Canon A-200 series Service Manual page 75

Before sending data, the keyboard unit always checks the logical level of the KOATA line. There­
fore, it does not send the next key data unless the PB7 signal of the CPU becomes HIGH after
the CPU receives the interruption signal and processes an 8-bit parallel data in the shift register.
Sending the soft reset signal to the keyboard unit is done by keeping the PB6 signal LOW for
more than 20 milli seconds.
By doing so, the KCLK line is forced to become LOW by U123. When the one chip microproces ­
sor in the keyboard unit detects this LOW level signal, it performs the soft reset.
When the soft reset signal is sent to the keyboard unit, the microprocessor in the keyboard will
begin to execute the self-diagnostic program, and then sends 1-byte return code to the keyboard
interface.
On the main P.C.B., the RESET signal is connected to the keyboard connector through U100
and U123, however, this signal is not used by the keyboard unit.
rcN14
100 123
....
rl
9
I
RESET
"
....
PClK
,
K-
RESET
- H
- G
j
F
r - -
Q R D Q R D ClK
-
U94
lK
•
- E
PAO-7
-
D
U116
CO
C~
Ul16
L-<: 0
Cl
123
i
K DATA
- c
-B
Sl
.....
-
A
QA
D
Q
IR1
U109
PB7
,
......
"
r
G1
G2
-
ClR
ClK C
0
r
123
L-!>,09
.......
PB6
"
Figure 6-21
72