RS485 communicatio

RS485 communicatio

[Linux PPP]

Hi all,
am currently working on PPP over serial interface (RS485) in linux 2.4.2-2. I believe RS485 half duplex system and hence only one can transmit at a time. And for RS485 we basicaly use Master-Slave or Primary-Secondary kind of communication. I don't know how to achieve the same using PPP since i need to have max of 10 nodes connected via serial interface. I tested with two nodes using PPP daemon it works fine. Following are the commands i issued

In PPP server: 
$usr/sbin/pppd -detach crtscts 10.10.10.100:10.10.10.101 115200 /dev/ttyS0 &

In PPP client side : 
$/usr/sbin/pppd call ppp-start 
where ppp-start file is copied into directory /etc/ppp/peers/ that had the following

-detach /dev/ttyS0 115200 crtscts
noauth

This point to point communication worked fine with RS485 interface. If i had to connect one more node what i need to do. Please clarify with the following 

i) Whether the existing pppd takes care of the RS485 with multi node , if so how do i manage giving commands
ii) If there is no direct support how do i go ahead. Is there any other layer 2 protocol allows me to acheive TCP/IP communicattion over RS485 which is my ultimatum.

I will be grateful if anybody of them could help me with my current problem.

Thanx in advance.

Karthik

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RE: RS485 communicatio

[Ed Vance]

On Wed, Mar 12, 2003 at 10:15 PM, linuxppp@indiainfo.com wrote:
> Hi all,
> am currently working on PPP over serial interface (RS485) in 
> linux 2.4.2-2. I believe RS485 half duplex system and hence 
> only one can transmit at a time. And for RS485 we basicaly 
> use Master-Slave or Primary-Secondary kind of communication. 
> I don't know how to achieve the same using PPP since i need 
> to have max of 10 nodes connected via serial interface. I 
> tested with two nodes using PPP daemon it works fine. 
> Following are the commands i issued
> 
> In PPP server: 
> $usr/sbin/pppd -detach crtscts 10.10.10.100:10.10.10.101 
> 115200 /dev/ttyS0 &
> 
> In PPP client side : 
> $/usr/sbin/pppd call ppp-start 
> where ppp-start file is copied into directory /etc/ppp/peers/ 
> that had the following
> 
> -detach /dev/ttyS0 115200 crtscts
> noauth
> 
> This point to point communication worked fine with RS485 
> interface. If i had to connect one more node what i need to 
> do. Please clarify with the following 
> 
> i) Whether the existing pppd takes care of the RS485 with 
> multi node , if so how do i manage giving commands
> ii) If there is no direct support how do i go ahead. Is there 
> any other layer 2 protocol allows me to acheive TCP/IP 
> communicattion over RS485 which is my ultimatum.
> 
> I will be grateful if anybody of them could help me with my 
> current problem.
> 

I believe Point-to-Point Protocol only supports point-to-point symmetric
links. Don't think there is any multi-point support in the protocol. IIRC,
PPP also requires a full duplex link, which is not available on an RS-485
link with more than two stations, even if it is a 4-wire link. 

I don't know of an easy way around this fundamental limitation.

Maybe somebody on the kernel list has a suggestion.

Cheers

---------------------------------------------------------------- 
Ed Vance              edv (at) macrolink (dot) com
Macrolink, Inc.       1500 N. Kellogg Dr  Anaheim, CA  92807
----------------------------------------------------------------

RE: RS485 communicatio

[Richard B. Johnson]

On Thu, 13 Mar 2003, Ed Vance wrote:

> On Wed, Mar 12, 2003 at 10:15 PM, linuxppp@indiainfo.com wrote:
> > Hi all,
> > am currently working on PPP over serial interface (RS485) in 
> > linux 2.4.2-2. I believe RS485 half duplex system and hence 
> > only one can transmit at a time. And for RS485 we basicaly 
> > use Master-Slave or Primary-Secondary kind of communication. 
> > I don't know how to achieve the same using PPP since i need 
> > to have max of 10 nodes connected via serial interface. I 
> > tested with two nodes using PPP daemon it works fine. 
> > Following are the commands i issued
> > 
> > In PPP server: 
> > $usr/sbin/pppd -detach crtscts 10.10.10.100:10.10.10.101 
> > 115200 /dev/ttyS0 &
> > 
> > In PPP client side : 
> > $/usr/sbin/pppd call ppp-start 
> > where ppp-start file is copied into directory /etc/ppp/peers/ 
> > that had the following
> > 
> > -detach /dev/ttyS0 115200 crtscts
> > noauth
> > 
> > This point to point communication worked fine with RS485 
> > interface. If i had to connect one more node what i need to 
> > do. Please clarify with the following 
> > 
> > i) Whether the existing pppd takes care of the RS485 with 
> > multi node , if so how do i manage giving commands
> > ii) If there is no direct support how do i go ahead. Is there 
> > any other layer 2 protocol allows me to acheive TCP/IP 
> > communicattion over RS485 which is my ultimatum.
> > 
> > I will be grateful if anybody of them could help me with my 
> > current problem.
> > 
> 
> I believe Point-to-Point Protocol only supports point-to-point symmetric
> links. Don't think there is any multi-point support in the protocol. IIRC,
> PPP also requires a full duplex link, which is not available on an RS-485
> link with more than two stations, even if it is a 4-wire link. 
> 
> I don't know of an easy way around this fundamental limitation.
> 
> Maybe somebody on the kernel list has a suggestion.
> 
> Cheers

TCP/IP only requires two-way communicaton. It does not even
have to be reliable. There are IP/SCSI adapters and fibre
channel adapters already in the kernel.

Therefore, you just make a driver that substitutes for a
network communications adapter and away you go. PPP does not,
in principle, require simultaneous two-way communications.
However, current implimentations expect that a modem is
attached.

You should be able to use any serial cmmunications device
for a PPP link although one would have to make a serial driver
that handles the TX/RX direction-change in a transparent manner
as well as the RS-485 "address" problem. If the drivers on each
connected host communicate with each other, i.e., resolve their
own address problems,, then the payload between these hosts can
be the communications channels for PPP.


Cheers,
Dick Johnson
Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
Why is the government concerned about the lunatic fringe? Think about it.

Re: RS485 communicatio

[whitnl73]

|
| Hi all,
| am currently working on PPP over serial interface (RS485) in linux
| 2.4.2-2. I believe RS485 half duplex system and hence only one can
| transmit at a time. And for RS485 we basicaly use Master-Slave or
| Primary-Secondary kind of communication. I don't know how to achieve
| the same using PPP since i need to have max of 10 nodes connected via
| serial interface. I tested with two nodes using PPP daemon it works
| fine. Following are the commands i issued
|
| In PPP server:
| $usr/sbin/pppd -detach crtscts 10.10.10.100:10.10.10.101 115200
| /dev/ttyS0 &
|
| In PPP client side :
| $/usr/sbin/pppd call ppp-start
| where ppp-start file is copied into directory /etc/ppp/peers/ that had
| the following
|
| -detach /dev/ttyS0 115200 crtscts
| noauth
|
| This point to point communication worked fine with RS485 interface. If
| i had to connect one more node what i need to do. Please clarify with
| the following
|
| i) Whether the existing pppd takes care of the RS485 with multi node ,
| if so how do i manage giving commands

AFAIK there is no usable protocol for multidtop serial lines.  Ring ten
times for system a, 9 got system b...

| ii) If there is no direct support how do i go ahead. Is there any other
| layer 2 protocol allows me to acheive TCP/IP communicattion over RS485
| which is my ultimatum.

Use a separate serial line between each pair of nodes, or make one a
sort of hub and run a line from it to each other node?
|
| I will be grateful if anybody of them could help me with my current
| problem.
|
| Thanx in advance.
|
| Karthik
|
| --

Lawson
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Re: RS485 communicatio

[whitnl73]

On Thu, 13 Mar 2003 whitnl73@juno.com wrote:

>
> AFAIK there is no usable protocol for multidtop serial lines.  Ring ten
> times for system a, 9 got system b...
>
> | ii) If there is no direct support how do i go ahead. Is there any other
> | layer 2 protocol allows me to acheive TCP/IP communicattion over RS485
> | which is my ultimatum.
>
> Use a separate serial line between each pair of nodes, or make one a
> sort of hub and run a line from it to each other node?
> |
> | I will be grateful if anybody of them could help me with my current
> | problem.
> |
> | Thanx in advance.
> |
> | Karthik
> |
> | --

Hmmm, what is this?

PPP multilink support (EXPERIMENTAL)
CONFIG_PPP_MULTILINK
  PPP multilink is a protocol (defined in RFC 1990) which allows you
  to combine several (logical or physical) lines into one logical PPP
  connection, so that you can utilize your full bandwidth.

  This has to be supported at the other end as well and you need a
  version of the pppd daemon which understands the multilink protocol.

  If unsure, say N.

Configure.help line 7235/19377 37%
>
Seem to be some things in man pppd about multilink, too.

> Lawson
> --
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>
>
>
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Re: RS485 communicatio

[Pavel Machek]

Hi!

> > I will be grateful if anybody of them could help me with my 
> > current problem.
> > 
> 
> I believe Point-to-Point Protocol only supports point-to-point symmetric
> links. Don't think there is any multi-point support in the protocol. IIRC,
> PPP also requires a full duplex link, which is not available on an RS-485
> link with more than two stations, even if it is a 4-wire link. 

Get scarabd (I don't know *where* it is), it can run TCP/IP over slip over
half-duplex link. Performance is not too good.
									Pavel
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[Johanka's followup: and *two* hearts?]

Re: RS485 communicatio

[Alan Cox]

On Sat, 2003-03-15 at 19:46, Pavel Machek wrote:
> > I believe Point-to-Point Protocol only supports point-to-point symmetric
> > links. Don't think there is any multi-point support in the protocol. IIRC,
> > PPP also requires a full duplex link, which is not available on an RS-485
> > link with more than two stations, even if it is a 4-wire link. 
> 
> Get scarabd (I don't know *where* it is), it can run TCP/IP over slip over
> half-duplex link. Performance is not too good.

Robin O'Leary wrote most of it, so its probably somewhere like
caderus.co.uk

Half duplex performance is fine at a sensible speed and with decent
reliability. On radio will collisions its a lot less pretty

Re: RS485 communicatio

[Stuart MacDonald]

From: "Linux PPP" <linuxppp@indiainfo.com>
> am currently working on PPP over serial interface (RS485) in linux
2.4.2-2. I believe RS485 half duplex system and hence only one can transmit
at a time. And for RS485 we basicaly use Master-Slave or Primary-Secondary
kind of communication. I don't know how to achieve the
[snip]
> This point to point communication worked fine with RS485 interface. If i
had to connect one more node what i need to do. Please clarify with the
following
>
> i) Whether the existing pppd takes care of the RS485 with multi node , if
so how do i manage giving commands

PPP is designed to connect two logical endpoints together. 485
multidrop is a one to many setup, which seems to me is logically
incompatible with PPP. At least, simultaneously.

..Stu

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