Troubleshooting
2-19
2.11.2
Digital I/O tests
The Digital I/O port on the Model 2002 consists of four open collector outputs and one TTL-
level input. Outputs originate from Port A of the 68302 microprocessor (U631), lines PA4
through PA7. PA4 drives Output #1, PA5 drives Output #2, PA6 drives Output #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:
Digital Output
68302 (U631) Port A
2596A (U612) IN, OUT
Output #1
PA4
IN4, OUT4
Output #2
PA5
IN3, OUT3
Output #3
PA6
IN1, OUT1
Output #4
PA7
IN2, OUT2
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 or may not fail depending on the firmware revision of the
instrument.
Test 103.1 through 103.4 — Digital output
Type
Pass/Fail
Fault message
Output <n> open/short failure
(<n> = 1 to 4)
Description
These tests make use of the fact that the 68302 MPU (U631) Port A registers are bidirectional
even though in normal use they are programmed only as outputs.
Diodes and resistors are connected around U612 so that OUT1 feeds back to PA5, OUT2 feeds
back to PA6, OUT3 to PA7 and OUT4 to PA4. (Note that signal numbers are scrambled at the
2596 open collector driver, U612. The numbers in the above description refer to the Digital out-
put signal numbers, not to the numbers on U612.)
During test 103.1, PA4 and PA6 are programmed as outputs, and PA5 and PA7 are programmed
as inputs. As PA4 (OUT1) is toggled high and low, the signal is verified at PA5.
During test 103.2, PA5 and PA7 are programmed as outputs, and PA4 and PA6 are programmed
as inputs. As PA5 (OUT2) is toggled high and low, the signal is verified at PA6.
Tests 103.3 and 103.4 test OUT3 and OUT4 in a manner similar to the above.
Drawing reference
Digital Board; 2002-140
Components
U631 PA4-PA7, U612, and associated components.
Test 103.5 — Digital input
Type
Pass/Fail
Fault message
Input continuity failure
Description
This test only verifies that the digital input signal is pulled high at PB8 of the microprocessor
(U631). Success of this test does not guarantee complete functionality of the input port.
Drawing reference
Digital Board; 2002-140
Components
U631 PB8 and associated circuitry.
