2 Fitting BBC BASIC

The job of fitting the BBC BASIC board in the ATOM requires removal and replacement of the bottom case section, plugging and unplugging ICs, plugging in the board itself, and one or more soldered joints. Users without constructional experience should refer the task to an Acorn dealer.

The BBC BASIC board fits on Side Two (the component side) of the ATOM board. It plugs into four of the existing IC sockets, which give it access to power and the necessary signals, and one flying lead connects to a pad on the ATOM board. To allow BBC and ATOM modes to be selected from the keyboard, three more flying leads must be connected. Whether these are used or not, the main procedure is as follows:

  1. Remove the bottom section of the ATOM case by turning the ATOM upside-down, unscrewing the two screws, and lifting off the bottom.
    Locate LK2 in the top left-hand corner of the ATOM board, next to the 6522 VIA, viewing the board with the keyboard section at the bottom. Make a link at this point with a short length of wire. The joints can be soldered on the component side with a bit of care.
    This link allows the 6522 VIA to generate interrupt requests, used by the additional MOS to implement ESC and the TIME and SOUND features.
  2. Three ICs must be removed from the ATOM board and two of these transferred to the BBC BASIC
    board. If a Utility ROM is fitted, this must also be transferred. The BBC BASIC board plugs into the sockets left empty. Using Figures 1 and 2 to locate the positions of the ICs on the ATOM and BBC boards, transfer them one-by-one according to the following table:
    Position on ATOM board IC6 IC8 IC23 IC24
    Position on BBC board IC9 IC15 - IC4
    IC type 74LS
    138    		 74LS
    00    		 74LS
    138    		 Utility
    ROM

    To remove an IC, carefully prise it out of its socket with a small screwdriver, or use an IC removal tool if available. when replacing an IC make sure all the pins are accurately positioned over their separate sockets before pressing home. All ICs have a polarising mark on their top surface, either a depressed dot to one side or a half moon shape. This must be next to the half moon shape marked in white on the board for the unit to work. Figure 2 shows the correct orientation of the ICs transferred to the BBC BASIC board.

  3. The BBC BASIC board should now be fitted by plugging the pins that extend downwards from the non-component side into the sockets on the ATOM board left empty by the last stage. Figure 2 shows where it sits in relation to the other components on the ATOM board. The top-right hand corner of the BBC BASIC board goes under the fold of the heatsink on the main board, so the right-hand end should be pushed home first, checking that the pins at both ends are positioned correctly. The unit may fail to work if any pin is not plugged in, so be careful not to bend any while inserting the board, and check for a stable fit before going on to the next stage.
  4. Take the end of the wire connected to the point marked 'nwds’ near the top left-hand corner of the BBC BASIC board, and solder it to the fourth pad from the left in the top row of the group of pads marked PL7 in the top left-hand corner of the ATOM board, as shown in Figure 3. If the bus expansion connector is already fitted in this position, the connection will have to be made on the other side of the board.
  5. If keyboard selection of ATOM BASIC and BBC BASIC is required, follow (a) below; otherwise follow (b).

    a) The three wires connected to the BBC BASIC hoard at the right-hand end of the top edge must be soldered to pads in the keyboard section of the ATOM board. Connect the two outside wires, marked 'ctrl in' and 'shift in', as shown in Figure 3, and the centre wire as shown in Figure 4. Figures 3 and 4 show details of the right and left-hand ends of the ATOM board respectively.

    (b) Solder a wire link joining the two left-most holes of the group of four holes marked LK1, near the top edge of the BBC BASIC board. This selects BBC BASIC, and it can be fitted across the other two holes instead to return to ATOM BASIC.
  6. Finally, check that all the these steps have been carried out correctly, and then replace the bottom case section and screws

Power-up the ATOM and press BREAK. If the link described in 6(b) link has been fitted, the ATOM will start up in the selected mode. ATOM BASIC starts up with its usual message, and BBC BASIC displays this:

BBC BASIC
>

If the keyboard mode-select option is being used, pressing BREAK with either the CTRL or one of the SHIFT keys held down will select BBC or ATOM BASIC respectively. Pressing BREAK alone resets the ATOM without changing mode.

Figure 1: Atom motherboard with chips to be removed highlighted

Figure 2: Conversion card fitted into Atom, with chips to be fitted highlighted

Figure 3: Connection to the Shift and Ctrl keys

Figure 4: Connection to the Common keyboard signal

Figure 5: Connection to the nwds signal

Next Chapter