Facit 4431 Service Instruction page 26

26
prox. 10Hz on the LEDs, and the signature ana-
lysis routine will be executed
continuously~
Except otherwise noted, these diagnostics de-
tect circuit nodes that are effectively shorted
to ground potential. In situations where there
is more than one possible cause of a fault con-
dition the diagnostic routine displays the er-
ror code of the first component tested.
Exemple: In the grid buffer test (Item 3 be-
Tow) a bad keyboard column driver would have
the same effect (as far as the diagnostic rou-
tine is concerned) as a bad grid buffer, but
since the grid buffer is tested before the key-
board drivers the grid buffer error code would
be displayed. The fault can then be located by
signature analysis.
TEST PROCEDURE
1 Upon POWER UP all seven LEDs will be lit for
0.5 seconds.
2 A checksum test is performed on the keyboard
EPROM (U3).
3 The grid buffer (U5) is tested as follows:
The twelve keyboard drivers are deactivated
and the grid buffer is read. If any low vol-
tage levels are detected the grid buffer is
faulty.
4 The modifier keys/country select buffer (U6)
is tested as follows:
The Shift and Control keys are read in
through their buffer. (The Caps and Num keys
lock so they are excluded, the country se-
lect Nibble is similarly ignored).
If a low voltage level is detected the spe-
cial keys buffer is faulty.
5 The country/product select code is displayed
on the LEDs for at least one second. (See
country and product select tables page 3).
6 Initialization is requested from the Main
Logic Board. If no response is detected with
in 5 seconds the keyboard assumes it is in
the test fixture and enters the keyboard
test mode. If a response from the MLB occurs
within 5 seconds of the initialization re-
quest then normal keyboard operation ensures
7 The keyboard test mode is entered only if
there is no communication between the key-
board and the main unit, which means that
either the keyboard is in the test fixture
that has the serial 1/0 lines tied together
or tha the serial 1/0 lines are faulty.
The bell will ring twice to signify the
start of the Keyboard Test Mode. The tech-
nician will then type the following twelve
character sequence:
X B G L Return 0 K LINE FEED D
J
F SET UP
After each keystroke the bell will sound if
the grid driver for that column of the key-
board is functioning properly.
Each keybaord column driver (AO through A11)
is sequentially activated and the keyboard
is continuously scanned until a key in that
column reads. When a key closure is detected
the column drivers are disabled and the grid
is read again. If a low voltage level is de-
tected the column driver is faulty.
If the bell fails to ring after any keystro-
ke then it is possible that either the co-
lumn driver or one line (row) of the grid
buffer is open. The problem can be localized
to the faulty component either by signature
analysis or by the following procedure:
Restart the keyboard test sequence by POWER
ON/OFF. Choose the keys for the 12 columns
from the same grid row. If keys from diffe-
rent column driver !Cs fail to ring the bell
then the grid buffer line for that row is
open. See Schematic Appendix.
8 The ASCII "DEL" character (7F} is sent out
over the serial line. If this character with
valid start and stop bits is received on the
input line then all seven LEDs are lit and
the signature analysis loop is executed. If
no character or an invalid character is re-
ceived then the serial 1/0 driver IC (U7} is
faulty.
ERROR CODES
(FLASHING)
000 0001
000 0010
000 0011
000 0100
000 0101
000 0110
000 0111
(NOT FLASHING)
111 1111
CAUSE
U3 EPROM CHECKSUM ERROR
U5 GRID BUFFER ERROR
U6 SPECIAL KEYS BUFFER ERROR
U7 KEYBOARD DRIVER ERROR
U8 KEYBOARD DRIVER ERROR
U9 KEYBOARD DRIVER ERROR
U7 SERIAL 1/0 RECEIVER-DRIVER
ERROR
PASSED ALL DIAGNOSTIC TEST
9.4 TESTING THE MONITOR
For a definite test to determine whether the
Monitor or Main Logic Board is at fault, check
the Power Supply for +12V and that the signals
running from the Main Logic to the monitor
through connector X4, see fig below. If these
signals are present and no Video or abnormal
video is observed, then the monitor control
circuit board or the complete monitor should be
replaced.