BeebMaster - Acorn Cheese Wedges

Acorn Cheese Wedges



The expansion units produced for the BBC Model B by Acorn have come to be known affectionately by Beeb enthusiasts as "cheese wedges" because of their colour and shape. They were intended to be half the width of the BBC Model B and with the same cross-section as the computer.

The first adapters came out in the spring of 1984. Curiously, the 6502 and Z80 second processors are referred to as early as 1982 in the BBC User Guide and the front cover to various Level 2 Econet books published in 1983 clearly shows the file server with a cheese wedge attached!

Below is what I hope to be an exhaustive list of the Acorn cheese wedges with a brief description of each:


6502 Second Processor

The original and, in my opinion, the best of all cheese wedges. At �199 when it came out in April 1984, the 6502 Second Processor cost half a new Beeb which is why it was not as popular as Acorn might have hoped. But those who could afford one could not fail to be amazed by what the little box could do.

Inside the cheese wedge is a 3MHz 6502C processor with 64K on board of RAM. The adapter connects via a 40-way ribbon cable to the Tube connector on the BBC. When the Tube is running, the current language is copied into the Second Processor's RAM leaving the rest of the memory available for user programmes.

By the addition of a 6502 Second Processor, all the limitations caused by filing systems and screen modes eating up the user RAM are completely eliminated - almost 32K of memory is available in any mode with any filing systems using the 6502 Tube. Running the special second processor version of BASIC, Hi-BASIC, frees up nearly 48K of RAM!!!

One of my 6502 Second Processors is pictured above, in the bottom left-hand corner.


Z80 Second Processor

I don't like the Z80 system and have spent most of the time since I bought my Z80 cheese wedge wondering why on earth anybody would want to turn their Beeb into a PC.

Nonetheless, the Z80 demonstrates the power of the Tube system on the BBC micro - not only could an extra processor the same as the Beeb's own CPU be added, but a totally different processor system could be connected up to change the whole nature of the computer.

The Z80 runs off discs in CP/N format, but as this is 10 sectors of 256 bytes per track, I can't see any advantage over using Acorn DFS.

My Z80 Second Processor is in the bottom right-hand corner of the picture above.


32016 Second Processor

This one is the stuff of legend. It is also known as the Cambridge Co-processor and the Acorn Scientific. It was boxed as the Cambridge Co-processor as the pictures above show, as well as the 32016 Second Processor.

The 32016 was a new type of processor which was a part 16-bit and part 32-bit processor. This was one of the most powerful of all the second processors issued. I think it had on-board RAM of 1 megabyte and it ran the Pandora Operating System, or PANOS.

I think there was also a larger version of the 32016 board with much more RAM, which was too large to fit inside a cheese wedge box.

My thanks are due to Matthew Cook for the photographs of the Cambridge Co-processor.


80186 Second Processor

I might be making it up, but I do think there was an 80186 second processor, as distinct from the Master 512 co-processor. However, since I can't seem to find any other references to it, and since I can't remember where I got the idea myself, it could be a total myth.


Teletext Adapter

Not all cheese wedges were second processors. The Acorn Teletext Adapter contained a teletext decoder and a bit of RAM to enable the Beeb to receive broadcast pages of teletext and, in its 1980s heyday, download programmes as part of the Telesoftware System. This is now sadly defunct.

The Teletext Adapter is my second favourite cheese wedge after the 6502. It connects to the 1MHz bus and is controlled by the Teletext Filing System ROM. Unfortunately this bumps up PAGE by quite a lot, even on the Master, so it is handy to have a 6502 cheese wedge attached as well.

I have a Teletext Adapter permanently connected to Station 1 although since November 2009, when the analogue TV signal was switched off in the Granada region, it hasn't been possible to view any live Teletext pages.

I now have several Teletext adapters and one of the spares is shown in the top right-hand corner of the picture above.


Prestel Adapter

The Acorn Prestel Adapter was an early form of modem which connected to a telephone line and the RS423 port of the BBC micro. When connected, you could use terminal emulation software such as that contained in the Prestel ROM to use Prestel, Viewdata, bulletin boards and all those kinds of things which went on in the 1980s. It was a sort of inter-net of the day, but no doubt more reliable and less expensive than the modern-day equivalent.

One of my Prestel Adapters, as yet untested, is shown top-left in the picture above.


Econet Bridge

The Acorn Econet Bridge has the distinction of being the only standalone cheese wedge, not needing to be plugged into a Beeb.

An Econet Bridge, put simply, is a communications unit between two Econet networks. There are limits on the size of Econets and when those parameters are exhausted, networks can be linked together to continue sharing fileservers by the use of one or more bridges.

The Acorn Econet Bridge is a miniature Beeb in itself containing a 6502-based system running the bridge software. It has a bit of RAM as a buffer to remember things and contains an Econet interface for each network. It has two Econet sockets to enable the bridge to be connected across the two networks. Each network is assigned a network number in the range of 1 to 127 to allow each station to retain a unique station number when taking the network number into account.

Econet Bridges don't turn up very often. I'm lucky have two of them, one of which you can see here.


ARM Evaluation System

This is rarest thing in the entire universe and would fetch at least a trillion guineas if one ever came up for sale on E-Bay.

It is nothing less than a whole Archimedes with 4MB RAM crammed into a cheese wedge with a Tube cable so you can plug it into your Beeb!!

I have in mind that there is also a thing called an A500 Second Processor, which is similar to the ARM Evaluation System, but I can't really confirm whether this is the case or not.


IEEE488 Adapter

Exactly what an IEEE488 device is I cannot say but it sounds important and Beebs can communicate with them when an IEEE488 adapter is attached to the 1MHz bus.

As far as I know, this adapter is the only one to have cheese-wedge and non-cheese-wedge varieties. The metal box type is the later variety and I am lucky enough to have one of each.

I think they were mainly used for attaching and controlling scientific instruments via a Beeb, but I don't really have much information on what devices could be attached and what you might be able to do with them.


RH Electronics Video Digitiser

Exactly what it says on the tin, well, plastic: a video digitiser made by RH Electronics, supplied in the Acorn BBC Microcomputer Expansion Box, or cheese wedge as everybody else calls it. This is the only non-Acorn cheese wedge I know about and you can see my unit here.



E-mail me if I have missed any from the list!



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