DComTalk.com

by **Eddie** » Fri Oct 28, 2005 8:30 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? 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.

by **Walter Roberson** » Fri Oct 28, 2005 8:30 pm

In article <1130531433.614105.297570@z14g2000cwz.googlegroups.com>,
Eddie <sedcline@yahoo.com> 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?

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

by **Dr. Anton T. Squeegee** » Sat Oct 29, 2005 11:20 am

In article <1130531433.614105.297570@z14g2000cwz.googlegroups.com>,
sedcline@yahoo.com says...

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.

<snippety>

Why try to fix something that is (probably) not broken?

RS485 may be "old" in the sense that it has been around for a
while, but my question for you would be "Does it fit your current needs
reliably, and can it also be reliably expanded to meet your future
needs?"

If the answer to both questions is 'Yes,' why go through the
expense, complexity, and hassle of "upgrading" to Ethernet?

This is curiosity on my part.

--
Dr. Anton T. Squeegee, Director, Dutch Surrealist Plumbing Institute.
(Known to some as Bruce Lane, ARS KC7GR,
kyrrin (a/t) bluefeathertech[d=o=t]calm -- www.bluefeathertech.com
"If Salvador Dali had owned a computer, would it have been equipped
with surreal ports?"

by **James Knott** » Sat Oct 29, 2005 10:53 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.

by **James Knott** » Sat Oct 29, 2005 10:55 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.

by **Al Dykes** » Sat Oct 29, 2005 11:08 pm

In article <MPG.1dcd3d08146453eb9896cd@192.168.42.131>,
Dr. Anton T. Squeegee <SpammersAreVermin@dev.null> wrote:

In article <1130531433.614105.297570@z14g2000cwz.googlegroups.com>,
sedcline@yahoo.com says...

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.

<snippety

Why try to fix something that is (probably) not broken?

RS485 may be "old" in the sense that it has been around for a
while, but my question for you would be "Does it fit your current needs
reliably, and can it also be reliably expanded to meet your future
needs?"

If the answer to both questions is 'Yes,' why go through the
expense, complexity, and hassle of "upgrading" to Ethernet?

This is curiosity on my part.

Agreed. And if you decide you *do* need ethernet it sounds like you
are looking at daisy-chaining switches down the line and only pulling
one wire.

IMO that's worse than thinwire in the probablity of having mysterious
time-killing problems that are solved only by wiring things properly
the second time.

All you're saving is the cost of wire, and that's the cheapest part of
the project unless you are dealing with distances close to the CAT5
limit.

/end of rant/

--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

by **Walter Roberson** » Mon Oct 31, 2005 12:05 pm

In article <WeOdnesrev9eYP7eRVn-ig@rogers.com>,
James Knott <james.knott@rogers.com> wrote:

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

by **Guest** » Mon Oct 31, 2005 12:20 pm

1. Eddie Oct 28, 8:30 pm show options

Newsgroups: comp.dcom.lans.ethernet
From: "Eddie" <sedcl...@yahoo.com> - Find messages by this author
Date: 28 Oct 2005 13:30:35 -0700
Local: Fri, Oct 28 2005 8:30 pm
Subject: ethernet point to point wiring
Reply | Reply to Author | Forward | Print | Individual Message | Show
original | Report Abuse

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.

Is I understand it the 7 hop limit was imposed along
with a requirement that each bridge should forward
each frame within one second (or drop it) as part of the
STP timing budget.

Today these numbers seem somewhat laughable
(1Gbps for 1 sec would require a 100M Byte buffer)
however as far as I know (you should check for yourself
since I have not read 802.1d for years) the standard
has never been changed in this respect.

There seems to me no reason not to increase the
number of bridge hops used if STP is not
involved.

A couple of things to ponder.

Support - If you tell your support channel that you are
not compliant with the standard then I strongly suspect
that they will be reluctant to assist.

Probability of packet loss - If you do not recover from
lost data then you should carefuly consider the
increased probability of lost packets.

Maintainability - I do not know what you do at present
however if you were to use low cost switches that
cannot report or record errors or problems
it will be very difficult to troubleshoot network
failures.

Good luck.

by **Al Dykes** » Mon Oct 31, 2005 12:20 pm

In article <1130777137.413908.293490@g47g2000cwa.googlegroups.com>,
<anybody43@hotmail.com> wrote:

1. Eddie Oct 28, 8:30 pm show options

Newsgroups: comp.dcom.lans.ethernet
From: "Eddie" <sedcl...@yahoo.com> - Find messages by this author
Date: 28 Oct 2005 13:30:35 -0700
Local: Fri, Oct 28 2005 8:30 pm
Subject: ethernet point to point wiring
Reply | Reply to Author | Forward | Print | Individual Message | Show
original | Report Abuse

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.

Is I understand it the 7 hop limit was imposed along
with a requirement that each bridge should forward
each frame within one second (or drop it) as part of the
STP timing budget.

Today these numbers seem somewhat laughable
(1Gbps for 1 sec would require a 100M Byte buffer)
however as far as I know (you should check for yourself
since I have not read 802.1d for years) the standard
has never been changed in this respect.

There seems to me no reason not to increase the
number of bridge hops used if STP is not
involved.

A couple of things to ponder.

Support - If you tell your support channel that you are
not compliant with the standard then I strongly suspect
that they will be reluctant to assist.

Probability of packet loss - If you do not recover from
lost data then you should carefuly consider the
increased probability of lost packets.

Maintainability - I do not know what you do at present
however if you were to use low cost switches that
cannot report or record errors or problems
it will be very difficult to troubleshoot network
failures.

You didn't say how many stations you want to wire but if it's a small
number, pull wire and do it right. It won't cost much.

If you are doing it for a large operation then do it right or you'll
spend your days under desks and with unhappy users and then your
successor will rewire the place, correctly.

--
a d y k e s @ p a n i x . c o m

Don't blame me. I voted for Gore.

by **Eddie** » Mon Oct 31, 2005 7:32 pm

Thanks to all for your comments. It appears that this solution won't
do for our application. Our typical installation would involve 10-30
controllers on a single RS485 multi-drop cable in the 1000+ft total
length range. With this layout and our polling master/slave protocol,
Modbus/RTU serial at 19.2kbaud, we typically get 1-2
updates/second/controller if requesting several bytes of data from each
controller. The Modbus/RTU serial protocol is only typically supported
at speeds up to 38.4kbaud, so a simple baud rate increase isn't a
soultion. A fairly seamless change for us would be to put all of these
controllers onto ethernet running Modbus/TCP protocol which wraps the
same serial command string into a TCP packet and uses ethernet for
transport. The major issue for our isnstallations with ethernet is the
home run wiring costs. Most of our wiring installations require
electrical conduit with a final installed cost of ~$10/ft which is much
larger than the cost of the cable itself. Therefore, anything we can
do to decrease the total installed feet of cable/conduit is a big cost
saver to our end users.

It sounds to me like the best solution for us if we want to utilize
ethernet is to put some centrally located multiple port switches in the
field and link them together to give the most effiecient layout.

Thanks again for your comments.

Ed

by **James Knott** » Wed Nov 02, 2005 3:05 am

anybody43@hotmail.com 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

When you reply to my messages, please quote something I actually said,
instead of making it appear as though I said something I didn't.

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