Date   : Fri, 29 Jul 2005 17:38:12 +0100
From   : jgh@... (Jonathan Graham Harston)
Subject: Re: Econet-Ethernet bridge

Jules Richardson <julesrichardsonuk@...> wrote:
> 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,
 
The Master I'm using at the moment is station number &047F -
network 4, station 127.
 
Econet packets all start:  stn  net  port  flag
 
The receiving software checks:
 
If stn<>this station, ignore this packet
If net<>&00, ignore this packet
If port<>one of my open ports, ignore this packet
 
That way a packet not destined for an open receive block on this
machine can be quickly ignored and further interrupts prevented.
 
The network system checks for network &00 as that means "this
network". A packet destined for a different network is transmitted
with the net byte set to a non-zero value. As it passed through a
bridge if the other side of the bridge is the destination network
the net byte gets set to &00 to indicate "this net".
 
RISC OS network software is also able to check that the net byte
is the network number of "this network".
 
> Presumably any kind of acknowledgement / timeout is done at a higher
> protocol level? (In other words a packet sniffer would be possible
> because the ADLC can run in purely passive mode and read entire packets
> destined for other stations)
 
That what various flavours of NETMON do.
 
> 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!
 
My Spectrum Econet interface is a Spectrum connected to a
Econetted BBC via a serial lead. With NFS in sideways RAM and the
'machine type' bytes appropriately poked, it even shows up on
*STATIONS !
 
-- 
J.G.Harston - jgh@...                - mdfs.net/User/JGH
There are three food groups: brown, green and ice cream.