Date   : Tue, 04 May 2004 00:48:16 +0100
From   : "John Kenyon" <etljwk@...>
Subject: Re: B+ Upgrades

> -----Original Message-----
> From: Majordomo List Manager [mailto:majordomo@...]On Behalf Of
> Christopher Thornley
> Sent: 03 May 2004 23:09
> To: 'John Kenyon'; 'bbc'
> Subject: RE: [BBC-Micro] B+ Upgrades
> Importance: High
> 
> 
>  Hi,
> DRAMS hmm that kind of complicates things.
> The photograph of the board shown on here has
> 2 x HB81464-12 (64KB x 4) === Very hard to find.
> 1 x SN74ALS10N === TriInput NAND
> 1 x 74LS158N   === QUAD 2-Input Multiplexer (8 to 4) 
>                    Basically a 4 pole 2 Position Switch
>                    (Obsolete) 

Without looking at a datasheet a 74LS158 should be something
similar to a 74LS157 (a device I would expect to see when
multiplexing A0-A15 into XA0-XA7... hint hint).
The only weird thing is that I would expect to see a pair of
LS157's - one doing A0-A3+A8-A11 > XA0-XA3 and the other
doing A4-A7+A12-A15 > XA4-XA7

Personally, having tried to decipher the 6502 second processor
diagram, I have almost given up trying to reverse engineer what
Acorn did with their DRAM refresh + clock-cycle-stretching-to-
suit-slow-component engineering.

> The connector below I assume 
> RD0 to RD07 are data I/O lines
Yes
> XA0 to XA7        are some type of address lines which can only access 2^8
> 256 bytes
Yes - see above
> nCAS and nRAS must be the column and row refresh etc of DRAMS
And also indicate whether XA0-7 contain X0-X7 or X8-X15
(where X0-X15 are the 16 bits required to address 64k of memory)
> nDW Not to sure about that one ???
Probably not Data Write - only the data sheets for a 4164 will tell
you that one.

> 
> Not to sure if any of these will be suitable equivalents if I can 
> track any
> down http://www.amiga-stuff.com/hardware/64kx4-dram.html 
> 
> I have also found this which might help me
> http://www.sxlist.com/techref/mem/dram/olmstead.html
> 
> I'll have a look but I don't think I'll be able to come up with a usable
> circuit which will work in a similar way to the old one.

Probably not, however you might want to consider Googling/Yahooing for
information on interfacing 30pin SIMMS/SIPPS to 8 bit computers.
That way all the complicated "how do I lay out a PCB for DRAM?" questions
get eliminated - you don't really need to use all of that 1Mb - just
use the application circuit and tie the unused address inputs to 0v.

/john