Texas Instruments 99/4A - Page 87

i< EYB 0 ARD ?■' REMOTE HANDHELD CONTROL UN IT 1idTERR AC I NG
Keyboard and ramcte control are scanned Linder software control .
The k"evboard is scanned b v i- select lines.. The 9 detect lines are
monitored by the TMS 9901 programmable system interface (see
tignrL 47. p. 127).
Two other select lines are used Icr selecting the remote handheld
control li n i t the 11 v e detect lines of these units are shared wit h
those at the keyboard.
HARDWARE DESCRIPTION
The keyboard and remote handheld
cessor via the TMS 9901 progra
and P4 of the 9901 are program
U302 (74LS156) , a dual 2 to 4
three to eight line decoder,
board and joystick port.
un 11
mmab 1 e
med
interfaces to the micropro —
s v s t e m interface. Fins F~ ' F 3
as outputs and used as input for
decoder. This IC is wired as a
Outputs are used to scan the kev-
Two outputs of U302 (pin 4 and 5) are used to select the remote
handheld control units. Full up resistors R322 and R329 are used
to supply enough current when the outputs are in the Ionic high
state (+5 volt). CR 301 in combination with R321 and CR302 with
R320 are used to get a good defined logic zero state. since R320
and R321 which are connected to the +5 volt supply generates a -.6
volt drop over CR301 and CR302.
KEYBOARD SCANNING
The keyboard is divided in two parts. Each part consisting of 4
rows with 5 keys. The outputs of the right four rows (INT 3. 4. 5
and 6) and the left four rows (INT 7. 8, 9 and 10) are connected
to the preprogrammed inputs of the TMS 9901 with the same names.
These inputs are scanned under software control to determine which
key is pressed down. The keyboard uses pull-up resistors for
putting all lines in the logic one state when no kev is hit. The
capacitors are used for reducing RF interference .
REMOTE HANDHELD CONTROL UNIT
The handheld unit uses the same inputs to the TMS 9901 as the
righthand keyboard side.
99 /4A KEYBOARD DESIGN
The 99/4A keyboard circuit is significantly changed from the
design used in the 99/4, although the principle of operation is
the same. Seven outputs are used on U302, and one additional in-
terupt line (PS), in order to scan the additional keys used on the
99 /4A keyboard.
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