Onsemi MC74HCT245ADT Octal Bus Transceiver: Datasheet, Pinout, and Application Circuit Guide

The MC74HCT245ADT from Onsemi is a high-speed CMOS logic octal bus transceiver, designed to facilitate bidirectional data transfer between data buses. It is part of the 74HCT family, which combines the low power consumption of CMOS technology with the high noise immunity and TTL compatibility of HCT. This integrated circuit is widely used in applications requiring non-inverting bidirectional interface between bus systems with different voltage levels or to simply amplify and buffer data lines.

Datasheet Overview: Key Specifications

The device operates over a broad voltage range, typically from 4.5V to 5.5V, making it ideal for 5V system environments. Key electrical characteristics from the datasheet include:

Logic Family: HCT (High-Speed CMOS, TTL compatible)

Number of Bits: 8 (Octal)

Direction Control: Managed by a single Direction (DIR) input pin.

Output Enable: An active-low Output Enable (OE) pin is used to disable the outputs, placing them in a high-impedance state to effectively isolate the buses.

TTL Compatibility: All inputs are TTL-level compatible, meaning they can be directly driven by TTL outputs without requiring pull-up resistors.

Low Power Consumption: Features typical CMOS low power dissipation with a maximum ICC of 80 µA.

High Noise Immunity: Characteristic of CMOS devices, it offers excellent noise immunity, with a worst-case noise margin of approximately 0.4V for VCC = 4.5V.

Pinout Configuration

The MC74HCT245ADT is available in a standard TSSOP-20 package. Its pinout is crucial for correct circuit design:

Pins 1 (DIR): Direction Control. A high level on DIR allows data from the A bus to the B bus. A low level does the reverse (B to A).

Pin 19 (OE): Output Enable (Active Low). When high, both A and B ports are placed in a high-impedance state.

Pins 2-9 (A1-A8): Side A Data I/O.

Pins 11-18 (B1-B8): Side B Data I/O.

Pin 10 (GND): Ground.

Pin 20 (VCC): Positive Supply Voltage.

Application Circuit Guide

A typical application circuit for the MC74HCT245ADT involves using it as a bidirectional buffer between a microcontroller and a peripheral data bus.

1. Power Supply: Connect VCC (Pin 20) to a stable +5V supply and GND (Pin 10) to the common ground.

2. Data Buses: Connect the 8-bit data bus from a microcontroller (e.g., a port) to the A pins (A1-A8).

3. Peripheral Connection: Connect the B pins (B1-B8) to the data lines of the target peripheral or another bus system.

4. Direction Control: Connect the DIR pin to a microcontroller GPIO. The logic state of this pin determines the data flow direction.

DIR = HIGH (Logic 1): Data flows from A (microcontroller) to B (peripheral).

DIR = LOW (Logic 0): Data flows from B (peripheral) to A (microcontroller).

5. Output Enable: Connect the OE pin to another microcontroller GPIO. To enable the transceiver's outputs, this pin must be pulled LOW (Logic 0). Pulling it HIGH tristates all outputs, effectively disconnecting the two buses. This is critical for shared bus architectures.

This setup allows a 5V microcontroller to communicate bidirectionally with multiple peripherals on a shared bus, with the transceiver providing the necessary current buffering and electrical isolation.

ICGOODFIND Summary

The Onsemi MC74HCT245ADT is a robust and versatile solution for bidirectional data bus interfacing. Its TTL-compatible inputs, 3-state outputs, and bidirectional flow control make it indispensable in systems like industrial controls, computer peripherals, and any design where multiple devices share a common data path. Its primary strength lies in its ability to isolate and amplify data signals while ensuring seamless communication between different parts of a system.

Keywords:

Bidirectional Buffer

Bus Transceiver

TTL-Compatible

High-Impedance State

Data Direction Control