The CNT line is used as a clock for the keyboard. On each transition of this line, one bit of data is clocked in from the keyboard. The keyboard sends this clock when each data bit is stable on the SP line . The clock is an active low pulse. The rising edge of this pulse clocks in the data. After a data byte has been received from the keyboard, an interrupt from the 8520 is issued to the processor. The keyboard waits for a handshake signal from the system before transmitting any more keystrokes. This handshake is issued by the processor pulsing the SP line low then high. While some keyboards can detect a 1 microsecond handshake pulse, the pulse must be at least 85 microseconds for operation with all models of Amiga keyboards. If another keystroke is received before the previous one has been accepted by the processor, the keyboard microprocessor holds keys in a 10 keycode type-ahead buffer.