Fluke PM6690, Service Manual, Page: 60 / 200

processor. See Figure 6-22. The loading starts when PROGN
is set low. The FPGA responds with a negative pulse on
INITN and setting DONE low. After loading 1442016 bits,
which takes approximately 2.2 s, the FPGA sets DONE high if
the loading was successful. If an error is detected, INITN is
set low. One clock pulse after DONE is set high all I/Os on the
FPGA are defined. If the loading of the FPGA is not success-
ful, the program just goes on with the rest of the startup
procedure.
The fan is set to 8.4 V. See Figure 6-23. Measure on J19 or
J28. The input amplifiers are initialized and a "click" from the
relays is heard. The I
2
C bus is used for controlling the relays.
Note: The I
2
C bus is of the utmost importance for the start of
the instrument. The FPGA, the LCD and the relays in
the input amplifiers all need a faultless I
2
C bus to work
properly.
Note: If the Flash PROM is exchanged, it must be replaced
by a preprogrammed Flash PROM. Voltage and
timebase calibration must be performed anew. The
utility program must be used for transferring the cali-
bration results to new factory calibrations. The serial
number and the oscillator option must also be pro-
grammed by the utility program.
The fan is kept at +8.4 V for the first 8.3 minutes. After that
the fan is temperature controlled. The processor reads the
temperature via the I
2
C bus every 10th second. IC U39 mea-
sures the temperature.
The keys on the display board are read over the I
2
C bus. If a
key is pressed, the I
2
C bus circuit U3 notices that and sends an
interrupt to the processor. Check at J13:9; low is interrupt.
The processor then scans the keys via the I
2
C bus to find the
depressed key. See Figure 6-30. During the scanning there
may appear some extra interrupts. This is not an error condi-
tion.
6-16 Troubleshooting
Figure 6-18
Important locations on PCB 1 during startup.
J19
R492
J28
U39
X33
Figure 6-19
Important locations on PCB 2 during startup.
X3
X2
X5
X1
R33
U3
R34
X4