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Asynchronous digital logic uses handshaking to ensure timing between circuits are robust to gate and wire delays. 2 and 4-phase handshakes are the two widely used handshaking protocols. Top: A sender and receiver are communicating over simple request and acknowledge lines. The sender drives the request line, and the receiver drives the acknowledge line. Middle: Timing diagram of two, two-phase communications. From the beginning, the sender initiates the first communication by raising the request line. The receiver then acknowledges the sender by raising the acknowledge line, which completes the first communication. Some time later, the sender initiates the second communication by lowering the request line. The receiver then acknowledges the sender by lowering the acknowledge line, which completes the second communication. Bottom: Timing diagram of a one, four-phase communication. From the beginning, the sender initiates the communication by raising the request line. The receiver then acknowledges the sender by raising the acknowledge line. The sender then acknowledges the receiver by lowering the request line. Finally, the receiver acknowledges the sender by lowering the acknowledge line, which returns all signal lines to their original state and completes the communication

By Sam Fok - Own Work CC BY-SA 4.0