RE: RS485 communication

["Cigol C" <linuxppp@indiainfo.com>]

Thannks for the info. Can i acheive IP over RS485 if i use SOCK_PACKET. I need some more info on this if u could provide that. If this option is set in the socket call will i have an option the choose the hardware interface.

----- Original Message ----- 
From: Chris Fowler 
Date: 15 Mar 2003 10:42:53 -0500 
To: Robert White 
Subject: RE: RS485 communication 

> I think using SOCK_PACKET an an ethernet chip may be the best choice. 
> You can use IP or you can use RWP (Rober White Protocol). 
> 
> 
> On Sat, 2003-03-15 at 03:07, Robert White wrote: 
> > 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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