Date   : Thu, 28 Jul 2005 13:15:04 +0100
From   : Rob <robert@...>
Subject: Re: Econet-Ethernet bridge

At 11:35 28/07/2005, Jules Richardson wrote:

>On Thu, 2005-07-28 at 11:32 +0200, Johan Heuseveldt wrote:
>
> > If NFS has determined from the first byte (or two) that this packet isn't
> > addressed to this machine, it gives the 68B54 a command to completely
> > ingore that frame.
>
>Yes, I hadn't realised that before, but the docs seem to imply that the
>protocol's capable of handling more than 256 devices, but Acorn chose
>not to do this I believe (I'm not aware of any Acorn machines that can
>have a station ID > 255). I haven't read the datasheet fully yet though,
>just skimmed through it.

I have a feeling that the second byte is used for network number.  Normally 
zero for small networks of course.

 >
> > And additional hardware for clocking the 68B54 from the clock remote
> > signals, and sensing clock presence (nDCD input), and contention logic
> > (nCTS input), and the line drivers of course.


there's not much needed for these, and they can be cribbed from the BBC 
circuits of course.


> >From memory I think there are 4 output lines (other than data) on the
>parallel port, 4 input control lines, plus an input IRQ line.
>
> > In addition to that, it must be possible to 'disconnect' or 'mask' the NMI
> > from the system NMI line. This is done by not propagating the low NMI 
> signal
> > from the 68B54 to the system's NMI line with extra hardware again.
>
>Could that be done in software though? OK, it's wasteful... depends on
>how often the chip sets NMI when the system technically doesn't need to
>respond though.

Yep, reading (actually any access to) strategic memory locations latches a 
flip-flop which is NAND'd with the IRQ line from the ADLC.



>This is starting to look less and less possible without additional
>hardware - I'm starting to see why the rumour is that Acorn's VAX Econet
>interface was actually a BBC doing the network processing and hooked up
>to the VAX by a different interface!


Has anybody got the circuit for Acorn's Econet-Econet Bridge (one on ebay 
at the moment) - what's in there, and how does that manage it?

Rob