Date   : Wed, 27 Aug 1997 08:33:18 -0700
From   : Angus Duggan <angus@...>
Subject: Sideways RAMs

Andrew de Quincey writes:
>
>Hi again,
>
>Another problem: 
>
>I was under the impression that the sideways RAM for the BBC were
>Static RAM chips plugged into one of the ROM sockets.....

Nope. The sockets are for ROMs, which are nearly, but not quite, compatible
with 27xxx series ROMs. There is no write enable line on the ROM sockets.

>However, when I tried this with two Static RAMs, the machine was not
>able to "see" the chip; i.e. when I looked at that area of memory, it
>looked like there was no chip plugged in there - just read constantly
>changing values from it....

Not surprising. I can't remember what signal goes to write enable, but it
won't be what you expect. You need some extra logic to determine whether
write enable should be on, and whether the socket your chip is in is the
correct one to write to. This is why there were sideways RAM boards (though I
built one myself initially, ISTR the extra logic was only a couple of gates).

a.
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