7400 Series Guide: 74HC91/74LS91 (8-bit shift register)

The 74×91 (ex 74LS91) is chip that has an 8-bit shift register with two AND gated serial inputs and complementary outputs.

In this guide, you will learn everything you need to know about this chip and how you can use gated shift registers in your own projects.

What does the 74HC91 / 74LS91 do?

The 74×91 gives you an 8-bit serial-in serial-out shift register. A shift-register is a memory circuit that that is used to store and move bits.

This chip has AND-gated serial inputs. An AND gate is a logic gate that outputs 1 (HIGH) only if all of its inputs are 1 (HIGH). This means that both the D (data) and the E (enable) input must be high to input a 1 into the shift-register.

How To Use This Chip

The 74HC91 comes in a 14-pin package, and you need to connect it to power before you can use it. Most 7400 ICs support a VCC voltage of 5V. One difference between the HC and LS version of the chip is that the 74HC91 supports 2V to 6V, while the 74LS91 only supports 5V.

The maximum current you can pull out of one output pin is 0.4 mA when the pin is high (sourcing) or 8 mA when the pin is low (sinking). This can differ between models, so check the datasheet of your model to verify.

Once you’ve connected it to power, you need to enable it be setting E (pin 11) high. Then you can start shifting bits into the shift register using the data input D (pin 12), plus the clock input (pin 9).

74×91 Pinout

The 74×91 has 14 pins and contains an 8-bit serial-in serial-out shift register with gated serial inputs. The pins are laid out as shown in the pinout diagram below:

Pin #	Type	Description
1	NC	Not in use (Not Connected).
2	NC	Not in use (Not Connected).
3	NC	Not in use (Not Connected).
4	NC	Not in use (Not Connected).
5	Power	Positive power supply (VCC). Connect to +5V power.
6	NC	Not in use (Not Connected).
7	NC	Not in use (Not Connected).
8	NC	Not in use (Not Connected).
9	Input	Clock input.
10	Power	Connect to ground (GND).
11	Input	Enable control signal.
12	Input	Data input.
13	Output	Output from the shift register.
14	Output	Output from the shift register (inverted).

Alternatives and Equivalents for 74HC91 / 74LS91

There are many versions of the 74×91 chip. They all have the same functionality, but with different specifications such as supported voltages and maximum current output.

Here’s a list of a few equivalents of this chip:

• 74HC91 (High-speed CMOS)
• 74HCT91 (High-speed CMOS, TTL compatible)
• 74LS91 (High-speed TTL)
• 74LVC91 (Low Voltage TTL)
• 74AC91 (Advanced CMOS)
• 74ALS91 (Advanced Low-Power Schottky TTL)
• 74F91 (Very High Speed)
• 74C91 (CMOS, similar to the 4000-series)

Some manufacturers also add a prefix, such as the SN74LS91 by Texas Instruments.

Can’t find the 74×91 anywhere? Then try one of the following IC alternatives:

• 74×595 – 8-bit shift register.
• 74×94 – 4-bit shift register.
• 74×95 – 4-bit shift register, parallel in, parallel out, serial input.
• 74×96 – 5-bit parallel-in/parallel-out shift register.
• 74×99 – 4-bit bidirectional universal shift register.
• 74×164 – 8-bit serial-in parallel-out (SIPO) shift register.
• 74×165 – 8-bit parallel-in serial-out (PISO) shift register.
• 74×166 – parallel-load 8-bit shift register.
• CD4006 – 18-stage shift register.
• CD4014 – 8-stage parallel in shift register.
• CD4015 – Dual 4-stage shift register.
• CD4021 – 8-stage parallel in shift register.
• CD4031 – 64-stage shift register.
• CD4035 – 4-stage parallel-in/parallel-out shift register.

If you can’t find the 74×91 IC in your local electronics store, don’t worry, you’ll most likely find it in one of the stores listed on this page of online stores where you’ll find components and tools for all your electronics projects.

Datasheet for the 74LS91 and 74HC91 chips

Download the PDF datasheet for your version of the 74×91 here:

• SN74LS91 (Texas Instruments)