Service manual

Service Manual
sequence 0123456789 is repeated across the top line of
the display. Every 4 digits represents a 32 bit word.
Watch for missing or corrupted display.
As the start of the screen memory is known to be at
physical address &2000000, it should be possible to
determine the exact device that is faulty by examining
the corruption pattern on the display.
The default 'memory size' is &100000 bytes (1 Mb),
however this may be cycled through 0.5, 1, 2 and 4Mb
memory sizes by pressing the 'M' key.
When using the ROMs on a machine having memory
content other than 1Mb, the video display may at first
appear out of line or incorrect. In this instance press the '
M' key repeatedly until the required memory size has
been selected.
The memory test is cyclic and on completion of each full
memory test a full stop ('.') will be displayed. The 4Mb
test takes about 29 seconds.
If for some reason the video display is completely blank
or unreadable (eg because of a video fault), a printed
output may be obtained by selecting option 1, the output
being produced at the printer port as well as on the VDU.
If an error is found in the memory, the display will show:
AT ADDRESS &nnnnnnnn WROTE &pppppppp READ
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
where nnnn is the faulty address, pppp is the data
written to that address and xxxxxxxx is the data read
back from that address in binary form.
The memory tests do not terminate unless an error is
found, in which case after reporting 8 or 9 errors, the test
will terminate.
An additional check is now made on the state of CMOS
RAM control lines C0 and C1. If either of these lines are
short-circuit to 0 Volts, the test ROMs will indicate this on
power-up.
Repetitive reset test
This section of test code is intended for use when the
main memory test menu fails to initialise.
To make use of this section of the ROMs the following
test equipment is required:
• Oscilloscope
• Signal or pulse generator
The purpose of the code is to produce certain signals
around specific areas of the PCB. These signals may
then be monitored using the oscilloscope to assess the
operation of that area of the circuit.
The code is written in a loop which should execute three
times before proceeding to the main memory test. For
this reason the machine must be reset repeatedly.
A suitable square wave or, preferably, a negative-going
pulse generator output at 10 KHz should be connected to
the reset line via a component connected to IOC IC58 pin
29.
After setting the border colour to white, the signals should
be observable in the following order.
Return to start for three executions.
Part 5 - Main PCB fault diagnosis Issue 2, June 1991 5-25