BBC Microcomputer Service Manual Oct 1985 Section 1 BBC Micro ...

6.5.2 Test ROM
The listing shown below is the object code for a ROM which could prove
extremely useful for fault-finding an apparently dead machine,
especially if you do not have a PET. There are three routines given,
but you could extend the idea for up to 8 different routines if you
wanted to do so. The idea is that a 2764 ROM is put in place of the
operating system ROM and the routine which the system starts on powerup
or break is determined by taking the address lines that would
normally be connected to A10, All and Al2 (pins 21, 23 and 2 of the
2764 respectively) and have some means of attaching them to +5 volts or
0 volts.
This can be done crudely by bending up the three pins so that they don'
t engage in the IC socket, and soldering on to them three leads
terminating in crocodile clips. These can then be used to select the
address by clipping on to the +5V and OV rails, being careful not to
let them short out. For a system that is to be used regularly for
fault-finding, it is wise to use either a DIL switch or better still a
thumbwheel switch, properly mounted.
The three routines given are:-
Routine "0": Provides a chip select pulse for each memory-mapped device
around the board in turn. The pin numbers at which each pulse should
appear are given in the program.
Routine "1": This sets up the teletext mode of graphics by programming
the 6845 and the video processor appropriately. Then codes 0 to 255 are
stored in the first four pages of video RAM. If there is a RAM fault
then the display will not be the succession of ASCII character which
you would expect, and by careful thought about which characters are in
error, you should be able to diagnose where the problem lies. The
pattern is periodically re-written so that intermittent faults will
show up and you can try temperature exercising any suspect chips.
Routine "2": This is similar to the previous routine but by
incrementing a location on the screen it checks the combination of
reading and writing; ie if the RAM can be written to but not read then
the character at location &7C01 will not cycle through 0 to 255 since
the read instruction will be in error. All these routines are working
in machine code at high speed and therefore it is easy to use an
oscilloscope to probe around the circuit to see what has gone wrong.
10 FOR N% = 0 TO 2 : REM ie 3 tests available
20 PROCtest(N%)
30 NEXT
40 *SAVE ROMIMAG 3000 + 2000
50 END
60
70 DEFPROCtest(N%)
80 offset% = &400*N%
90 0% = &3000 + offset%
100 P% = &F800
110 table% = P% + &200
120 !(&33FC + offset%) = &F800 : REM RESET vector
130 opt% = 5
140 IF N% = 0 PROC strobe select lines
150 IF N% = 1 PROC DRAM test
160 IF N% = 2 PROC RNW exercise
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