Keithley 485 Instruction Manual page 38

RAM Test-If the RAM test fails, the Model 485 will lock up
with all zeroes displayed. Replacing U112 may correct prob-
lem.
Non-Volatile RAM Test-If the NVRAM test fails, the
following message will be displayed.
I
ctrr
This is a sufficient message indicating that the instrument is
probably not properly calibrated since calibration constants
are stored in the non-volatile RAM. The Model 488 will lock
up at this point if the test fails, but operation may be restored
for troubleshooting by pressing any front panel momentary
button. The flashing "CAL" annunciator will indicate that the
unit failed the NVRAM test.
At this point try calibrating the instrument with the constants
already entered by simultaneously pressing in the REL and
LOG buttons until "CAL" is displayed, and then again until
"Stor" is displayed. If error gets corrected, indicating that the
NVRAM is probably good, a full calibration will be needed. If
the error persists try replacing the NVRAM chip (U123).
Again, the Model 485 must undergo a complete calibration
after the problem is corrected.
Sequential Display Test-Segments and annunciators are
sequentially displayed in eight steps. Use Figure 5-1 for
segment identification. The steps are as follows:
a
f
I I
b
a C
d
Figure 5-1. Segment Identification
1. The "a" segments of the digits, and the LOG and CAL an-
nunciators are displayed.
2. The "b" segments of the digits are displayed.
3. The "c" segments of the digits are displayed.
4. The "d" segments of the digits, and the m, ZERO CHECK
and RMT annunciators are displayed.
5. The "e" segments of the digits, minus sign end the REL,
and A annunciators are displayed.
6. The "f" segments of the digits, end the AUTO and BAT
annunciators are displayed.
7. The "g" segments of the digits, and the RCL and n annun-
ciators are displayed.
8. The decimal points, most significant digit and the ST0 and
A annunciators are displayed.
Troubleshooting Test Modes-The troubleshooting mode
is designed to switch on various switching FETs, relays and
logic levels to allow signal tracing through the instrument.
The first displayed mode will reflect the function and range.
For example, assume that "OAl" is displayed. The "1" cor-
responds to the 2nA range, and the "0" is the test number.
The test number can be changed by pressing in the LOG but-
ton. Table 53 lists the test modes for all ranges.
To update the test mode, select the new range and hold in
the LOG button until range number changes.
Troubleshooting consists of selecting the desired test mode
and using the data found in Table 5-3 to signal trace the cir-
cuit.
NOTES:
1. When a different range is selected the LOG button must be
pressed and held in to update the display with the cor-
responding test mode.
2. Do not use AUTO or ZERO CHECK in the self-diagnostic
mode.
3. To get out of the self-diagnostic mode, cycle power.
5-4