Date   : Fri, 29 Jul 2005 18:21:12 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Econet-Ethernet bridge

On Jul 29 2005, 13:22, Johan Heuseveldt wrote:

> It doesn't seem logical to me that only one of the four sockets could
> be upgrade from 2K to 8K. Having only 8K puts restrictions on the
> packet size. I think Mike got it right; see later.
> But a good schematic would give the answer.

It might just have been to do with costs; when the bridge was designed,
four 2K chips were cheaper than one 8K, but the designer at Acorn
probably realised that would be reversed.  I don't know the answer,
however; I'm just guessing.

> > > as with the original BBC OS which was 4 x 4K ROMs and later a
> > > single 16K ROM.
>
> Ah yes, all jumpers there.
>   If you gave that a serious look, you'll notice that the system for
>   selecting a SWRom, is 'kidnapped' for the (M)OS with 4 x 4K ROMS,
>   now controlled by address lines A12/A13.
>
>   The 4 sockets for SWRoms (IC52, 88, 100, 101) are then used for the
>   MOS in 4 x 4K, and only a single SWR (possibly BASIC) was possible
>   in the 5th socket (IC51), which normally is for the MOS 1.20 ROM.
>   IIRC it was MOS 1.00 which was in 4 x 4K?

No, you're right in principle but slightly off on the detail.  MOS 0.1
and earlier versions (in the earliest machines sent to selected
schools) were originally in 2732 EPROMs.  Then MOS 0.1 became available
as a masked ROM, and MOS 1.2 was a masked ROM when it came out.  There
were very small numbers of development systems with versions in
between, but by then it was possible -- and economioc -- to get a pair
of 2764s on a carrier or to provide a 27128.

> There was the *VIEW command [1] to watch the screen of another Beeb,
> which data size could be 20K.
> OK, most (school) network managers were not happy with it, I believe,
> as it has a disastrous effect on Econet performance, and wouldn't
> allow it.

It was more to do with the way pupils tended to use it, than with the
overhead :-)

> The 2K RAM devices are 24 pins, while the 8K are 28 pins. This means
> that the +5V supply needs to be switched from socket pin26 to socket
> pin28. Is this taken care of by a jumper setting, or is a
modification
> necessary?

Not required.  Pin 26 on an 8K x 8 RAM is normally an active-high
chip-select that is typically connected directly to +5V.  And of course
pin 28 doesn't matter to a 2K x 8 :-)


-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York