Having released both the RAMFS software and circuit diagrams to EUG to build an 8M RAM disk over the past few EUG issues, anyone who still hasn't had the nerve to built it on strip board can now order the board ready made!

The next few paragraphs describe the board's features so if you're not really that bothered about the ins and outs skip straight to the bottom line...

As I hinted in the original article, you can eliminate three chips by replacing the read buffer and write latches (74LS373 and 74LS241) with read back latches, in particular the 74ALS990 is suitable.

By choosing surface mounted components instead of through hole DIL parts the board size can be further reduced to just 3" by 3.5" which includes a full Megabyte of RAM. The filing system supports up to 8 Megabytes and a fully decoded address signal is available on a 21 way header should you need it.

Optional terminating resistors may be fitted if there are no other devices plugged into the 1MHz bus, or these can be omitted and the integral loop through connector used to "daisy chain" the other 1MHz bus devices together.

A 1.3mm power plug supplies the board with +5v, though there are tracks for an optional voltage regulator if some other supply (eg. +12v) is closer to hand.

Two 32 pin sockets to use either 1x4Mbit, 1x1Mbit, 2x4Mbit or 2x1Mbit chips depending on how much RAM is around 130mA average during RAM accesses.

Building the board is simply a matter of placing the parts as detailed in the article in this issue, though confidence with surface mount parts is essential!

The Bottom Line
These boards are available now, and you'll find a photo of both my stripboard prototype and the finished PCB online at www.sprow.co.uk/bbc

Pricing
OPTION 1: The blank PCB, you source the components (Maplin, Farnell, etc), you build the board as per the article on this disk, plug it in and away you go: £18 + £0.54.

OPTION 2: The blank PCB plus all of the components, you build the board as per the article on this disk, plug it in and away you go: £40 + £0.81 P&P. With the 128k byte option, add £8. With the 256k byte option, add £16. With the 512k byte option, add £22. With the 1Mbyte option, add £44.

OPTION 3: The blank PCB plus the 74ALS990 latches, in case you find the latches hard to find. You source the remaining components (Maplin, Farnell, etc), you build the board as per the article, plug it in and away you go: £27 + £0.54 P&P.

OPTION 4: Buy the finished board. The option if you're not happy working with surface mount. Receive a built and tested board, plug it in and away you go. £64 + £0.81 P&P. With the 128k byte option, add £8. With the 256k byte option, add £16. With the 512k byte option, add £22. With the 1Mbyte option, add £44.

Kits of parts and finished boards contain everything except the cost of the RAM, the optional voltage regulator, and the one optional address decoder chip.

Unfortunately the economies of scale don't really help here, so unless someone buys about twenty of these the price will be quite high - the PCB alone has to recoup the one off tooling charges. For most I think the 256k option is enough (a Beeb disk only holds 200k anyway!).

Component prices are based on Farnell 'one off' prices. I'm quite sure the resourceful among you can find parts cheaper - so feel free to use option 3 if you think you can...

In any case, get your orders in quickety quick!

Robert Sprowson, EUG #56