What is the best communication mechanism with collision detection/avoidance for my topolog...

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Hello All, Here is my sensor topology..  


 _____________                        _____________                
|                                    |            |
|         Tx |------------|--------->| Rx         |
|            |            |          |            |
|ATMEGA128   |            |          | ATMEGA8    |
|            |            |          |            |
|         Rx |------------|------|-->| Tx         |
|____________|            |      |   |____________|
                          |      |
                          |      |            _____________                
                          |      |           |            |
                          |      |---------->| Rx         |
                          |      |           |            |
                          |      |           | ATMEGA8    |
                          |      |           |            |
                          |------|---------->| Tx         |
                          |      |           |____________|
                          |      |
                          |      |           _______________                
                          |      |           |              |
                          |      |---------->| Rx           |
                          |      |           |              |
                          |      |           |ATMEGA8       |
                          |      |           |              |
                          |------|---------->| Tx           |
                          |      |           |______________|
                          |      |
                          |      |
                          |      |           _______________                
                          |      |           |              |
                          |      |---------->| Rx           |
                          |                  |              |
                          |                  |ATMEGA8       |
                          |                  |              |
                          |----------------->| Tx           |
                                             |______________|

  
Here, the atmega128 is the main controller and all other atmega8 are connected to the main controller using rs485 cable.

What is the best communication protocol for this to detect and avoid collision.?

I have 20 atmega8 connected in a row..(i.e all the tx pins of atmega8 are connected together and finally connected to the Rx of the main controller) and distance between the any atmega8 or atmega128 is 3 meters....

Re: What is the best communication mechanism with collision detection/avoidance for my topology?
On Sunday, 3 December 2017 12:35:23 UTC+5:30, T Obulesu  wrote:
Quoted text here. Click to load it



Hello All, Here is my sensor topology..  


 _____________                        _____________                
|                                    |            |
|         Tx |------------|--------->| Rx         |
|            |            |          |            |
|ATMEGA128   |            |          | ATMEGA8    |
|            |            |          |            |
|         Rx |------------|------|-->| Tx         |
|____________|            |      |   |____________|
                          |      |
                          |      |            _____________                
                          |      |           |            |
                          |      |---------->| Rx         |
                          |      |           |            |
                          |      |           | ATMEGA8    |
                          |      |           |            |
                          |------|---------->| Tx         |
                          |      |           |____________|
                          |      |
                          |      |           _______________                
                          |      |           |              |
                          |      |---------->| Rx           |
                          |      |           |              |
                          |      |           |ATMEGA8       |
                          |      |           |              |
                          |------|---------->| Tx           |
                          |      |           |______________|
                          |      |
                          |      |
                          |      |           _______________                
                          |      |           |              |
                          |      |---------->| Rx           |
                          |                  |              |
                          |                  |ATMEGA8       |
                          |                  |              |
                          |----------------->| Tx           |
                                             |______________|

  
Here, the atmega128 is the main controller and all other atmega8 are connected to the main controller using rs485 cable.

What is the best communication protocol for this to detect and avoid collision.?

I have 20 atmega8 connected in a row..(i.e all the tx pins of atmega8 are connected together and finally connected to the Rx of the main controller) and distance between the any atmega8 or atmega128 is 3 meters....

Re: What is the best communication mechanism with collision detection/avoidance for my topology?
On Sat, 2 Dec 2017 23:06:51 -0800 (PST), T Obulesu

Quoted text here. Click to load it


"Best" is dependent on your requirements.  Start with a simple
approach that's easy to understand and implement:  Have the master
poll all the slaves in a loop.  If that has issues you find
unacceptable, perhaps load on the master, latency on the polls, power
consumption due to excessive polls (especially if the actual data rate
is very low), then consider other schemes, all of which will be
considerably more complex.

There's no real possibility in your scheme for collision detection,
since the slaves cannot talk to each other, so they couldn't even pass
a token around, so a non-polled scheme would have to depend on the
detection of failed transmissions.  A sufficiently robust packet
structure and check sum would allow the master to discard any garbled
transmissions, and a lack of an ack could cause a slave to retransmit
after a timeout and a random delay.

If the data rates are really low, you could consider a slotted
approach, where each slave has a slot where it's free to transmit.
That would require synchronized clocks (or a clock distributed in some
way, that *could* be a periodic transmission from the master - and
then enough clock synchronization so that all the slaves can stay in
their slots), and that messages are limited to those that fit into the
slot.

Re: What is the best communication mechanism with collision detection/avoidance for my topology?
Il 03/12/2017 08:06, T Obulesu ha scritto:
Quoted text here. Click to load it

Do you use only a "rs485 cable" (and I don't know what it is) or do you  
have RS485 transceivers? Because connecting more TX lines together isn't  
a good thing.

It isn't the same topology, (it is related to a more common two-wires  
RS485 bus)... recently I encountered the uLan[1] project. It seems  
interesting.

I couldn't understand the collision detection/avoidance mechanism.

[1] http://ulan.sourceforge.net/index.php

Re: What is the best communication mechanism with collision detection/avoidance for my topology?
Il giorno domenica 3 dicembre 2017 08:06:56 UTC+1, T Obulesu ha scritto:

Quoted text here. Click to load it

If yyou don't need something very fast or realtime, a master-slave protocol (for example modbus) is easy to implement and you don't have collision.

Bye Jack

Re: What is the best communication mechanism with collision detection/avoidance for my topology?
Am 03.12.2017 um 08:06 schrieb T Obulesu:


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[...]

That topology, as shown, it broken.   You've connected your master Tx to  
one slave Rx, but also several slave Tx'es.  And vice versa.  I'll  
assume you really meant

+------------+                         +------------+
|         Tx |>------------------*---->| Rx         |
|            |                   |     |            |
|ATMEGA128   |                   |     | ATMEGA8    |
|            |                   |     |            |
|         Rx |<-----------*------|----<| Tx         |
|____________|            |      |     +------------+
                           |      |
                           |      |     +------------+
                           |      *---->| Rx         |
                           |      |     |            |
                           |      |     | ATMEGA8    |
                           |      |     |            |
                           *------|----<| Tx         |
                           |      |     +------------+
                           .      .
[etc.]

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The type of cable is much less relevant than you probably think.  The  
real question is what kind of line interfaces (a.k.a. transceivers)  
you're using on those lines.  Let's assume they're RS485 transceivers.

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None, because with this kind of topology, there really shouldn't be any  
collisions in the first place, so nothing to detect or avoid.  You have  
a master controller, so how about letting it actually be the controlling  
master of the bus?

The topology you described practically begs for a query-response  
protocol: master tells all slaves who is to talk next, and optionally  
what it's to talk about.  The addressed device (master itself, or one  
slave) then talks; all others only listen.  Repeat until the cows come home.

Collisions are what you get if there's not one master, but many of them.  
  That's when you really need collision mitigation techniques.  If  
that's where you really want to go, fell free to google CSMA/CA and  
CSMA/CR and the protocols mentioned in connection with those.  One good  
choice might then be CAN --- and it would save you half the wiring, too.

Re: What is the best communication mechanism with collision detection/avoidance for my topology?


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This corrected form is essentially 4 wire RS-485 style communication,
in which all Rxs are always active, the master Tx is always active,
but one slave Tx only is active, all others are in three state.

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Or use a "master" token, passing token from one master to the next
master when the first has done its job.

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Was 3 m the total network size or the distance between each node ? In
the first case, even open collector TTL could be used, but the power
should be delivered from the master in the same cable. There should be
no ground connections at the slaves to avoid TTL ground potential
problems.

To save in wiring, use a similar topology as 2 wire RS-485. Just add
an NPN transistor at each Tx pin in the network, connect collectors
together and a common pull-up to +5 V. Connect each Rx pin to the
collector of the corresponding collector (i.e. the low end of the
pull-up resistor.  

Make sure you have a echo-cancel routine to avoid hearing your own
transmissions. It is also a good idea to use a protocol, in which the
request and response messages are different so tat the inactive slaves
know, if there is a master request or a reply from an other slave on
the line.


Re: What is the best communication mechanism with collision detection/avoidance for my topology?
Old and venerable... http://bfy.tw/FRQl
All the info you need ;-)
Best Regards, Dave

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