HP Dc7700 HP Compaq dx7300 and dc7700 Business PC Technical Reference Guide, 1 - Page 179

Keyboard C.2.1 PS/2-Type Keyboard Transmissions The PS/2-type keyboard sends two main types of data to the system; commands (or responses to system commands) and keystroke scan codes. Before the keyboard sends data to the system (specifically, to the 8042-type logic within the system), the keyboard verifies the clock and data lines to the system. If the clock signal is low (0), the keyboard recognizes the inhibited state and loads the data into a buffer. Once the inhibited state is removed, the data is sent to the system. Keyboard-to-system transfers (in the default mode) consist of 11 bits as shown in Figure C-2. Tcy Tcl Tch Clock Data Start Bit Th-b-t (LSb) Data 0 Data 1 Data 2 Data 3 Data 4 Data 5 Data 6 (MSb) Data 7 Parity Bit Stop Bit Parameter Minimum Nominal Maximum Tcy (clock cycle) 60 us -- 80 us Tcl (clock low) 30 us 41 us 50 us Tch (clock high) 30 us -- 40 us Th-b-t (high-before-transmit) -- 20 us -- Figure C-2. PS/2 Keyboard-To-System Transmission, Timing Diagram The system can halt keyboard transmission by setting the clock signal low. The keyboard checks the clock line every 60 µs to verify the state of the signal. If a low is detected, the keyboard will finish the current transmission if the rising edge of the clock pulse for the parity bit has not occurred. The system uses the same timing relationships during reads (typically with slightly reduced time periods). The enhanced keyboard has three operating modes: ■ Mode 1-PC-XT compatible ■ Mode 2-PC-AT compatible (default) ■ Mode 3-Select mode (keys are programmable as to make-only, break-only, typematic) Modes can be selected by the user or set by the system. Mode 2 is the default mode. Each mode produces a different set of scan codes. When a key is pressed, the keyboard processor sends that key's make code to the 8042 logic of the system unit. The When the key is released, a release code is transmitted as well (except for the Pause key, which produces only a make code). The 8042-type logic of the system unit responds to scan code reception by asserting IRQ1, which is processed by the interrupt logic and serviced by the CPU with an interrupt service routine. The service routine takes the appropriate action based on which key was pressed. Technical Reference Guide www.hp.com C-3