As I read through all the stuff like PLCs and industrial computers, I usually see the interfaces are Fieldbus (CAN?), serial, but I rarely see an industrial network whose backbone is wired Ethernet?
Why is wired Ethernet not used? Is it because TCP/IP over Ethernet cannot meet stringent deadlines? Is it because all the relays and motors in an industrial environment would create so much noise and EMI on an Ethernet line?
Likewise, is the WLAN (802.11a/b/c/g/etc) technology starting to find its way into such environments? Or is it too not suitable for all the conditions in such an environment.
A PLC may have I/O of many types and is a Programmable Logic Controller. It is a computer with a software layer that allows it to be programmed in relay ladder logic and other means like "C" and BASIC. They come with many types of comm port variations including Ethernet, CAN (and other busses), RS232/422/485 to name a few. I/O can include discrete / analog / different Buss's / and PLC specific and are configurable via add on card selection or expanders.
The 802.11 is NOT a WLAN, it is Wi-Fi. See info at
PLCs and DCS had their own networks long before the 'open standards' were open standards. Now that Ethernet is available it has a bit of catching up to do before it meets the requirements of the old stuff. It has to PROVE is superior according to the priorities of process control before anyone will touch it. There are two levels of industrial networks. The higher level network links equipment. It is not so different from IT applications and Fault Tolerant Ethernet is slowly coming into use.
The lower level networks link individual instruments. There is far more to it than simple bit transfer. Foundation Fieldbus has the following features:
1 - two wires INCLUDING power.
2 - capable of working intrinsically safe.
3 - a predefined list of parameters that are universally understood by all vendors.
4 - ability to do loops for single point failure immunity.
5 - a defined set of industrially hardened connectors.
6 - guaranteed scan rates
7 - guaranteed power draw
8 - certified functionality. i.e. devices are tested and certified as fully functional.
9 - deliberately limited functionality to make it immune to malicious code, i.e. virus.
High performance in terms of bandwidth is not a concern at all but cost of cabling is.
A bit of both. I once worked at an airport in which ethernet was and still is used to transmit message between some 13 PLCs. On this
10Mbit network the traffic was typically only 60 killobit second sometimes rising to 100kbit when we used the ethernet to download or monitor to the PLC. Generally for PLC applications so long as you ethernet does not excede 5% or 500kbit it is effectively deterministic in that collisions are so rare and so qucikly resolved that the PLC won;t notoce. The only traffic was bar codes and bag serial numbers as the basgs were conveyed over the massive system.
This was a 10base5 network. One day some people with SUN work stations came alone and the cables just happened to be in the right place and unbeknowst to us they connected into out system which we had carefully isolated from the rest of the network with an intelligent router that only repeated messages that needed to be. About twice a day these 3 Sun workstations synchrosnied their database and Generated about 4.5Mbits for 15-30 seconds.
For that period bags got misorted becuase barcode scanner could not be passed on in time.
So ethernet will work for industrial applications: you just have to make sure you control who gets on it.
Also look at networks such as CAN which is the basis of devicenet. This was developed by Bosch and released to all for the control of cars: ABS brakes, engine management systems, blinkers, gearboxes, sun roofs, stability programs, disppalys, drive by wire etc etc.
It's hard to immagine ethernt being used in the envirnonment of a cars engine bat or it being cheap enough.
Ethernet is starting to be used more and more for industrial control applications. The older protocols are still around because they have so much momentum - people have been using FieldBus, RS232, RS485 and RS422 long before Ethernet ever existed. There are many advantages to using Ethernet over any of the older protocols and most manufacturers of control equipment know this and are converting over. The only issue is that the installed base of older systems is huge therefore the older systems are not likely to go away for some time. The good thing is that is is possible to migrate to an Ethernet backbone from the older protocols quite easily by using converters like "Serial Device Servers" that let you connect serial devices directly to Ethernet networks. In another 10 years, I am pretty sure that Ethernet will be the dominant backbone for industrial control systems. The writing is on the wall.
For some good articles about this as well as information on some good software tools, visit
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