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High speed RAM disc for BBC Microcomputer range

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A background

Having released both the RAMFS software and accompanying manual in February 2001's edition of EUG to build an (up to) 8 mega byte RAM disc, anyone who still hasn't had the nerve to build it on strip board can now order the board ready made!

Depending on whether you feel happy soldering surface mounted components you may want to go for a ready made and tested board or start with a partly assembled kit to finish yourself.


The board in question

It certainly is possible to build the circuit from the circuit diagrams on a very long piece of stripboard as was required for the prototype:

Prototype
The prototype board (256kB RAM fitted)

However the original article hinted that you can eliminate 3 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.

Two 32 pin sockets house either 1x4Mbit, 1x1Mbit, 2x4Mbit or 2x1Mbit chips depending on how much RAM is needed in your case. Current consumption is 125mA quiescent, rising to around 130mA average during RAM accesses.

When assembled the result is a high speed high capacity solid state RAM disc:

Non prototype
The ready made board (shown with RAM chips removed)


Availability

Orders can be placed online, or alternatively sent by post.

The RAM disc boards are available now, and are offered in various different options depending on how confident you are at soldering. For most users, the 256kB RAM option matches the normal 200kB DFS floppy disc close enough:

Option: Price: Comments:
The custom PCB plus the 74ALS990 latches
£35.50 inc postage
Incase you find the latches hard to find
You source the remaining components (Maplin/Farnell/etc...)
You build the board as per the article and layout
Does not include RAM chips (see later)
Buy the finished board
£60.80 inc postage
The option if you're not happy soldering or working with surface mount
Receive a built and tested board
Plug it in and away you go
Does not include RAM chips (see later)
RAMFS on EPROM
£5.50 inc postage
Latest version of RAMFS programmed into ROM

Additional RAM options

All of the above options exclude the cost of the RAM devices which are dictated by the amount of storage you think you may need. When fitting the RAM to the PCB a pair of option links determine which type of RAM chip is in use, though the actual amount of RAM is automatically detected by the software on power up.

Option: Price: Comments:
128k byte
£4.00 single chip
Only when bought at the same time
Links need altering from '4' to '1' position
256k byte
£7.00 two chip
Only when bought at the same time
Links need altering from '4' to '1' position
512k byte
£7.00 single chip
Only when bought at the same time
Leave links in '4' position
1M byte
£13.00 two chip
Only when bought at the same time
Leave links in '4' position

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