Service manual

Service Manual
Part 5 - Main PCB fault diagnosis
This chapter deals with fault diagnosis and repair of the
main PCB at component level.
The larger part of this chapter describes how to use the
integral test software which is incorporated in the
computer's ROMs, and includes details of the power-on
self-test (POST), the fault display and the diagnostic
interface.
The remainder of the chapter gives details of how to
repair a 'dead' computer (see the section entitled
Repairing a 'dead' computer on page 5-23).
Test equipment you will need
You will need
• an Acorn Probe test kit (part number 0386,804) which
contains the following:
• display adaptor
• interface cable
• interface cable with grabber connectors (for
attaching to machines without a diagnostic
connector)
• test disc (for use in a host machine)
• probe test ROMs (4) (0286,834 to 0286,837)
• host machine fitted with a user port (if you are using
the external diagnostic interface)
• frequency counter
• 100 MHz oscilloscope
• DC voltmeter
• earth continuity tester
• serial port loopback plug – see Appendix C - Serial
port loopback plug
• headphones (32 Ohm impedance)
• chip extraction tools (68/84 pin)
• standard items such as soldering/desoldering
workstation, screwdrivers, pliers, etc.
IMPORTANT: Use anti-static precautions (ie antistatic
matting and wrist-straps) at all levels of servicing.
Integral test software overview
The integral test software is invoked through the ARM
reset vector, and will automatically select a test mode
defined by the type of reset conditions and the presence
or absence of external diagnostic equipment.
When no external equipment is connected, a standard
user start up is performed. The test software will
examine a status register in IOC to determine whether
the reset was a soft reset or a power-on reset:
• If a soft reset occurred (Reset key operated or re-
execution of the reset vector) then no further test
operations take place and RISC OS is immediately
started. This ensures that the most common type of
reset operation is not delayed by operation of the
POST.
• If the reset appeared to have been caused by a power-
on operation, a short test sequence (the POST) is
started. This is accompanied by changes of the screen
colour to indicate test progress. If faults are detected,
these are indicated by a blinking LED on the floppy
disc drive (if fitted). If no faults are found, this test
sequence will last between 2 and 12 seconds (
dependant on memory configuration).
You can use three types of test interface to modify this
standard operation. These are:
• the test link
If you have fitted the test link, the POST will be
performed regardless of whether the IOC power-on bit
is set. This is useful to force repeated test operations
on reset without power cycling, and to force a test
sequence if the power-on circuitry is faulty.
• the display adapter
If you have fitted the display adapter, the POST
sequence is forced and the test execution is
accompanied by a series of status messages on the
attached display which indicate test progress and
results. These results may be used to suggest which
areas of the system are malfunctioning, although they
will rarely identify an actual faulty component.
• the external diagnostic interface
If you are using the external diagnostic interface, the
integral test software will perform no further automatic
operations, but will await commands from a second (
host) computer to perform further tests, enter
exercising loops etc. You can request the POST
sequence. The status messages that are normally sent
to the display adapter will instead appear on the host
machine's screen.
Power-on self-test (POST)
Note 1: During the POST, the screen mode is set to suit
a simple 15kHz monitor (Monitor 0, Sync 0). This will
produce a signal unsuitable for VGA or High Resolution
monitors, resulting in an unsynchronised screen display.
If a stable display is not shown on a type 0 monitor, this
may indicate either a video system fault, or some more
fundamental fault which stops the test software itself from
running.
Note 2: The various power-on key combinations should
be held until the message in stage 5 (or the red screen
border resulting from a power-on delete operation)
appear – the keys will be ignored if released before the
self-test sequence has completed.
Part 5 - Main PCB fault diagnosis Issue 2, June 1991 5-1