74HC595 Shift Register Symbol
Definition: The 74HC595 Shift Register symbol represents an 8-bit serial-in, parallel-out (SIPO) shift register integrated circuit that converts a serial SPI-compatible data stream into eight simultaneous parallel output bits (QA–QH), used in circuit diagrams to indicate output expansion for microcontrollers, with pins DS (serial data), SHCP (shift clock), STCP (storage/latch clock), /MR (master reset), QA–QH (parallel outputs), and QH' (serial cascade output), as standardised under IEEE 315-1975 / ANSI Y32.2 rectangular IC symbol conventions.
Also known as: 74HC595, 595 shift register, serial-to-parallel converter, SIPO shift register, SPI output expander, 8-bit shift register, 74HCT595, SN74HC595.
What the 74HC595 Shift Register symbol means
The 74HC595 Shift Register symbol marks a point in a circuit diagram where serial data from a microcontroller (such as an Arduino or ESP32) is expanded into eight parallel output lines, enabling control of up to eight LEDs, relays, or other digital loads using only three microcontroller I/O pins. Data is clocked into the shift register one bit at a time via DS on rising SHCP clock edges; after all eight bits are loaded, a STCP latch clock pulse transfers them simultaneously to the output latches QA–QH.
In circuit schematics, the 74HC595 symbol appears wherever parallel output expansion is required while preserving microcontroller pins. Multiple 74HC595 ICs can be daisy-chained: the QH' serial output of the first device feeds the DS input of the second, allowing any number of output bits using the same three control wires.
How to identify the 74HC595 Shift Register symbol
The 74HC595 symbol is a rectangle with input pins on the left (DS serial data, SHCP shift clock, STCP latch clock, /MR active-low master reset) and output pins on the right (QA through QH, eight parallel outputs, plus QH' serial output for cascading). VCC and GND supply pins appear at the top and bottom or are grouped on one side. The part number '74HC595' or 'SN74HC595' is printed inside or beside the rectangle, and the designator letter 'U' identifies it as an IC.
Function in a circuit
The 74HC595 shift register shifts data serially through eight D-type flip-flops on each rising SHCP clock edge, with bit values entering at DS and propagating toward QH'. After eight clock pulses, a rising edge on STCP latches the eight stored bits into the output storage register, updating QA–QH simultaneously and cleanly. The /MR pin (active-low) asynchronously clears all shift register stages to 0 when asserted low. The QH' output passes the last-shifted bit to a downstream 74HC595 for cascading. The output stage can source or sink up to 35mA per pin, sufficient to drive LEDs directly.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-12 defines rectangular qualifying symbols for shift registers using 'SRG8' notation (8-stage shift register). The 74HC595 in IEC-format schematics is shown as a block labelled 'SRG8' with appropriately labelled clock and data inputs. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 / ANSI Y32.2 specifies the rectangular IC symbol with functional pin labels and a 'U' reference designator. The 74HC595 is universally drawn as a simple labelled rectangle in ANSI-style schematics, matching the datasheet pin layout. |
| Key difference | IEC 60617-12 uses the standardised internal label 'SRG8' to indicate an 8-bit shift register, while IEEE 315 / ANSI Y32.2 uses part-number labelling inside the rectangle. Both use the same rectangular block outline. |
Terminals / pins
| Pin | Name |
|---|---|
| ds | DS |
| shcp | SHCP |
| stcp | STCP |
| mr | /MR |
| qa | QA |
| qb | QB |
| qc | QC |
| qh | QH |
Typical values
Supply voltage: 2V–6V (74HC series). Output current: up to 35mA source or sink per pin, 70mA total. Maximum clock frequency: 25MHz at 4.5V. Logic levels: VIH ≥ 3.15V, VIL ≤ 1.35V (at 4.5V). Propagation delay: ~13ns typical. Operating temperature: -40°C to +125°C (74HC), -40°C to +85°C (consumer grade).
Where the 74HC595 Shift Register symbol is used
- LED matrix and bar-graph driver circuits controlled by a microcontroller with limited I/O pins
- Relay and output expander modules adding 8 or more digital outputs to Arduino, ESP32, or Raspberry Pi
- Seven-segment display multiplexing, driving segment lines via a cascaded pair of 74HC595 ICs
- Daisy-chained LED strip controllers, where multiple 595 ICs control separate groups of LEDs over one SPI bus
- Output expansion in PLC simulator boards and educational electronics platforms
- Addressable switch banks in industrial panel interfaces without consuming extra MCU pins
- Shift register DACs and bit-pattern generators in test and measurement circuits
Example
In an Arduino LED matrix project, a 74HC595 symbol is connected with DS to Arduino digital pin 11 (MOSI), SHCP to pin 13 (SCK), and STCP to pin 10 (CS); the eight QA–QH outputs each drive an LED through 470Ω resistors. A second 74HC595 is daisy-chained via QH' → DS to control eight more LEDs, extending to 16 outputs with the same three Arduino pins — the 74HC595 symbol on the schematic immediately shows the serial-to-parallel expansion architecture.
Key facts
- The 74HC595 shift register symbol represents an 8-bit SIPO (serial-in, parallel-out) IC that converts three-wire SPI serial data into eight parallel digital outputs (QA–QH).
- Data is shifted in on rising edges of SHCP and latched to QA–QH outputs simultaneously on a rising edge of STCP, preventing intermediate output glitches during shifting.
- The /MR pin (active-low master reset, pin 10) asynchronously clears all eight shift register stages to logic 0 when pulled low.
- The QH' serial output passes the 8th bit through to a downstream 74HC595, allowing unlimited cascading with only three microcontroller pins regardless of the number of ICs.
- Each output pin (QA–QH) can source or sink up to 35mA, enabling direct LED drive without additional transistors for typical indicator LEDs.
- The 74HC595 is compatible with 3.3V and 5V microcontrollers; the 74HCT595 variant includes TTL-compatible input thresholds for 5V logic.
- The symbol is drawn per IEEE 315-1975 / ANSI Y32.2 as a rectangular IC block with the reference designator 'U' and labelled input/output pins.
Frequently asked questions
What does the 74HC595 shift register symbol mean in a schematic?
The 74HC595 symbol means a serial-to-parallel output expander IC is used at that point. Three input lines (DS data, SHCP shift clock, STCP latch clock) receive serial data from a microcontroller and the IC converts it to eight simultaneous parallel output bits (QA–QH), expanding the microcontroller's output capability without using additional I/O pins.
What does the 74HC595 symbol look like?
The 74HC595 symbol is a rectangle with input pins on the left — DS (serial data in), SHCP (shift clock), STCP (storage/latch clock), and /MR (active-low reset) — and eight output pins on the right labelled QA through QH, plus QH' (cascade serial output). VCC and GND are also shown. The part number '74HC595' appears inside or beside the block.
How many pins does the 74HC595 symbol have?
The 74HC595 symbol shows the following functional pins: DS (serial data in), SHCP (shift register clock), STCP (storage register clock/latch), /MR (active-low master reset), QA–QH (8 parallel outputs), QH' (serial cascade output), VCC, and GND — totalling 16 pins in the physical PDIP or SO-16 package.
What is the difference between SHCP and STCP on the 74HC595?
SHCP is the shift register clock — data on the DS pin is captured into the internal shift register on each rising SHCP edge. STCP is the storage register (latch) clock — a rising edge on STCP transfers the current contents of the shift register to the output latch, updating QA–QH simultaneously. Keeping STCP low while clocking SHCP prevents outputs from changing during serial data loading.
How do you cascade multiple 74HC595 ICs?
To cascade 74HC595 ICs, connect QH' (serial output) of the first IC to DS (serial data input) of the second IC. All ICs share the same SHCP and STCP lines. After 16 SHCP pulses (for two ICs), all 16 shift register bits are loaded; a single STCP pulse then latches all 16 outputs simultaneously. This extends the output count to any multiple of 8 using the same three microcontroller pins.
What standard defines the 74HC595 symbol?
The 74HC595 symbol follows IEEE 315-1975 / ANSI Y32.2 rectangular IC symbol conventions for North American schematics. IEC-format schematics use IEC 60617-12 notation with 'SRG8' inside the block to indicate an 8-bit shift register. Both draw the same rectangular outline with labelled pins.
Can the 74HC595 drive LEDs directly?
Yes. Each QA–QH output of the 74HC595 can source or sink up to 35mA, which is sufficient to drive standard indicator LEDs (typically 10–20mA) through a current-limiting resistor (typically 220–470Ω at 5V supply). For higher-current loads such as relays or high-brightness LEDs, a transistor or MOSFET driver stage is added after each output.
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