Why dont PLCs use ethernet??

I know NOTHING or very little abt PLC and automation. But the subject interests me.
Im curious..... why don't ALL plc's talk via ethernet?
Aren't some PLCs based on a proprietary networking protocol? If yes.... why?
Is ther a primer on PLCs that you can also point me to that explains the "basics"?
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Many PLCs do use Ethernet. If you read introductory material on Ethernet/IP (a TCP/IP extension with automation in mind) you'll get answers why plain Ethernet was not very popular with automation folks.
The main reason Ethernet has not been popular is the lack of determinism, but there are other reasons.
www.ethernet-ip.org
Andrew

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OK thanks Andrew.
Is this new industrial version of Ethernet being accepted very well? Is it taking off?
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It's not all that new, but "yes" and "yes".
The two "main" players are Ethernet/IP and Modbus/TCP and each have their uses and applications. The industrial world (processing real-time data) is very different from the IT world (copying files around) and thus requires completely different protocols - but the hardware mostly remains the same.
Cameron:-)
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Really dumb question.....cause I don't really know what Im talking abt....
but on those PLCs that do NOT use industrial ethernet...
.. do they use each use a proprietary protocol that differs from maker to make and thereby making it impossible to mix and match PLCs?
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Sometimes they do and sometimes they don't - it depends entirely upon the marketing strategies of the manufacturer in question. But these days it is most always possible to mix and match PLCs if you really must - no manufacturer wants to lose a sale just because the next guy can't talk to his product.
That's where System Integrators come in...
Cameron:-)
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Using switches and 100BaseTX can reduce the problems caused by non- determinism. This article is pretty interesting:
http://www.sensorsmag.com/articles/1100/22/index.htm
Joel Moore
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snipped-for-privacy@yahoo.com wrote in message
<snip>

http://www.plcs.net /
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Very simple. DCSs and PLCs used networks, distributed computing, structured programming, pre-tested code and a whole host of other recent inventions long before the names were invented. They still work in existing systems.
The value of upgrading a working system that continues to produce product is zero. The cost of shutting down these systems to upgrade their systems is extremely high. Do you see any incentive to change?
The question now is, "Why don't they use it now?" The answer is also simple. All new products do use it. They were just waiting until feature creep came up with features that were of value to industrial control systems.
The answer to your last question is that with proprietary systems, the user does not care *how* they work as long as the *do* work. His objective is not to design neat new networks. His objective is to make gas, beer, cookies, hubcaps, etc. Its different now. With 'open' systems you have to build your own networks. This dumps a lot of extra work onto the end user and is another reason for their resistance.
Walter.
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PLC's use many types of communication links, including Ethernet (RS232-RS485-RS422..., different busses). Typically the mid and hi end units have Ethernet option or com plug-in. Price of Ethernet on <$1,000 low end PLC's is preventing it being done, but is coming fast and is on > $ 1,000 PLC's and PLC CPU's, but mainly by plug-in card and DRIVERS are also needed. SCADA is driving it's use in PLC's and the factory floor for speed of data logging, plus newer IT people understand it. I would also look for USB-2 to become common.

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