Maintenance—492/492P Service Vol. 1 (SN B030000 & up)
Power-Up Self-Test Mode
The microcomputer enters a self-test mode when the in
strument is turned on if this mode is selected (Fig. 4-24). In
this mode the instrument does not operate normally. The
microcomputer performs the following steps, stopping the
test to indicate the source of any problem found by blinking
an LED on the Processor board.
Addresses are specified as hexadecimal numbers in this
description.
Step 1. At power-up, the microcomputer vectors to the
self-test in the ROM at the top of address space U2028 on
the Memory board. The microcomputer first verifies the
checksum of U2028. If the routine for this step runs, but
does not obtain the correct checksum for the ROM, the rou
tine halts and blinks the ROM 17 LED, DS1044.
This step uses only U2028 and no other memory; so if
the test does not blink the LED and does not proceed to
step 2, U2028 is probably the culprit. Consider first, howev
er, that the correct ROM must be installed, both phases of
the clock on the Processor board must be present, and the
microcomputer system (exclusive of the instrument bus or
GPIB) must be operating correctly. If in doubt about the
6800 microprocessor, its bus, or the microcomputer bus,
skip to the instructions under Microcomputer Test Mode, to
exercise the microcomputer in a more simple manner.
Step 2. The microcomputer next checks the condition of
RAM. This step does not rely on the RAM being ok to ex
ecute. The procedure is: the microcomputer loads the bit
pattern 01010101 into a RAM location, reads the location,
and compares what is returned to what was stored. The
microcomputer then repeats this test with the pattern
10101010.
The microcomputer attempts to test all RAM addresses.
If it finds an error on the Memory board, it stops the test and
pulses the RAM LED, DS1042—once for an error in U2035,
twice for an error in U2032, and three times for an error in
both RAM IC’s; these IC’s are on the Memory board. If the
microcomputer finds an error on the GPIB board, it pulses
the LED 7, 8, or 9 times in a similar manner for low
RAM—U1046 and U1037—or 9, 10, or 11 times for high
RAM—U1042 and U1032. The microcomputer continues to
repeat the number of pulses after an error is found.
Step 3. The microcomputer proceeds to checksum all the
ROMs. A checksum is stored in the header of each ROM.
This is compared to a checksum formed by the successive
8-bit sum of each byte in the ROM starting at the fourth
location in the ROM. The upper eight bits of the ROM’s
address (stored at the first location) are also added to the
checksum. Thus, if a ROM is installed in the wrong socket,
it's checksum does not verify.
The ROM sockets, including those on the GPIB board,
are checked starting at the lowest address (U1012 on the
Memory board). The check starts by looking at the MSB in
the first and fourth locations in the ROM’s address range. If
both bits are one, it is assumed that no ROM is installed.
When a defective ROM is found, the routine discontinues
the test and pulses repeatedly the ROM LED, DS1038, to
indicate the ROM socket where an error was found:
ROM Socket Board
Pulses
U1012
Memory
1
U1017
Memory
2
U1023 Memory
3
U1028 Memory
4
U2012
GPIB 6
U2019
GPIB 7
U2025
GPIB 8
U2031
GPIB
9
U3012 GPIB 10
U3019 GPIB
11
U3025 GPIB
12
U3019 GPIB
13
U2012 Memory 14
U2017 Memory 15
U2023 Memory 16
U2028
Memory
17
If this step fails, it can be forced to continue checking
ROMs; just turn power off and unplug the defective ROM,
then turn power on to restart the self-test.
Step 4. The microcomputer checks part of the instru
ment bus PIA, U3016 on the Processor board. First the
microcomputer writes to the A control register and then
reads back from the register. Next it repeats these oper
ations with the A data direction register. If either of these
attempts fail, the routine stops and pulses the bus LED,
DS1036.
Step 5. This step checks part of the GPIA, U2047 on the
GPIB board (if installed). The microcomputer resets the
GPIA and checks to see that the GPIA is not addressed to
talk or listen. The GPIA is then set to listen-only mode and
checked to see that it is addressed to listen. Then the GPIA
is set to talk-only mode and checked to see that it is ad
dressed to talk. If any part of this step fails, the test stops
and pulses repeatedly the GPIB LED, DS1034.
REV JUN 1983
4-37
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