onsemi MC74ACT245DWG Octal Bus Transceiver: Datasheet, Pinout, and Application Circuit Guide
The MC74ACT245DWG from onsemi is a high-performance octal bus transceiver that serves as a fundamental building block in modern digital systems. As part of the advanced ACT family, it is designed for bidirectional asynchronous communication between data buses, providing an essential interface in applications requiring voltage translation, buffering, and signal isolation.
This device is built with high-speed, high-drive CMOS technology, combining the benefits of low power consumption with the robust drive capability of TTL outputs. It is capable of operating over a wide voltage range of 4.5V to 5.5V, making it ideal for 5V system environments. The transceiver features non-inverting 3-state outputs and can handle bidirectional data flow controlled by two management pins: the Direction Control (DIR) and the Output Enable (OE).
A key strength of the MC74ACT245 is its ability to isolate bus segments. When the Output Enable pin (OE) is driven high, the outputs are placed in a high-impedance state, effectively disconnecting the two buses. This prevents data collisions and allows multiple devices to share the same bus in a multiplexed system. The direction of data flow is determined by the DIR pin; a high level on DIR allows data to pass from the A bus to the B bus, while a low level reverses the direction.
Pinout Configuration (DWG: SOIC-20 Package)
The MC74ACT245DWG is housed in a 20-pin Wide SOIC package. Its pinout is crucial for correct PCB layout:
Pins 1 (DIR) and 19 (OE) are the control pins.
Pins 2-9 (A1-A8) form one 8-bit bus port.
Pins 11-18 (B1-B8) form the second 8-bit bus port.
Pin 10 (GND) and Pin 20 (VCC) are for ground and power supply, respectively.
Key Datasheet Specifications
Logic Family: ACT (Advanced CMOS Logic)
Supply Voltage Range (VCC): 4.5V to 5.5V
High-Level Output Current: -24mA
Low-Level Output Current: 24mA
Propagation Delay (max): 10ns at 5V
Operating Temperature Range: -40°C to +85°C
Package: 20-Lead SOIC (DWG)
Typical Application Circuit Guide
A common application is level shifting between a 5V microcontroller and a 3.3V peripheral. While the ACT245 is a 5V part, its inputs are TTL-compatible, meaning a 3.3V logic high (~2.4V and above) is correctly recognized as a HIGH. This allows a 3.3V device to safely drive its inputs. However, its outputs are a full 5V, so to protect a 3.3V input on a receiving device, a simple current-limiting resistor (e.g., 100Ω - 1kΩ) in series with each data line is often sufficient to clamp the current and allow the 3.3V device's internal protection diodes to safely handle the voltage.
For pure 5V system bus interfacing, the circuit is straightforward:
1. Connect VCC to +5V and GND to system ground.
2. Connect the DIR pin to a control signal to set the desired data direction.
3. Drive the OE pin low to enable the outputs. It should be pulsed high to tri-state the bus during hot-swap or multiplexing events.
4. Decouple the power supply with a 0.1µF ceramic capacitor placed close to the VCC and GND pins.
ICGOOODFIND: The onsemi MC74ACT245DWG remains a highly reliable and efficient solution for bidirectional bus buffering and isolation. Its robust drive strength, fast switching speed, and simple control interface make it an indispensable component for system designers working on legacy 5V systems or requiring a robust interface between different voltage domains. Its wide availability and proven performance ensure its continued use in industrial control, automotive electronics, and communications infrastructure.
Keywords:
Bidirectional Bus Transceiver, Octal Interface, 3-State Output, Voltage Level Translation, Bus Isolation
