PESA RCP-48X, Руководство пользователя

Страница: 27 / 33

FUNCTIONAL
4
DESCRIPTION
Section 4
page 4.6
Functional
RCP–48X
remainder of the ID bits (ID1-ID5) are either floating or grounded by the
switchcard. Floating pins are pulled high by pullup resistors on the CPU
board. The CPU may use these bits to detect what switchcard is attached.
Keyboard Scan
The RCP–48X switchcard contains circuitry capable of scanning up to 64
pushbuttons. The scan circuit is arranged as an eight row by eight column
array. While the circuitry is capable of serving 64 pushbuttons, the printed
circuit board will only accept 50 pushbuttons. To scan the keyboard, the
microprocessor on the CPU board performs read cycles that enable SEL1.
This occurs for addresses in the range of 800h to FFFh. SEL1 provides a
low-active chip select for a 3 to 8 line decoder (U3 pin 5). A second low-
active chip select is provided by address bit A3 at U3 pin 4. The three
least significant address bits (A0-A2) are connected to the input of the
decoder (U3 pins 1, 2, and 3). One of the eight low-active outputs of the
decoder is selected by placing the appropriate address on the input of the
decoder. Since partial decoding is used, the keyboard circuitry is mapped
to several addresses within the SEL1 address range. The software in the
CPU only uses the lowest available addresses to access the keyboard.
Table 4–2 contains the addresses used to access each row of the key-
board circuit.
Address Row U3 pin #
800h KB_ROW0 15
801h KB_ROW1 14
802h KB_ROW2 13
803h KB_ROW3 12
804h KB_ROW4 11
805h KB_ROW5 10
806h KB_ROW6 9
807h KB_ROW7 7
Table 4–2 Keyboard Memory Map
The CPU scans all eight rows of the keyboard. Each row of pushbuttons
contains up to eight individual switches. Example: KB_ROW0 will simulta-
neously enable pushbuttons S1-S8. If any of these switches is pressed,
the low-active signal will be passed through the pushbutton contacts to
one of the eight column signals (KB_COL0-KB_COL7). If the pushbutton
is not pressed, the switch contacts are broken and the column signal will
be pulled high by resistor pack RP1. The SEL1 signal also enables U4 to
place the KB_COL signals on the data bus. Thus, by performing a read
Switchcard Continued: