Fluke 6080A Service Manual page 125

A chip select for the I/O circuitry is also generated by U11. Two additional levels of
decoding generate the individual device selects. If U11 is operating correctly, but the
decoded chip select is not properly generated, three internal diagnostic tests may be of
use.
I/O Diagnostic Tests
If the data write selects to the display latches, IEEE-488 talker/listener IC (U28), the
module I/O control circuitry, or the control outputs are not generated properly,
momentarily ground TP1 on the controller. This initiates a diagnostic routine that
continuously writes the data byte 10101010 (binary) to each decoded I/O write
location (labeled W or RW in Figure 6B-2). Momentarily grounding TP2 performs the
same action, but writes the data byte 01010101 (binary) instead.
These diagnostic tests write the data bytes very fast so that an oscilloscope can be easily
triggered. Inspect the various I/O select signals and their relationship to the data and
address signals. Normal software activity is halted so the instrument power must be
cycled to terminate the test. The display should show an odd combination of digits and
segments since it is displaying an alternating bit pattern rather than the normal display
data. Although the module I/O selects are generated by the tests, it may be easier to test
the module I/O circuitry using the latch test once the microprocessor circuitry is fully
functional.
If the data read selects to the IEEE-488 talker/listener IC (U28) or the status input
buffers are not generated properly, momentarily ground TP3. This initiates a
diagnostic routine that continuously reads data from each decoded read position
(labeled R or RW in Figure 6B-2). Cycle the instrument power to terminate this
diagnostic function.
READ SELECTS (R), WRITE SELECTS (W), READ/WRITE SELECTS (RW)
Figure 6B-2. Address Decoding
TROUBLESHOOTING AND REPAIR
DIGITAL CONTROLLER
6B-21.
6B-7