Acorn Computers Limited BBC A, Руководство по оборудованию, Страница: 43 / 47

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3.23 General Guidelines For Use Of The 1MHZ Bus.
Acorn Computers Ltd. have issued certain guidelines which should be adhered to when designing
devices to attach to the 1MHz bus, these are summarised as follows;
i) The maximum permissible loading on any one line of the 1MHz bus is 1 low power Schottky TTL load.
In practise this will mean that all lines will have to be buffered by the peripheral device. It may be possible
when using an expansion box that the expansion box back-plane will provide the necessary buffering
requirements. The Control Universal expansion box is to be recommended in this respect.
ii)
Each of the logic lines on the 1MHz bus except NIRQ, NRST and NNMI should have the facility to be
terminated by a potential divider composed of two 2k2 resistors placed between 0 volts and +5 volts.
Each logic line is then connected to the junction of its potential divider. In this way, bus reflections etc. will
be minimised.
Chapter 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 cross-point 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.