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

Started by December 3, 2017
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....
On Sunday, 3 December 2017 12:35:23 UTC+5:30, T Obulesu  wrote:
> 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.... 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....
On Sat, 2 Dec 2017 23:06:51 -0800 (PST), T Obulesu
<obulesu.t@gmail.com> wrote:

>On Sunday, 3 December 2017 12:35:23 UTC+5:30, T Obulesu wrote: >> 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....
> > > >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.... "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.
Il 03/12/2017 08:06, T Obulesu ha scritto:
> 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....
>
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
Il giorno domenica 3 dicembre 2017 08:06:56 UTC+1, T Obulesu ha scritto:

> 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.... 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
Am 03.12.2017 um 08:06 schrieb T Obulesu:


> _____________ _____________ > | | | > | Tx |------------|--------->| Rx | > | | | | | > |ATMEGA128 | | | ATMEGA8 | > | | | | | > | Rx |------------|------|-->| Tx | > |____________| | | |____________| > | | > | | _____________ > | | | | > | |---------->| Rx | > | | | | > | | | ATMEGA8 | > | | | | > |------|---------->| Tx | > | | |____________|
[...] 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.]
> Here, the atmega128 is the main controller and all other atmega8 are connected to
the main controller using rs485 cable. 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.
> What is the best communication protocol for this to detect and avoid collision.?
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.
On Tue, 5 Dec 2017 21:23:41 +0100, Hans-Bernhard Br&#2013266166;ker
<HBBroeker@t-online.de> wrote:

>Am 03.12.2017 um 08:06 schrieb T Obulesu: > > >> _____________ _____________ >> | | | >> | Tx |------------|--------->| Rx | >> | | | | | >> |ATMEGA128 | | | ATMEGA8 | >> | | | | | >> | Rx |------------|------|-->| Tx | >> |____________| | | |____________| >> | | >> | | _____________ >> | | | | >> | |---------->| Rx | >> | | | | >> | | | ATMEGA8 | >> | | | | >> |------|---------->| Tx | >> | | |____________| >[...] > >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.] > >> Here, the atmega128 is the main controller and all other atmega8 are connected to
the main controller using rs485 cable. 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.
>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. > >> What is the best communication protocol for this to detect and avoid collision.? > >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.
Or use a "master" token, passing token from one master to the next master when the first has done its job.
> 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.
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.
Old and venerable... http://bfy.tw/FRQl
All the info you need ;-)
Best Regards, Dave