.

.
Navigation

Pin connections	Cascading
Truth table	NOR gate bistable
Basic operation
Other gates made with NOR	LINKS . . .

Back to Contents   Back to Beastie Zone

.
Pin connections

The 4001 has four separate 2-input NOR gates which you can use independently.

Up

.
Truth table

The truth table of each individual gate is:

where '0' represents a LOW voltage, and '1' represents a HIGH voltage.

Up

.
Basic operation

You can investigate the behaviour of a single NOR gate using this circuit:

The inputs of the gate must be connected, either to LOW or to HIGH, and must not be left open circuit. This is the function of the input switches with their pull-down resistors. To avoid loading the output of the gate, a transistor switch indicator circuit should be used. It is good practice with CMOS circuits to insert a decoupling capacitor, 47 µF or 100 µF, across the power supply. (This helps to prevent the transfer of spikes along the power supply rails.)

Don't forget - - Connect pin 14 of the 4001 to +9 V and pin 7 to 0 V.

This is the test circuit built on prototype board:

4001

In the prototype circuit, it is not essential to make connections to the unused gates. However, in any final circuit, all unused CMOS inputs must be connected either to HIGH or to LOW. Make it an absolute rule that CMOS inputs are never left open circuit.

There is no problem with CMOS outputs. Worry about the inputs and leave any unused outputs unconnected.

Up

.
Other gates made with NOR

An important property of NOR gates is that they can be linked to perform the functions of other logic gates. In fact, any logic function can be implemented using only NOR gates:

If you have designed a system which contains a NOR gate integrated circuit, it can be convenient and cost-effective to implement other logic functions using spare NOR gates which would otherwise be unused.

Up

.
Cascading

What happens when NOR gates are linked like this?

It is easy to work out the truth table for this circuit. When you do this, you will discover that the circuit does not give the truth table for a 3-input NOR gate. Unlike AND and OR gates, NOR gates cannot be cascaded in this way.

It is possible to make a 3-input NOR gate using 2-input NOR gates, but the circuit requires an extra gate:

3-input, 4-input, and 8-input NOR gates are available in integrated circuit form, as listed in the links section.

Up

.
NOR gate bistable

A useful application of NOR gates is in making a SET/RESET bistable, or latch. Here is the circuit:

Once again, notice that you must have a switch/resistor voltage divider at the inputs to the NOR gates. With a NOR gate latch, the inputs are held LOW and pulsed HIGH. This means that you need pull-down resistors. (Compare this circuit with a SET/RESET latch bistable built using NAND gates.)

To avoid loading the outputs of the latch, transistor switch/LED indicators are used. It is a common mistake to connect LEDs directly to the latch outputs:

Even with current-limiting resistors connected in series with the LEDs, this circuit might not work because the voltages at the outputs of the latch are pulled downwards and may not count as HIGH when they are supposed to be HIGH.

If you connect LEDs without current-limiting resistors, the circuit cannot possibly work.

This is the correct circuit built on prototype board:

4001

Up to previous stage

Up

.
Links

Cross references in the Beastie Zone:

NOR GATE
BISTABLE

4000, 3-input NOR + inverter, 4002 4-input NOR, 4025 3-input NOR, 4078 8-input NOR

Back to Contents   Back to Beastie Zone