Keithley 2002 Repair Manual page 75

Troubleshooting
Components
Bit patterns
Test 306.3 — High ohms voltage source
Type
Fault message
Description
Drawing reference
Components
Bit patterns
2-62
U220, U223, U232, and associated components.
—U811—
00001101
—U224—
00010111
—U400— —U432—
01111011 10000000
*Bits associated with register IC terminals as follows:
Q Q Q Q
87654321 87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.
Pass/Fail
High ohms voltage source
The HIOHMSRC line at pin 7 of U231 is approximately 14.4V. This voltage is generated by the
Ohms Reference U230 switching OHMREFHI onto C227 that is relative to REFBUF. HIOHM-
SRC is then switched through U244 pin 6 to 7, through Q226, through the 100k Ω leg of the input
divider, and through Q242 to BUFFIN. The 20V range MUX signal path is now used. Q205 is
turned on so that the signal reaches R204. Q203 is always on because its gate and source are tied
together by R241. 7.2V appears at U217 pin 3. Since U217 is a unity gain buffer, 7.2V also ap-
pears at U222 pin 7. The U222 control lines are configured to switch the pin 7 input to the U222
output and to the A/D buffer in the X1 configuration.
Analog Board; 2002-100
U222, U231, U244, and associated components.
—U811—
00001101
—U224—
00010111
—U400— —U432—
01111011 10000000
*Bits associated with register IC terminals as follows:
Q Q Q Q
87654321 87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.
Bit pattern*
—U810—
00000011
—U206—
01100000
—U203—
11010110
—U411—
11111011
Q Q Q Q
87654321 87654321
Bit pattern*
—U810—
00000011
—U206—
01111000
—U203—
10101110
—U411—
11111011
Q Q Q Q
87654321 87654321
Register
—U809— AD_STB
11100111
—U207— MUX_STB
00001111
—U221— R1_STB
11101001
—U406— R2_STB
00000100
Register
—U809— AD_STB
11100111
—U207— MUX_STB
01101111
—U221— R1_STB
11111100
—U406— R2_STB
00000100