NI Digital System Development Board User Manual | © National Instruments | 23
Figure 6. Keyboard Scan Codes
A host device can also send data to the keyboard. Table 8 shows a list of some common
commands a host might send.
The keyboard can send data to the host only when both the data and clock lines are high (or idle).
Because the host is the bus master, the keyboard must check to see whether the host is sending
data before driving the bus. To facilitate this, the clock line is used as a clear to send” signal. If
the host drives the clock line low, the keyboard must not send any data until the clock is released.
The keyboard sends data to the host in 11-bit words that contain a '0' start bit, followed by 8-bits
of scan code (LSB first), followed by an odd parity bit, and terminated with a '1' stop bit. The
keyboard generates 11 clock transitions (at 20 to 30 kHz) when the data is sent, and data is valid
on the falling edge of the clock.
Table 8. Keyboard Commands
Command Action
ED Set Num Lock, Caps Lock, and Scroll Lock LEDs. Keyboard
returns FA after receiving ED, then host sends a byte to set
LED status: bit 0 sets Scroll Lock, bit 1 sets Num Lock, and bit
2 sets Caps lock. Bits 3 to 7 are ignored.
EE Echo (test). Keyboard returns EE after receiving EE.
F3 Set scan code repeat rate. Keyboard returns F3 on receiving
FA, then host sends second byte to set the repeat rate.
FE Resend. FE directs keyboard to re-send most recent scan code.
FF Reset. Resets the keyboard.
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