isolating a signal until it is needed
- Building a multiplexer from buffers
Electrical summary
| Package | 14-pin DIP |
| Supply voltage | 5 V (4.75 V to 5.25 V) |
| Supply current, typical | 11 mA |
| Output type | Three-state — drives high and low, or disconnects when disabled |
| Output current, low | sinks up to 24 mA |
| Output current, high | sources up to 2.6 mA |
| Input current, low | 0.4 mA out of each input held low |
| Fan-out | one output can drive about 60 LS inputs |
Typical figures for quick reference when planning a board. Check the manufacturer's datasheet for limits and timing before committing a design. The 74HC125 and 74HCT125 use the same pin numbers, with different electrical values.
Try the 74LS125 on a breadboard
The Breadboard Simulator has the 74LS125 in its parts library: seat it on a virtual breadboard, wire it, power the board and probe the pins before you build the real thing.