Troubleshooting
2-24
Digital I/O tests
The Digital I/O on the Model 2001 consists of four open collector outputs, and one
TTL-level input. Outputs originate from Port A of the 68302 microprocessor (U626),
lines PA4 through PA7. PA4 drives Output #1, PA5 drives Output #2, PA6 drives Out-
put #3 and PA7 drives Output #4. These signals are buffered by a 2596A open collector
driver (U612).
The following table summarizes how the lines of Port A of the microprocessor and the
IN/OUT designations of the 2596A driver correspond to the digital output lines:
The single Digital Input is buffered by protection circuitry (CR619, CR626, R610, R733)
and read by the 68302 at PB8.
NOTE
Digital I/O tests may fail or not be run depending on the hardware
and Þrmware revisions of the instrument.
Test 103.1 through 103.4 – Digital output
Pass/Fail
Defective digital output port
These tests make use of the fact that the Port A registers of the 68302 microprocessor
(U626) are bidirectional even though in normal use they are programmed as outputs
only.
Diodes and resistors are conÞgured around the 2596 driver (U612) so that Output #1
feeds back to PA5, Output #2 feeds back to PA6, Output #3 feeds back to PA7 and Out-
put #4 feeds back to PA4.
During test 103.1, PA4 is programmed as an output and PA5 is programmed as an in-
put. As PA4 (Output #1) is toggled from high to low, the signal is read (veriÞed) at PA5.
For test 103.2, PA5 is programmed as an output and PA6 is programmed as an input.
As PA5 (Output #2) is toggled from high to low, the signal is read (veriÞed) at PA6.
Tests 103.3 and 103.4 check Outputs #3 and #4 in a similar manner.
Success of these tests assures the basic functionality of Port A, the 2596 and associated
components.
Digital
Output
68302 (U626)
Port A
2596A (U612)
IN, OUT
Output #1
Output #2
Output #3
Output #4
PA4
PA5
PA6
PA7
IN4, OUT4
IN3, OUT3
IN1, OUT1
IN2, OUT2
Type
Failure analysis
Description
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