Texas Instruments 990 - Page 123

Table 2-7. Fault Isolation Procedures When Unable to Load Diagnostic
~
Step
Symptom
Probable Cause
Troubleshooting Procedure
Normal Indication
If Abnormal
\0
~
Diagnostic does not
Constant keyboard
Scope circuit.
Logic 1 at P2-66
Trace back until fault is
Ul
~
execute
interrupt
{Pl-66 for 2nd
located. Replace faulty
N
~
controller).
component(s).
\o
-....)
0
-
2
Diagnostic does not
Master clear not
Scope circuit.
Logic 1 at P2-66
Trace back until fault is
load, test LED
functioning or
{Pl -66 for 2nd
located. Replace faulty
lighted
always active
controller).
component(s).
3
Diagnostic does not
CRU interface signal
Master clear system
CRUBIT12 = 0
Remove CPU or CPU
load
shorted to another
(cycle chassis power
CRUBIT13 = 0
buff er. Ground each
signal or to ground
off and back on)
CRUBIT14 = 0
input. Check effect of
CRUBIT 15 = 0
grounded input on other
CRUBITOUT = 0
input. Check continuity
STORECLOCK- = 1
to locate short circuit
MODSELA- = 1
path. Repair.
N
I
MODSELB- = 1
N
-
CR UBITINT = 1
NKBINT = 1
4
Diagnostic does not
CRU interface signal(s)
Same as for step 3
Same as for step 3.
Remove VDT controller.
load, CRU lines not
shorted to ground
Check CRUBIT, 12-15,
shorted to each
and CRUBITOUT for
other
ground.