Using one ethernet network for all communications


Thread Starter

Nimi Hurvitz

We are using one ethernet network for both plant pc network carying web and office communication and PLC network that is carying messages between our PLC's.
We have about 70 PC's and 30 PLC's on a 100mps ethernet network.
We started getting some communication errors recently and I thought it might be related - any comments

Nimi Hurvutz

George (Jim) Hebbard

I was originally discouraged from Ethernet because it was not deterministic.

The thought of a Collision Detection type of protocol called to mind a message about the office dinner on Friday colliding with a request to shut down an overloading pump turned my stomach.

Then I learned about routers and switches. Essentially, a message from the power plant that a turbo is about to go down, and to back off is a
Ball-mill area (power consumer) can be forced to be delivered while the meal announcement is routed another way or delayed.

Perhaps you should consider this?



Johan Bengtsson

Well, the first comment is to NOT use the same network for plant and and office.

The plant network should really be connected to as few things as possible. For informaiton that really have to be passed between those two networks they should pass thru some kind of firewall or other thing stopping all trafic except the one you really want to pass.

This is especially thrue if you have hubs (insted of switches) in your network.

/Johan Bengtsson

P&L, Innovation in training
Box 252, S-281 23 H{ssleholm SWEDEN
Tel: +46 451 49 460, Fax: +46 451 89 833
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Pravin Fatnani

Please specify your connection scheme for completeness. What configuraion of swtiches, hubs etc. is being used? I am sure you are using
switches to separate the traffic on PLC segments and PC segments. Please comment.


Pravin Fatnani
CAT/ Indore/ India


Curt Wuollet

It would, of course, be better to have these seperate. Especially if the office nodes run under MS servers as I have found these to be
extremely chatty even when there is little business traffic. It is likely that a rouge application or a Windows client gone astray
could be causing problems. To answer more specifically would require information on topology. The general solution is fortunately fairly inexpensive and consists of using switches to logically group nodes that do primarily local traffic each on their own segment, thus keeping local traffic off the backbone while still permitting full communications between groups. This is fairly easy to do if you understand the traffic patterns. Quite often simply replacing a hub with a switch at the busiest part of the network can work wonders. If the PLC's are sending large amounts of data frequently (unlikely
as most simply can't) it may pay to analyze the traffic and switch there also. Many office applications don't make any effort at all to pace data and with today's PC's can burst enough data onto the network to cause delays. Ethereal and the like are your friends.



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As an answer to these types of requirements Siemens has developed 100/10 Mbps network components with features like optical, redundancy, SNMP etc. all in Industrial ruggid designs.
Conventional network topologies will start to fail with these requirements, since the moment of failure is undefined and the results of failure are also random a thorough network design becomes more important.



Jocko Harmet

Nimi Hurvitz,
The PLC's use very little bandwidth and are probably not contributing to your problem. Some things that can cause the PLC's however to cause problems with your network are: Electrical noise from improperly routed cable, and exceeding the maximum distance for your network media. CAT5 cable is only good for 100 meters without a repeater, and the various types of Fiber have various maximum run lengths.

Jocko Harmet
Tamtech, Inc.
Phoenix, Arizona USA
[email protected]
If your devices are polling each other on a fairly quick basis, it is entirely possible to tie up the network with traffic. The first thing that needs to be clairifed is: are you using hubs or switches? Switches can seriously reduce network traffic by removing the need for broadcast
messages. A switch only sends a message out the single port where it finds the IP address of the destination, not broadcast over all ports like a hub. The best method for this is to segment the network and have the PLCs on a seperate network, with perhaps a router to allow the web browsing from the PCs to the PLCs, but I seriously would not have it all on one network, without some kind of management by switches and routers.

Interwave Technology ( (disclaimer: Interwave is a client of mine) won the 2001 Wonderware Open Award (see the Wonderware
website ( for details) for a very innovative and clever way of dealing with the necessity of using ethernet for plant data and
interfacing with the office systems.

Walt Boyes

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Peter Nachtwey

Not a good idea if you mean they are all computers are on the same sub-net.
Don't use hubs. Use full duplex switches. Full duplex greatly reduces the chances of collisions. In an extreme case you may need a router or switch that can block multicast packets may also be required. Some industrial appication layers like Ethernet/IP makes use of multi cast i/o packets that will flood the whole net.

I would consider something like this office subnet office gateway office mask

Do the same for the subnets below for engineering for work cell A for work cell B

This way traffic on each sub net will not interfere with the other sub nets. However a office computer can access a work cell computer if specifically addressed.

Now if you have many office computers accessing work cell computers you may still have a problem.

In this case a computer with two or more Ethernet cards where the data is stored for each work cell so that the work cell computers can't be accessed directly. Obviously each Ethernet card has its own sub net. We have used this approach on our server until the cost of switches became reasonable.
If you MUST have the PLCs and PCs on the same network, then use switches instead of hubs. Otherwise keep them on their own networks. If data needs to be interchanged between them, use a switch to bridge.

Lynn August Linse

At 12:39 PM 9/18/01 -0700, you wrote:
>We are using one ethernet network for both plant pc network carying web
>and office communication and PLC network that is carying messages between
>our PLC's.
>We have about 70 PC's and 30 PLC's on a 100mps ethernet network.
>We started getting some communication errors recently and I thought it
>might be related - any comments

The big problem I see with PC traffic - meaning general Windows applications - is that they rarely place a priority on "frugal" use of
network resources. For example there are proven printer drivers from big names like Lexmark and HP which may attempt plug-n-play behavior by ARP'ing (ie: sending a broadcast to) every IP in your subnet every 10 minutes or so. Of course these broadcasts will go through switches treating it much like a hub. And these have proven in working factory systems we've seen to
disrupt some PLC with Ethernet cards which cannot process/discard the ARPs fast enough to keep their ladder logic from missing scan-cycles. There
could even be some multi-user games a few of your employees play at lunch time which use broadcast to "share" moves. You don't know - that's the big
problem with Windows - you won't know unless you go down and wait the traffic frame by frame with an Ethernet sniffer.

While switches may help, they cannot protect from broadcast traffic.

I'd recommend you talk to Cisco or other true router vendors to segment the PLC segments completely at the IP level from the PC segments. This means setting up subnets & "routing" between the 2 halves. It's not so cheap and easy, but the only way to keep broadcast and excessive Windows chatter out of your more frugal PLC network.

If cost of a commercial router is out of your budget, if you have a skilled Linux user an older PC running only Linux routing code should make a
suitable router (last time I saw our network topology, Lantronix had 4 Linux servers running in parallel to "route" between our production network and "test" network where we can test potentially faulty TCP/IP products without affecting real work).

Best Regards

Lynn August Linse, [email protected]
3 Rue Monet, Foothill Ranch CA 92610
Ph: 949-300-6337 Fx: 612-677-3253