"The CMOS RAM card gives 8K bytes of non-volatile RAM which
can be used to store data that is retained when the system is switched off. The
card is a 100mm x 160mm Eurocard and connects to the standard Acorn bus using a
64-way connector. A +5V supply is all that is required.
On the card are 16 CMOS RAM ICs which provide a contiguous
two blocks of memory, address decoding links allow these two blocks to be memory
mapped from any even block boundary, e.g. 0000, 2000, 4000 (hex). Address and
data bus buffers then interface the memory ICs to the Acorn bus. A 3.6v battery
provides the minimal current requirement of the RAMs so that they hold their
data when the +5v goes down. This battery is charged up from the +5v supply when
it is on.
Data retention will be at least 60 days. As soon as the +5V
begins to go down the bus buffer chips are disabled stopping invalid data from
being stored in the RAMs by a dying CPU. There is also provision for an external
power fail signal which can be driven from a mains failure detector off the
card. Finally there is provision for a write protect switch and a card isolate
switch which puts the card into power down mode prior to unplugging it from the
Acorn system. Using this, the card can be used to transfer data between systems.