Texas Instruments 990 Manual page 124

Step Symptom
Raster stable, screen
blank, timing errors,
previous word select
error, keyboard self-
test failure, test LED
not lighted
0
<§:
-
Q )
--
Table 2-8. Fault Isolation Procedures for Apparent CRU Interface Problems
Probable Cause Troubleshooting Procedure
Normal Indication
CRU base for diag-
Reinitialize. CRU base correct
nostic not properly
initialized, CRU bit
decoder, multiplexer
Input buffers or
Exercise interface
See figure 2-6.
byte selector
with CD-C verb.
(figure 1-36)
If Abnormal
Exec:utie IT verb, enter
correct CRU base and
interrupt level.
See figure 2-6.