Messages in this thread | | | Subject | RE: RS485 communication | From | Chris Fowler <> | Date | 15 Mar 2003 10:42:53 -0500 |
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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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