NOR gate and the 74HC02: a beginner's guide, with one wiring trap to avoid

Some gates shout when anything happens. A NOR gate does the opposite: its output is high only when everything is quiet. In this post you will learn how a NOR gate works, wire a 74HC02 chip in Mokxi to make an "all clear" light, avoid a pinout trap, and then build NOT, OR, AND and a one-bit memory out of NOR gates alone.

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What a NOR gate does

NOR means "NOT OR". An OR gate gives a 1 when at least one input is 1. A NOR gate flips that, so its output is 1 only when both inputs are 0.

A	B	Y (A NOR B)
0	0	1
0	1	0
1	0	0
1	1	0

Compare this with a NAND gate, where a 0 on either input forces the output high. On a NOR gate the controlling value is a 1: a single 1 on either input forces the output low, whatever the other input is doing. Mokxi's chip model follows that rule, so a 74HC02 gate with one input high and the other left floating still gives a clean 0.

Like NAND, NOR is a universal gate: any logic circuit can be built from NOR gates alone.

The trap: the 74HC02 is not a 74HC00

If you have wired a 74HC00 NAND chip, your fingers know the pattern: inputs on pins 1 and 2, output on pin 3. The 74HC02 puts the output first on every gate. The Texas Instruments SN74HC02 datasheet lists it like this, and so does Mokxi's 74HC02 part:

Gate	Output	Inputs
1	pin 1 (1Y)	pins 2 and 3
2	pin 4 (2Y)	pins 5 and 6
3	pin 10 (3Y)	pins 8 and 9
4	pin 13 (4Y)	pins 11 and 12

Ground is pin 7 and the supply is pin 14, as on the 74HC00. Wire a 74HC02 as if it were a 74HC00 and you drive your signals into the chip's output pins. Mokxi's 74HC02 help page lists this as the first common mistake, and it is worth a glance at the pin table before you wire anything.

Build an "all clear" light

Picture a workshop with two doors. A lamp should be lit only while both doors are shut. Treat "door open" as a 1, and the lamp is a NOR of the two doors.

You need a 74HC02, a Power part (VCC), a Ground part (GND), two Slide switches, a 1 kilohm resistor and an LED. Add each one with the + button (or the A key) and the search box, as described in the placing parts guide.

1. Wire Power to pin 14 and Ground to pin 7. A Mokxi logic chip has real supply pins, so it does nothing until both are connected.
2. For each slide switch, wire pin 1 to Ground and pin 3 to Power. Its middle pin, 2, then moves between low and high when you click it while the simulation runs.
3. Wire the first switch's middle pin to chip pin 2, and the second switch's middle pin to chip pin 3.
4. Wire chip pin 1 (the output) to the resistor, then to the LED's anode (A). Wire the LED's cathode (C) to Ground.
5. Press Run. With both switches at ground the LED is lit. Click either one up to 5 V and it goes out.

I ran the same circuit in Mokxi's headless simulator for all four switch positions and got 1, 0, 0, 0 on the output, exactly the truth table above. With the 1 kilohm resistor the LED draws about 3 mA, a little under the 4 mA at which the datasheet quotes the chip's output levels, so the glow is gentle but easy to see.

Every other gate, from NORs

Everything below uses the four gates in one chip, using the pin table above.

NOT from one NOR

Tie both inputs of a gate together. A 0 on both gives a 1, and a 1 on both gives a 0. Put your input on pins 5 and 6 and read pin 4.

OR from two NORs

An OR is a NOR followed by a NOT. Put A on pin 2 and B on pin 3. Wire pin 1 to pins 5 and 6, and read pin 4.

AND from three NORs

This is the mirror image of building OR from NANDs: invert both inputs, then NOR them. If both inputs are 1, both inverted inputs are 0, and only then does the NOR give a 1.

1. A goes to pins 2 and 3. B goes to pins 5 and 6.
2. Wire pin 1 to pin 8, and pin 4 to pin 9.
3. Read pin 10.

I checked these three in Mokxi as well. NOT gave 1 then 0, OR gave 0, 1, 1, 1 and AND gave 0, 0, 0, 1.

Two NORs make a memory

Cross-couple two NOR gates and the circuit remembers one bit. This is an SR latch, where S means "set" and R means "reset".

1. Gate 1: R goes to pin 2. Pin 3 takes the output of gate 2.
2. Gate 2: S goes to pin 5. Pin 6 takes the output of gate 1.
3. So pin 1 is Q, and pin 4 is the opposite output.

Drive S and R with two slide switches and watch Q. Here is what I saw:

• Both low at power-up: Q and its opposite output are both unknown. A real latch also starts in a random state, and Mokxi shows that honestly rather than inventing a starting value.
• S high, then low again: Q goes high and stays high.
• R high, then low again: Q goes low and stays low.
• Both high at once: both outputs go low. That is the forbidden state, so avoid it.

Unlike the NAND latch, the NOR latch is active high: you pulse a 1 to set or reset it. If you want to compare the two, flip-flops and latches explains how they differ.

Things to try next

• Make a NAND from NORs. Build AND as above, then add gate 4 as an inverter. That uses every gate in the chip.
• Chain a latch to an LED. Put an LED and a 1 kilohm resistor on pin 1, set it with a switch and see it stay lit after you let go.
• Compare with the 74HC00. Build the same all-clear light with a NAND and inverters, and count how many gates each way needs. The logic gates guide has the background.

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