ethernet point to point wiring

ethernet point to point wiring
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Subject Author Date
ethernet point to point wiring Eddie 10-28-2005
Posted by Eddie on October 28, 2005, 1:30 pm
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I am looking to replace an old serial RS485 multidrop point to point
communications line with ethernet but would like to retain the wiring
savings in a point to point layout. On RS485 we can connect 32 devices
with series point to point wiring. I have done some research that
indicates that if I used a switch at each device I would be limited to
a maximum of 7. Is this correct, or are new switches able to exceed
that number? If so, how many can I realistically connect in a point to
point topology. The network is connecting industrial PLC control
systems.

Thanks in advance for you help.



Posted by Walter Roberson on October 28, 2005, 8:40 pm


:I am looking to replace an old serial RS485 multidrop point to point
:communications line with ethernet but would like to retain the wiring
:savings in a point to point layout. On RS485 we can connect 32 devices
:with series point to point wiring. I have done some research that
:indicates that if I used a switch at each device I would be limited to
:a maximum of 7. Is this correct, or are new switches able to exceed
:that number?

The ethernet standards for Spanning Tree Protocol place timing limitations
that are such that with worst case timing scenarios, only 7 switches
can be used.

If your devices are newer or your distances shorter, then -in practice-
larger topoligies might work for you.

And if your topology is fixed and you can be fairly sure that no
loops will be created even by accident, then you could turn spanning
tree off.

:If so, how many can I realistically connect in a point to
:point topology.

Using some routing instead of pure layer 2 switching.
--
Okay, buzzwords only. Two syllables, tops. -- Laurie Anderson


Posted by James Knott on October 29, 2005, 6:55 pm


Walter Roberson wrote:

> The ethernet standards for Spanning Tree Protocol place timing limitations
> that are such that with worst case timing scenarios, only 7 switches
> can be used.

I thought spanning tree was only an issue if multiple paths exist to any
point.



Posted by Walter Roberson on October 31, 2005, 5:05 pm


>Walter Roberson wrote:

>> The ethernet standards for Spanning Tree Protocol place timing limitations
>> that are such that with worst case timing scenarios, only 7 switches
>> can be used.

>I thought spanning tree was only an issue if multiple paths exist to any
>point.

I believe that would fall under this sentance of mine that you snipped:

"And if your topology is fixed and you can be fairly sure that no
loops will be created even by accident, then you could turn spanning
tree off."


In theory, there are no loops in our LAN. In practice, people accidently
introduce loops... including me getting distracted about which cable
end is which and so ending up plugging them both into the same stack.
--
"No one has the right to destroy another person's belief by
demanding empirical evidence." -- Ann Landers


Posted by James Knott on October 29, 2005, 6:53 pm


Eddie wrote:

> I am looking to replace an old serial RS485 multidrop point to point
> communications line with ethernet but would like to retain the wiring
> savings in a point to point layout. On RS485 we can connect 32 devices
> with series point to point wiring. I have done some research that
> indicates that if I used a switch at each device I would be limited to
> a maximum of 7. Is this correct, or are new switches able to exceed
> that number? If so, how many can I realistically connect in a point to
> point topology. The network is connecting industrial PLC control
> systems.

The only ethernet method that works on a bus like that, is the old coax
based 10base5 or 10base2. They are considered obsolete now, so you'll have
to search for used gear. Switches are available in various sizes. I've
heard of 64 port models. With switches (or hubs), you can cut down on
cabling by placing small switches close to groups of users and running a
single cable back to a central switch.

BTW, cable is cheap and it doesn't cost much more to pull multiple cables at
the same time.



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other useful resources:
The Federal Communications Commission (FCC)
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