How to install RISC OS 3 on Acorn A310:
---------------------------------------

* What's needed:
 - 4 RISC OS 3 chips (the references are 0270, 251-01 to 0270, 254-01 for
RISC OS 3.0. It'll be something different with RISC OS 3.10). We'll call them
ROM1 to ROM4, fitted in sockets labelled IC24 to IC27 on the motherboard.
The socket for IC24 is located on the right side at the rear of the machine
whereas IC27 is on the far left side.

Step 1: Remove the RISC OS 2 roms with a flat screwdriver. Be careful not to
bend the pins (it helps if you lift each side a little bit at a time). When
the roms are removed from the sockets, store them in an electrostatic-safe
area: plug them into antistatic foam (best) or aluminium-coated polystyrene
(if you don't have the former). The aluminium foil prevents any charge
build-up which could be fatal to the roms.

Step 2: Check how the rom pins are numbered:

       Small dot determines lowest pins
       V

     1 o         32
     2           31 <-- Here
     3           30 <-- Here
     4    RO3    .
     .           .
     .    ROMs   19
    15           18
    16           17

Step 3: Bend pins #30 and #31 (watch out ! not 31 and 32) of all 4 roms
90 degrees outward.
It has to be done like this since the sockets don't provide the 2 highest
bits of the rom adress (they have to be fetched elsewhere) but tie to the
ground instead.

Step 4: We have to connect pins 30 and 31 of the roms to two signals from
IC28 (this a 74HC573 chip directly soldered to the motherboard, at the left
of the backplane connector).

First locate pins 16 and 17 on IC28

      Small dot indicating lowest pins
      V
    1 o        20
    2          19
    3          18
    4          17   <-- here
    5   IC28   16   <-- here
    6          15
    7          14
    8          13
    9          12
   10          11

Step 5: Connect all pins #30 of all 4 roms together (as close as possible to the chip)

Step 6: Connect all pins #31 of all 4 roms together (as close as possible to the chip)

Step 7: Connect ROM4 pin #30 to IC28 pin #17 (ROM4 is closest to IC28)

Step 8: Connect ROM4 pin #31 to IC28 pin #16 (ROM4 is closest to IC28). Yes,
there's a crossover between wires but it's correct this way !

Step 9: Look at links LK12 on the motherboard (2x2 block of pins). Cut track
between 1 and 2, between 3 and 4 (very superficially, careful). Connect with a wire
pin 1 to 3, and 2 to 4. In case your A310 has jumpers instead of copper
tracks, then obviously you don't have to cut anything.

Step 10: Do a DELETE-power-on (this will reset the CMOS ram contents). If all
went well, you'll see the RISC OS 3 splash screen on start-up.

Yann Perrais (yann.perrais@libertysurf.fr)