7402 (74LS02, 74HC02): the quad NOR gate chip, live

The 7402 is a 14-pin chip with four independent 2-input NOR gates: each output nY is 1 only when both of its inputs are 0. Unlike most quad gate chips, it puts each gate's output first: outputs on pins 1, 4, 10 and 13. The 74LS02 and 74HC02 share this pinout. This live chip has all four gates.

The pinout is the thing to remember about the 7402. On the 7400, 7408, 7432 and 7486 each gate reads input, input, output from pin 1, but the 7402 reads output, input, input, so a 7402 dropped into a 7400's socket drives its outputs against the signals meant to be inputs. Check the pinout below before you wire it. NOR is a universal gate like NAND: NOT is a NOR with its inputs tied, OR is NOR followed by NOT, and AND is the NOR of the two inverted inputs (see NOT, OR and AND from NOR gates). Two NOR gates cross-coupled are the classic SR latch, the first memory circuit. Each gate here is a real CMOS NOR, two PMOS in series and two NMOS in parallel. All 256 combinations of the eight inputs are checked before the page is built.

Live circuit: click the inputs to toggle them and watch the outputs update. It runs the real simulator, right here on the page.

Open this circuit in the lab →

What you need

4× NOR: four independent 2-input NOR gates, Y = NOT (A OR B), numbered 1 to 4 as on the datasheet

Step by step

Power the chip: VCC on pin 14 and GND on pin 7 (5 V for the 74LS02; 2 V to 6 V for the 74HC02).
Each gate has two inputs, nA and nB, and an output nY; find their pins in the pinout below.
Tie every unused input to VCC or GND: TI's 74HC datasheets say unused inputs must be terminated, because a floating CMOS input can drift and draw current.
Leave 1A and 1B at 0: 1Y is 1. Turn either input on and 1Y drops to 0.

7402 pinout

Pin	Name	Pin	Name
1	1Y	14	VCC
2	1A	13	4Y
3	1B	12	4B
4	2Y	11	4A
5	2A	10	3Y
6	2B	9	3B
7	GND	8	3A

NOR truth table (each gate)

A	B	Y
0	0	1
0	1	0
1	0	0
1	1	0

Pin numbers are for the DIP and SOIC packages (TI SN74HC02 and SN74LS02 datasheets, checked 2026-10-25); TI's SN5402 flat package is wired differently again. See also the NOR truth table.

You can also read the full theory in the lesson on the NOR gate. Prefer to build it yourself from scratch? Open the digiwleea lab and place the gates on the canvas, or start from the logic gate simulator.

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Each of these is one step from a transistor to a working CPU. Build every gate from CMOS transistors and work all the way up in the free digiwleea lab, with the free theory reference alongside.

Open the 7402 quad NOR gate chip in the lab →

Frequently asked

What is the 7402 pinout?

Pin 1 1Y, 2 1A, 3 1B, 4 2Y, 5 2A, 6 2B, 7 GND, 8 3A, 9 3B, 10 3Y, 11 4A, 12 4B, 13 4Y, 14 VCC. The outputs are on pins 1, 4, 10 and 13, unlike the 7400 family's usual input, input, output order.

Can I replace a 7400 with a 7402?

Not pin for pin: besides being a different gate, the 7402 has its outputs where the 7400 has inputs, so the wiring has to change.

Is NOR a universal gate?

Yes. NOT, OR and AND can all be built from NOR gates alone, so any logic function can.

What is the difference between the 74LS02 and the 74HC02?

The same four NOR gates and pinout. The 74LS02 is TTL for a 5 V supply; the 74HC02 is CMOS, runs from 2 V to 6 V and uses less power.