The BBC Micro in Detail

The BBC Micro was released as the BBC Microcomputer in December 1981 and quickly became known affectionately as the "BEEB". The machine was popular in the UK, especially in the educational market: where ultimately, about 80% of British schools had at least one. A key advantage for the BBC Micro in the educational market was its very robust construction (unlike the Spectrum which competed for the education contract with the BBC).  Both casing and keyboard were built solidly and able to cope with the abuse heaped upon it by school children.

The Model A had 16K of user RAM, while the Model B came equipped with 32K. The processor in the BBC was the 6502, and like other, similarly equipped machines such as the Apple and the early Commodore models, the RAM was clocked twice as fast as the CPU. Alternating access was given to the CPU and the video display circuits which gave the BBC Micro a fully unified memory address structure without speed penalties.

Most competing Z80 based microcomputers such as the Amstrad CPC and ZX Spectrum which had memory mapped displays, incurred CPU speed penalties depending on the actions of the video circuits.
The BBC included a number of extra I/O (input/output) interfaces such as serial and parallel printer ports and an 8-bit general purpose digital I/O port. There was also an expansion connector - the "1 MHz bus" that enabled other hardware to be connected. Extra ROMs could be fitted (four on the PCB or sixteen with expansion hardware) and accessed via paged memory. An Econet network interface and a disk drive interface were also available as options.
In addition, an Acorn proprietary interface named the "Tube" allowed a second processor to be added. Three models of second processor were offered by Acorn, based on the 6502, Z80 and 32016 CPUs. The Tube was later used in third-party add-ons, including a Z80 board and a hard disk drive from Torch that allowed the BBC machine to run CP/M programs.

BBC Micro Motherboard versions
Both the Model A and the Model B were built on the same main board, so the Model A could be upgraded to a Model B without too much difficulty. Despite being much cheaper, the Model A didn't sell well and was phased out after the introduction of the Acorn Electron.

There were five developments of the main BBC micro circuit board that addressed various issues through the models production, from 'Issue 1' through to 'Issue 7' with variants 5 and 6 not being released.

Technical Details

•CPU: 6502

•Speed: 2MHz

•RAM: 16KB (Model A), 32KB (Model B)

•Special Features: Multiple graphics modes inc Teletext and 640 X 256

•Local price at launch - £335 (Model B)

BBC Model B+
In 1985, Acorn introduced the BBC Model B+, increasing the total RAM to 64 kB.
The extra RAM was configured as two blocks -  a block of 20 kB, dedicated solely for screen display (shadow RAM) and a block of 12 kB of special "Sideways" RAM.

The Model B+ also included floppy-disk support as standard, but this caused more problems than the benefit it added!
The built-in Western Digital 1770 floppy-disk controller was mapped to different addresses to the popular Intel 8271 controller previously used making it fundamentally incompatible with any software which used direct access techniques to the 8271 controller.
This made some very popular software such as "Castle Quest" (and later "Repton Infinity") unplayable on the new machine.

Acorn attempted to alleviate this in later versions ( 2.20 or later) of the 1770 DFS, via a 8271-backward- compatible Ctrl-Z+Break option.

Following on from the B+ 64K, , the Model B+128 came with an additional 64 kB ( 4 × 16 kB "Sideways" RAM banks) to give a total RAM of 128 kB.

Problems and Issues

Timing Issues
There can be specification timing issues with issue 3 circuit boards which can be cured by replacing the CPU with a Rockwell 6502A chip, or by replacing IC14 (a 74LS245) with either another 74LS245 or the faster 74ALS245".

Smoking and smelling power supply
By far the most frequent problem today is the now infamous "popping" power supply. During normal operation, there is a "pop" and a foul, n acrid burning smell, followed by smoke coming from the power supply vents.

The problem is that the X2 100nF filter capacitor (C2) has perished with age, usually because of small cracks in the plastic between the layers of foil. When exposed to mains voltage they break down - melting the plastic and burning the foil. It's counterpart, (C1), is also prone to failing but is a X2 10nF value filter capacitor.  Both are shown in the yellow boxes in the picture to the left.

These are a little tricky to replace as the power supply needs to be removed and dismantled before the capacitors can be removed.

New capacitors are available in the shop.

Dry startup capacitor
When switching on the BBC, you hear a single, continuous, audible "straining" sound and the machine will fail to start.

This is very frequently a faulty capacitor (C9) in the power supply which deteriorates over time - you may also have noticed that the BBC has been taking longer and longer to turn on until it finally fails.

A replacement is provided along with a set of the two X" filter capacitors in the shop.


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