Keyboard - HP 9020 Service Manual

2-30
Theory of Operation
Keyboard
The keyboard is a microprocessor-controlled assembly consisting of two boards, a keyboard elec-
tronics board and a keyswitch board (Figure 2-19). The assembly plugs into three ribbon cable
connectors mounted in the base of the computer. The assembly connectors are as follows:
C - 50-pin; lOP bus
PC - 50-pin; power and miscellaneous signals
TB - 14-pin; optional input device interface
The keyboard electronics board and keyswitch board are physically interconnected by screws
and a 20-pin ribbon cable. The ribbon cable is the keyboard bus which functionally intercon-
nects the boards.
The keyswitch board contains the keyswitches and minimal electronics needed to scan the key
matrix. The keyboard electronics board contains an 8048 microprocessor which scans the key
matrix, the real-time clock/non-volatile memory (RTC/NVM), the programmable tone generator
(beeper) the optional input device, and the lOP bus interface.
Select code 6 (SC6) provides access to the printer, keyboard, RTC/NVM, optional input device,
and the beeper. Each of these devices has one or more interface register(s) assigned to it. The
device can be read from or written to by accessing the assigned interface register. The interface
registers are designated as RX registers.
To eliminate bus conflicts with the printer, which has its own bus interface circuitry, the lOP bus
drivers are enabled only when registers R1, R3, R9, R10, R11, R12, and R15 are accessed. The
printer's bus drivers are enabled when registers RO, R1, R2, R4, R5, R6, and R7 are accessed. R1 is
shared, with the keyboard driving the lower 12 hits and the printer driving the upper 4 bits. For
R12, only the lower 8-bit bus buffer is enabled on the keyboard for both reads and writes.
NSTATUS and NFLAG for all of select code 6 are driven from the keyboard. Printer STATUS and
FLAG are sent to the keyboard on the 50-pin ribbon cable carrying power to both assemblies.
The keyboard electronics board contains a nickel-cadmium (NICAD) battery assembly to maintain
the RTC/NVM data during periods of no mainframe power. The battery assembly has a minimum
life expectancy of approximately 4 years. It can maintain the data during a power-off condition for a
minimum of 10 days.
The keyboard circuitry, contained on the keyboard electronics board, can be divided into six main
subsections. Five of these subsections interface to a comtnon internal bi-directional data bus. The
remaining subsection interfaces the data bus to the lOP bus. The six subsections are:
• lOP Bus Interface
• Miscellaneous Status and Control
• Keyboard and Input Device Scanning
• Beeper
• RTC/NVM Logic and Enable
• Power-Up/Power-Down Sense Circuitry