From: "Robert White" <rwhite@casabyte.com>
To: "Chris Fowler" <cfowler@outpostsentinel.com>
Cc: "Ed Vance" <EdV@macrolink.com>, "'Linux PPP'" <linuxppp@indiainfo.com>,
       <linux-serial@vger.kernel.org>,
       "'linux-kernel'" <linux-kernel@vger.kernel.org>
Subject: RE: RS485 communication
Date: Sat, 15 Mar 2003 00:07:22 -0800
Message-ID: <PEEPIDHAKMCGHDBJLHKGCEGACDAA.rwhite@casabyte.com>
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Yes, that, but that is only part of it.

The RS485 is a proper bus, so this custom program (or programs) will have to
act as full bus arbiters and a kind of router.  Each PPP daemon must receive
ONLY the data that its peer daemon transmits.  That means that each slave
must know to ignore the data not destined for it.  Further, the master,
which would have multiple PPP instances running on it, will need to decide
which of those instances get which of the receiving bytes.

So just like an Ethernet transceiver puts a protocol frame around the data
to get it to the destination, the transport program will have to put
envelopes around the data.  THEN the master transport program will tell each
slave when and how many of its envelopes it may send.  The only way that can
work (because there is no "ring" you can't pass a "token") is for the master
to ask each slave in turn: "Got anything to send?"

This usually devolves to a sequence of "#1, say your piece", "#2 say your
piece" etc.  That is a very bad performance model.

So every frame of data will need to be arbitrarily wide, meaning a length
code, and will need an in-multiplexor address.

So the master, for instance, will say "slave 1, go".  The slave 1 will send
a packet (not necessarily a PPP packet, as the multiplexor will have
overhead data etc.)

The master will look at the address and decide which local pty the data is
for and send it there.  (Think a simple byte pump here)

When that pty has response data, and when the master says "slave 0 (e.g. me)
go" it will frame a message that slave #1 will receive and put through to
its local pty.  Slave 1 also has the job of ignoring data for slaves 2
through N and the Master (Slave 0).

In short, he has to write a distributed application that pumps data into and
out of a broadcast medium, and makes sure that each participant gets only
the data intended for itself.  (This is what both the Ethernet hardware
layer, and the IP protocols do.)

In communications you almost always put protocols inside of protocols to
some significant depth.

For instance, when you play Unreal Tournament 2003:
Unreal Tournament's data is carried by UDP,
The UDP is carried by IP,
The IP is carried by the Ethernet hardware access layer (raw Ethernet),
Those packets may go to your cable modem which either wraps the Ethernet
hardware packets or decodes them and reencodes the IP  into whatever it
does.

>From there, if your cable modem is doing PPPoE there are even more layers.

This guy will only have to write a multiplexing layer, but it won't be fun.

Then again, the Ethernet people have done all that, which is why it is
cheaper and easier to just get the Ethernet hardware and use it.

Rob.

-----Original Message-----
From: Chris Fowler [mailto:cfowler@outpostsentinel.com]
Sent: Thursday, March 13, 2003 3:31 PM
To: Robert White
Cc: Ed Vance; 'Linux PPP'; linux-serial@vger.kernel.org; 'linux-kernel'
Subject: RE: RS485 communication


Are you saying that for him to to use PPPD that he will have to write a
program that will run on a master and tell all the slave nodes when they
can transmit their data.  In this case it would be ppp data.  Hopfully
in block sizes that are at least the size of the MTU ppp is running.

Chris


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