onsemi MC74ACT245DWR2G Octal Bus Transceiver: Datasheet, Pinout, and Application Circuit Guide
The MC74ACT245DWR2G from onsemi is a high-performance, octal bus transceiver that serves as a fundamental building block in modern digital systems. Fabricated with Advanced CMOS Technology, this IC is designed for asynchronous two-way communication between data buses, making it indispensable for interfacing subsystems operating at different voltage levels or for simply boosting the current drive of a microprocessor's I/O ports.
This device is part of the ACT family, which is known for its TTL-compatible inputs and CMOS-compatible outputs, providing both low power consumption and high-speed operation. The '245' designation signifies its function as an 8-bit (octal) transceiver with 3-state outputs, allowing multiple devices to share a common bus without conflict.
Key Datasheet Specifications
Understanding the absolute maximum ratings and recommended operating conditions from the datasheet is critical for reliable design.
Supply Voltage (VCC): 4.5V to 5.5V (Fully operational at 5V ±10%)
Logic Family: ACT (Advanced CMOS with TTL-compatible inputs)
High-Level Output Current (IOH): -24mA
Low-Level Output Current (IOL): 24mA. This robust drive strength is a key feature, enabling the IC to directly drive LEDs or other relatively high-current loads.
Propagation Delay (tp): Typically 8.5ns at 5V, ensuring high-speed data transfer.
Operating Temperature Range: -40°C to +85°C, suitable for industrial applications.
Package: The "DW" suffix denotes a 24-pin SOIC wide-body package, which is suitable for automated assembly processes.
Pinout and Functional Description
The MC74ACT245DWR2G features 24 pins with a symmetrical layout.
Pins 1 (DIR) and 19 (OE): These are the two control pins that dictate the device's operation.
DIR (Direction Control): Determines the direction of data flow. A HIGH on DIR allows data from the A bus to the B bus. A LOW on DIR allows data from the B bus to the A bus.
OE (Output Enable, Active LOW): This pin enables the device's outputs. When OE is HIGH, all outputs are placed in a high-impedance state (3-state), effectively disconnecting the device from the bus. This is essential for bus-sharing applications.
A0-A7 (Pins 2-9): One 8-bit bidirectional data bus.
B0-B7 (Pins 11-18): The other 8-bit bidirectional data bus.
VCC (Pin 20): Supply Voltage (5V).
GND (Pin 10): Ground.
Application Circuit Guide
A typical application involves using the transceiver to interface a microcontroller with a peripheral device or another bus.
Basic Bidirectional Interface:
1. Connect the 5V supply to VCC (Pin 20) and ground to GND (Pin 10). Use a 0.1µF decoupling capacitor placed close to the VCC pin to suppress high-frequency noise.
2. Connect the 8-bit data bus from your source (e.g., a microcontroller) to the 'A' side (Pins 2-9).
3. Connect the target device or bus to the 'B' side (Pins 11-18).
4. Connect the DIR pin to a microcontroller GPIO. The logic level of this pin will control the data direction.
5. Connect the OE pin to another microcontroller GPIO. The transceiver is active only when this pin is pulled LOW. To simplify control, OE can be permanently tied to GND if the bus is not shared with other devices, but this is rare.
Key Design Considerations:
Bus Contention: Always ensure the OE pin is used correctly to prevent multiple devices from driving the bus simultaneously, which can cause excessive current draw and potential damage.
Termination: For long PCB traces or high-frequency operation, series termination resistors (e.g., 22Ω to 33Ω) may be required on the output lines to dampen signal reflections and prevent ringing.
Unused Inputs: Although the ACT family has CMOS inputs, they are TTL-compatible and include pull-down structures. However, it is still good practice not to leave inputs floating.
ICGOODFIND Summary
The onsemi MC74ACT245DWR2G is a versatile and robust solution for bidirectional bus interfacing. Its high-speed performance, significant output drive capability, and 3-state outputs make it an ideal choice for a wide range of applications, including data communication networks, computer peripherals, industrial control systems, and as a general-purpose logic buffer. Its ability to isolate and drive buses is fundamental to ensuring signal integrity and system reliability in complex digital designs.
Keywords: Bidirectional Bus Transceiver, 3-State Output, MC74ACT245DWR2G, Octal Buffer, Bus Interface IC
