4 LINK SETTINGS AND FUNCTIONS

The BBC Microcomputer has a number of "selectable" links. There are 33 of these links on main printed circuit boards (main PCB's) up to issue 31 subsequent issues having 39 links. It may be that future issues will have more than these 39 links available. This section explains the functions of the various links within the machine.

The links are situated at various positions on the main PCB, taking the form of soldered wire, printed track or shorting jumpers. Table 4.1 at the end of this section lists each of the links and provides grid reference information as to their approximate position on the main PCB. This table also identifies the type of link together with the upgrade or expansion with which it deals. Figure 4.1, at the end of this chapter, is a shadow diagram of the main PCB which shows the positions of the various links.

In addition to the links on the main PCB there are 8 more links that can be fitted to the machine, these are situated in the bottom right hand corner of the printed circuit board on which the keyboard is mounted. The keyboard links are shown on the photographs at the end of chapter 5. These keyboard links are not normally fitted on models A or B but some may have to be installed if the floppy disc expansion is to be fitted. For further details see chapter 5.

In order to simplify matters and minimise any ambiguities, the orientation of the various links will be described in terms of the cardinal points of the compass (see shadow diagram fig 4.1).

With the machine placed in front of you in the normal position (with the keyboard facing towards you) the rear of the machine will be referred to as NORTH and the front of the machine as SOUTH. The left and right hand sides will be referred to as WEST and EAST respectively. This is shown diagrammatically in fig 4.1 and on the photographs at the end of chapter 5.

As with all modifications that are to be made to the machine the microcomputer should be completely disconnected from the mains supply before the top cover is removed.

It must be stressed that by making modifications to the links on the main PCB you are likely to invalidate the manufacturers guarantee. If you are in any doubt you should consult your local Acorn dealer.

The top cover can be released by removing the four large fixing screws (which may be marked "FIX") using a No.2 crosspoint screwdriver. The positions of these screws can be seen from the photographs at the end of chapter 5. Two are situated at the rear of the machine and the remaining two are on the underside of the machine beneath the keyboard.

Depending on the particular links that are to be modified it may be necessary to remove the keyboard in order to gain access. Links S4, 58, S10, S14, S17, S18, and S30 are those in question on boards up to issue 3. Boards from issue 4 onwards also have links S35, S36, S37 and S38 beneath the keyboard.

At this stage one must consider the particular links that are to be modified. Table 4.1 at the end of this section specifies whether a particular link is soldered wire, copper track or a jumper.

If any of the soldered wire links are to be modified then the main PCB may have to be removed from the base to allow access to the underside of the PCB.

4.1 Keyboard PCB Removal

The keyboard assembly can be detached by removing the two nuts and bolts situated on either side of the keyboard PCB (please note that some main PCB's prior to issue 4 have two keyboard securing bolts on the left hand side of the keyboard PCB) . The location of these bolts can be seen from the photographs in chapter 5. The next step in removing the keyboard assembly is to carefully unplug the interconnecting ribbon cable from the main PCB (PL13) and also the loudspeaker connector from PL15.

The keyboard can now be safely set aside to reveal the main PCB beneath.

4.2 Removing the Main PCB

Before the main PCB can be removed the two wires from the BNC "video out" socket on the rear of the case must be disconnected. On early issue boards these two wires were soldered but some later issues use push fit connectors. It is important to note which of the two wires is the signal wire and which is the ground wire, so that they can be replaced correctly. It is suggested that the ground wire be marked by placing a piece of coloured insulating sleeving over the wire. On some late issue boards black (0V) and red (signal) wires are fitted as standard.

The next step is to remove each of the seven power supply cables from the main PCB. It is important to take care when removing these cables as a reasonable amount of force may be required. It is particularly important not to bend the flat PCB pins backwards and forwards because they can easily fracture.

All that remains now, is to remove the four screws which secure the main PCB to the base with a No.2 crosspoint screwdriver. The positions of these screws are shown in the photographs at the end of chapter 5. The PCB can now be removed from the base and carefully placed on a work surface which should be clean and free from static charge. The ideal working surface would be an electrically conductive sheet (perhaps aluminium foil) connected via a 1 Megohm resistor to earth. Chapter 2, gives more details on static precautions and working environment. As some of the following description involves handling of MOS devices it is strongly recommended that the reader should be familiar with the section on static hazards as detailed in that chapter.

Before describing the function of each individual link on the main PCB, the procedure for re-assembling the microcomputer will be described.

4.3 Re-Assembling the Machine

Re assembly is essentially the reverse of the dismantling procedure with the main PCB being replaced before the keyboard PCB. There are, however, a number of points to note

1. Do not forget to replace the two wires on the BNC "video out" connector correctly i.e. the ground wire to the tab and the signal wire to the centre pin.
2. When reconnecting the seven power supply wires back to the main PCB be sure to connect the positives and negatives correctly, the red positive wires connect to the pins marked "VCC" and the black negative to the pins marked "0V". The mauve wire connects to the -5V tab. Also make sure that the flat PCB pins fit securely into the socket and are not simply tucked down the side of the plastic cover which shrouds the receptacle. This happens surprisingly regularly the usual symptoms being that the microcomputer becomes microphonic: i.e. when the microcomputer is mechanically shocked the video display "flickers".
3. Be sure not to over-tighten the self tapping screws which are used to fasten the main PCB to the case. If over tightened either the plastic case or the PCB may be damaged or distorted. Also check that when tightened the heads of the screws do not touch any of the copper tracks on the PCB. In latter issue boards this may become a problem. It is a good idea to fit nylon or fibre washers under the heads of the screws. This is done on later issue machines.
4. Conduct a detailed visual inspection of the PCB's paying particular attention to those areas which have a high population of discrete components such as resistors and capacitors. These areas are mainly around the UM1233 video modulator in the NORTH EAST corner of the main PCB, the DIN sockets at the rear of the machine and the "Econet" circuitry, if fitted.
   It is very common when working on the PCB for some of these resistors and capacitors to become bent over and consequently the wires on adjacent components may short together. Take great care to ensure that this does not happen.
   It is likely that on later issue PCB's the number of discrete components will be considerably reduced.
5. It is a good idea to remove the flux residues that will be left behind after soldering. This is best accomplished by using a proprietary flux removing agent such as RS Components stock number 555-134 together with a fairly stiff bristle brush. A cut down artists paint brush is quite suitable.
   Chapter 1 gives information on de-fluxing procedures.
6. Check that the heat sink on IC6 (video ULA) is still correctly positioned.
   It is likely to have been disturbed whilst the work was being carried out on the board.
7. Visually check the entire PCB for traces of solder or fine pieces of wire etc. which may have dropped onto it whilst it was being worked on. Use a bright light and take your time.
8. Conduct the above visual check again.

4.4 Link Functions

4.4 Keyboard PCB Links 1 to 8

As mentioned above there is a facility provided on the BBC Microcomputer's keyboard PCB to allow jumpers to be installed, the position of the holes that are there to take these jumpers can be seen in the photographs at the end of chapter 5.

At the present time when the machines are manufactured there are no links fitted across these holes on either the model A or B machine.

A machine which was purchased with the floppy-disc upgrade already fitted may, however, have certain of the links inserted. This will depend upon the type of floppy disc drives that were sold with the microcomputer.

The keyboard links are extremely useful and the functions that they control are likely to be ones that the average user would want to change fairly frequently. For this reason it is not a good idea to use soldered wire links, as these are difficult to alter. Indeed if the links were unsoldered and resoldered a number of times the PCB would eventually become damaged.

The ideal solution to this problem is to use an eight-way dual in-line switch (DIL) such as the one shown in the photograph at the end of chapter 5.

These switches are available from a number of electronic hobbyist sources or from RS Components part no. 337-560. They should fit readily into the holes provided without any modifications. On some keyboard PCB's the copper track is not roller tinned and this may cause slight difficulty when soldering.

As with all modifications to the microcomputer that involve dismantling the machine and soldering, take great care and read chapter 2 before starting any work.

The link functions themselves are detailed in tables 4.3 to 4.6 shown below. Note that the links are numbered from WEST to EAST 1 to 8 respectively.

Table 4.3 - Link functions

Table 4.4 - Floppy disc drive type selection

TABLE 4.1

Note that the grid reference column in the above table refers to figure 4.1.
T = copper Track.
W =Wire link.
J = Jumper.

TABLE 4.5 Auto boot selection

TABLE 4.6 Screen mode on power up and break

FIGURE 4.1 LINK SHADOW DIAGRAM