74 Series Logic ICs


There are several families of logic ICs numbered from 74xx00 onwards with letters (xx) in the middle of the number to indicate the type of circuitry, eg 74LS00 and 74HC00. The original family (now obsolete) had no letters, eg 7400.

This page covers a selection of the many ICs in the 74 series, concentrating on the most useful gates, counters, decoders and display drivers. For each IC there is a diagram showing the pin arrangement and brief notes explain the function of the pins where necessary. For simplicity the family letters after the 74 are omitted in the diagrams below because the pin connections apply to all ICs with the same number. For example 7400 NAND gates are available as 74HC00, 74HCT00 and 74LS00.

If you are using another reference please be aware that there is some variation in the terms used to describe pin functions, for example reset is also called clear. Some inputs are 'active low' which means they perform their function when low. If you see a line drawn above a label it means it is active low, for example:  reset-bar  (say 'reset-bar'). The Links page lists datasheet websites.

Recommended book: TTL Cookbook

Rapid Electronics: 74 series ICs


74 series families

The 74LS (Low-power Schottky) family (like the original) uses TTL (Transistor-Transistor Logic) circuitry which is fast but requires more power than later families. The 74 series is often still called the 'TTL series' even though the latest ICs do not use TTL!

The 74HC family has High-speed CMOS circuitry, combining the speed of TTL with the very low power consumption of the 4000 series. They are CMOS ICs with the same pin arrangements as the older 74LS family. Note that 74HC inputs cannot be reliably driven by 74LS outputs because the voltage ranges used for logic 0 are not quite compatible, use 74HCT instead.

The 74HCT family is a special version of 74HC with 74LS TTL-compatible inputs so 74HCT can be safely mixed with 74LS in the same system. In fact 74HCT can be used as low-power direct replacements for the older 74LS ICs in most circuits. The minor disadvantage of 74HCT is a lower immunity to noise, but this is unlikely to be a problem in most situations.

For most new projects the 74HC family is the best choice. The 74LS and 74HCT families require a 5V supply so they are not convenient for battery operation.


Open Collector Outputs

Some 74 series ICs have open collector outputs, this means they can sink current but they cannot source current. They behave like an NPN transistor switch.

The diagram shows how an open collector output can be connected to sink current from a supply which has a higher voltage than the logic IC supply. The maximum load supply is 15V for most open collector ICs.

Open collector outputs can be safely connected together to switch on a load when any one of them is low; unlike normal outputs which must be combined using diodes.

open collector output


74HC and 74HCT family characteristics

The CMOS circuitry used in the 74HC and 74HCT series ICs means that they are static sensitive. Touching a pin while charged with static electricity (from your clothes for example) may damage the IC. In fact most ICs in regular use are quite tolerant and earthing your hands by touching a metal water pipe or window frame before handling them will be adequate. ICs should be left in their protective packaging until you are ready to use them.

Recommended book: TTL Cookbook

Rapid Electronics: 74 series ICs


74LS family TTL characteristics

Recommended book: TTL Cookbook

Rapid Electronics: 74 series ICs



Quad 2-input gates

The 74132 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals.

quad 2-input gates

7402 quad 2-input NOR

The 7402 IC is shown separately because it has an unusual gate layout.

7402 quad 2-input NOR gates

Triple 3-input gates

Notice how gate 1 is spread across the two sides of the package.

triple 3-input gates

Dual 4-input gates

NC = No Connection (unused pin).

dual 4-input gates

7430 8-input NAND gate

NC = No Connection (unused pin).

7430 8-input NAND gate

Hex NOT gates

The 7414 has Schmitt trigger inputs to provide good noise immunity. They are ideal for slowly changing or noisy signals.

hex NOT gates

7490 decade (0-9) ripple counter
7493 4-bit (0-15) ripple counter

These are ripple counters so beware that glitches may occur in any logic gate systems connected to their outputs due to the slight delay before the later counter outputs respond to a clock pulse.

The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain.

The counter is in two sections: clockA-QA and clockB-QB-QC-QD. For normal use connect QA to clockB to link the two sections, and connect the external clock signal to clockA.

For normal operation at least one reset0 input should be low, making both high resets the counter to zero (0000, QA-QD low). Note that the 7490 has a pair of reset9 inputs on pins 6 and 7, these reset the counter to nine (1001) so at least one of them must be low for counting to occur.

Counting to less than the maximum (9 or 15) can be achieved by connecting the appropriate output(s) to the two reset0 inputs. If only one reset input is required the two inputs can be connected together. For example: to count 0 to 8 connect QA (1) and QD (8) to the reset inputs.

7490 and 7493 ripple counters

NC = No Connection (unused pin).

# on the 7490 pins 6 and 7 connect to an internal AND gate for resetting to 9.

For normal use connect QA to clockB and connect external clock signal to clockA.

Connecting in a chain

Please see below for details of connecting ripple counters like the 7490 and 7493 in a chain.


74390 dual decade (0-9) ripple counter

The 74390 contains two separate decade (0 to 9) counters, one on each side of the IC. They are ripple counters so beware that glitches may occur in any logic gate systems connected to their outputs due to the slight delay before the later counter outputs respond to a clock pulse.

The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means a counter output can directly drive the clock input of the next counter in a chain.

Each counter is in two sections: clockA-QA and clockB-QB-QC-QD. For normal use connect QA to clockB to link the two sections, and connect the external clock signal to clockA.

For normal operation the reset input should be low, making it high resets the counter to zero (0000, QA-QD low).

Counting to less than 9 can be achieved by connecting the appropriate output(s) to the reset input, using an AND gate if necessary. For example: to count 0 to 7 connect QD (8) to reset, to count 0 to 8 connect QA (1) and QD (8) to reset using an AND gate.

74390 dual decade counter

For normal use connect QA to clockB and connect external clock signal to clockA.

Connecting in a chain

Please see below for details of connecting ripple counters like the 74390 in a chain.


74393 dual 4-bit (0-15) ripple counter

The 74393 contains two separate 4-bit (0 to 15) counters, one on each side of the IC. They are ripple counters so beware that glitches may occur in logic systems connected to their outputs due to the slight delay before the later outputs respond to a clock pulse.

The count advances as the clock input becomes low (on the falling-edge), this is indicated by the bar over the clock label. This is the usual clock behaviour of ripple counters and it means means a counter output can directly drive the clock input of the next counter in a chain.

For normal operation the reset input should be low, making it high resets the counter to zero (0000, QA-QD low).

Counting to less than 15 can be achieved by connecting the appropriate output(s) to the reset input, using an AND gate if necessary. For example to count 0 to 8 connect QA (1) and QD (8) to reset using an AND gate.

74393 dual 4-bit counter

Connecting in a chain

Please see below for details of connecting ripple counters like the 74393 in a chain.


Connecting ripple counters in a chain

The diagram below shows how to link ripple counters in a chain, notice how the highest output QD of each counter drives the clock input of the next counter.

connecting ripple counters

74160-3 synchronous counters

These are synchronous counters so their outputs change precisely together on each clock pulse. This is helpful if you need to connect their outputs to logic gates because it avoids the glitches which occur with ripple counters.

The count advances as the clock input becomes high (on the rising-edge). The decade counters count from 0 to 9 (0000 to 1001 in binary). The 4-bit counters count from 0 to 15 (0000 to 1111 in binary).

For normal operation (counting) the reset, preset, count enable and carry in inputs should all be high. When count enable is low the clock input is ignored and counting stops.

The counter may be preset by placing the desired binary number on the inputs A-D, making the preset input low, and applying a positive pulse to the clock input. The inputs A-D may be left unconnected if not required.

The reset input is active-low so it should be high (+Vs) for normal operation (counting). When low it resets the count to zero (0000, QA-QD low), this happens immediately with the 74160 and 74161 (standard reset), but with the 74162 and 74163 (synchronous reset) the reset occurs on the rising-edge of the clock input.

Counting to less than the maximum (15 or 9) can be achieved by connecting the appropriate output(s) through a NOT or NAND gate to the reset input. For the 74162 and 74163 (synchronous reset) you must use the output(s) representing one less than the reset count you require, e.g. to reset on 7 (counting 0 to 6) use QB (2) and QC (4).

74160-3 counters

* reset and preset are both active-low
preset is also known as parallel enable (PE)

Connecting in a chain

Please see below for details of connecting synchronous counters like the 74160-3 ICs in a chain.


Connecting synchronous counters in a chain

The diagram below shows how to link synchronous counters such as 74160-3, notice how all the clock (CK) inputs are linked. Carry out (CO) is used to feed the carry in (CI) of the next counter. Carry in (CI) of the first 74160-3 counter should be high.

connecting synchronous counters

74192 up/down decade (0-9) counter
74193 up/down 4-bit (0-15) counter

These are synchronous counters so their outputs change precisely together on each clock pulse. This is helpful if you need to connect their outputs to logic gates because it avoids the glitches which occur with ripple counters.

These counters have separate clock inputs for counting up and down. The count increases as the up clock input becomes high (on the rising-edge). The count decreases as the down clock input becomes high (on the rising-edge). In both cases the other clock input should be high.

For normal operation (counting) the preset input should be high and the reset input low. When the reset input is high it resets the count to zero (0000, QA-QD low).

The counter may be preset by placing the desired binary number on the inputs A-D and briefly making the preset input low. Note that a clock pulse is not required to preset, unlike the 74160-3 counters. The inputs A-D may be left unconnected if not required.

74192-3 up/down counters

* preset is active-low

Connecting in a chain

Please see below for details of connecting these up/down counters in a chain.

Connecting up/down counters in a chain

The diagram below shows how to link 74192-3 up/down counters with separate up and down clock inputs, notice how carry and borrow are connected to the up clock and down clock inputs respectively of the next counter.

connecting 74192-3 up/down counters

74HC4017 decade counter (1-of-10)
74HC4020 14-bit ripple counter
74HC4040 12-bit ripple counter
74HC4060 14-bit ripple counter with internal oscillator

These are the 74HC equivalents of 4000 series CMOS counters. Like all 74HC ICs they need a power supply of 2 to 6V. For pin connections and functions please see:


7442 BCD to decimal (1 of 10) decoder

The 7442 outputs are active-low which means they become low when selected but are high at other times. They can sink up to about 20mA.

The appropriate output becomes low in response to the BCD (binary coded decimal) input. For example an input of binary 0101 (=5) will make output Q5 low and all other outputs high.

The 7442 is a BCD (binary coded decimal) decoder intended for input values 0 to 9 (0000 to 1001 in binary). With inputs from 10 to 15 (1010 to 1111 in binary) all outputs are high.

Note that the 7442 can be used as a 1-of-8 decoder if input D is held low.

Also see: 74HC4017 and 4017 both are a decade counter and 1-of-10 decoder in a single IC.

7442 BCD to decimal (1 of 10) decoder

7447 BCD to 7-segment display driver

The appropriate outputs a-g become low to display the BCD (binary coded decimal) number supplied on inputs A-D. The 7447 has open collector outputs a-g which can sink up to 40mA. The 7-segment display segments must be connected between +Vs and the outputs with a resistor in series (330ohm with a 5V supply). A common anode display is required.

Display test and blank input are active-low so they should be high for normal operation. When display test is low all the display segments should light (showing number 8).

If the blank input is low the display will be blank when the count input is zero (0000). This can be used to blank leading zeros when there are several display digits driven by a chain of counters. To achieve this blank output should be connected to blank input of the next display down the chain (the next most significant digit).

The 7447 is intended for BCD (binary coded decimal) which is input values 0 to 9 (0000 to 1001 in binary). Inputs from 10 to 15 (1010 to 1111 in binary) will light odd display segments but will do no harm.

7447 BCD to 7-segment display ddriver

74HC4511 BCD to 7-segment display driver

This is the 74HC equivalent of the CMOS 4511 display driver. Like all 74HC ICs it needs a power supply of 2 to 6V. For pin connections and functions please see 4511.