Troubleshooting
2-66
Test 308.3 — 2M overload ohms
Type
Pass/Fail
Fault message
2M Overload OHMS
Description
The 9.6µA ohms source is switched in with the /OHMD line at U223 and U220. U232 pin 2 to
3 connects REFBUF to U233 pin 3, and 7.2V appears across R277. (For a more detailed descrip-
tion of the circuit setup, see test 304.1.) In this test, there is no path provided for current to flow
outside of the ohms source. Current flow is downward through the parallel path of Q211 and
Q212, through CR213, VR204, and CR214 to common. U238 sees the open-circuit ohms volt-
age of about 6.6V at pin 3. It is used in a unity-gain configuration, and its output, which is the
COMPLIANCE signal, goes to comparator U215. U232 pin 15 to 14 is open, and the divider in
this case is made up of R235 and R234. This divider places pin 5 of U215 at 5.7V. Since the
COMPLIANCE line at 6.6V is more positive than the 5.7V on /OVLD_OHM, the line is pulled
low at about -15V. All of the OVLD lines are latched by U202, parallel loaded into U201, and
serially input to the A/D board at J1026 pin 15. There is no A/D measurement for this test. The
pass/fail criteria is based on the value of the /OVLD_OHM bit.
Drawing reference
Analog Board; 2002-100
Components
U220, U223, U232, U233, and associated components.
Bit patterns
Bit pattern*
Register
—U400—
01111011
—U811—
00001101
—U224—
00010111
—U432—
10000000
—U810—
00000011
—U206—
01110000
—U203—
10101010
—U411—
11111011
—U809—
11100111
—U207—
11001111
—U221—
11101011
—U406—
00000100
AD_STB
MUX_STB
R1_STB
R2_STB
*Bits associated with register IC terminals as follows:
Q
Q
Q
Q
Q
Q
Q
Q
87654321
87654321
87654321
87654321
IC pins: Q8=11, Q7=12, Q6=13, Q5=14, Q4=7, Q3=6, Q2=5, Q1=4.
