Date   : Wed, 08 Feb 2006 10:51:25 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: 1 MHz Bus backplane

Mike wrote:
> I've seen a few references to a 1 MHz bus backplane system.
> 
> Does anybody have any details of the configuration. I have details of a
> Control Universal 1MHz interface that uses a 64 way eurocard connector, and
> their diagram is marked 'Acorn Bus'. Does anyone know if they are one and
> the same?

Hmm, interesting. Acorn's System 2, 3, 4 and 5 machines are all Eurocard 
format (System 1 boards are same form factor but lack the bus connector). 
However, system clock on the CPU card (and hence bus clock) is 
jumper-selectable for 1, 2 or 3MHz.

The Atom uses the same bus expansion connector as the System backplane, but 
presumably has a fixed speed clock line.

CU also did a Eurocard system based around BBC B functionality (see post 
archive on this list) - I've got lots of boards and the bus *looks* to be the 
same (sadly no docs other than Control FORTH).

Makes sense that they'd retain the bus actually, as CU did make boards to put 
in System machines - I know I've got a couple of CU memory boards in one of my 
System boxes.

Possibly for the later CU-only system though the bus was only ever run at 1MHz?

Acorn System bus connector as follows:

Side A     Side B
   +5V   1  nc
   A15   2  nc
   A14   3  PB7
  NWDS   4  PB6
  NRDS   5  PB5
  NRST   6  PB4
    A8   7  PB3
    A7   8  PB2
    A6   9  PB1
    A5  10  PB0
    A4  11  CB2
    A3  12  CB1
    A2  13  12MHz
    A1  14  8MHz
    A0  15  6MHz
    D7  16  4MHz
    D6  17  3MHz
    D5  18  2MHz
    D4  19  nc
    D3  20  nc
    D2  21  1MHz
    D1  22  NBGt
    D0  23  nc
   A13  24  nc
   A12  25  NBRq
   A11  26  MR
   A10  27  nc
    A9  28  NIRQ
   CLK  29  NNMI
  R/NW  30  SYNC
NBLK0  31  nc
    0V  32  0V


CPU clock is pin 29, side A - the other clock signals are just there for 
anything that needs them.

cheers

Jules