Why not Ethernet vs. RS-485? 1

[bdn2004]

I'm looking at the devices that will attach to the PLC and note that they will communicate Ethernet or RS-485. The engineering firm picked RS-485 to the devices, and connected Ethernet to the LAN. I'm thinking why not just use Ethernet everywhere? The devices are only about 50' away. There is not a speed/processing issue either.

 

[itsmoked]

For 50 feet Ethernet would make more sense. Ethernet also gives you automatic galvanic isolation.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

If ethernet is used for the (field) devices, a switch is needed, another point of failure.

Ethernet communication with the LAN could be down and communication with the (field) devices remains intact, because that link over RS-485.

 

[bdn2004]

More basics....

I understand I need CAT 6 cable for Ethernet, but it can also do RS485 right? Is Ethernet like a parallel connection and RS485 serial? or is that too much a simplification?

And Modbus is one way to transmit the information via RS485 right? Although you can do Modbus over Ethernet. That seems to blow the parallel/series simplification out of the water.

All these terms seems like they jumble together to me. Is it a little like the term grounding in electrical - which seems to cover way too many aspects of a similar concept?

You do realize I asked this question, I am assuming you both are correct and they are opposite answers. Oh the world of IT....

 

[Henneshoe]

Ethernet can run at multiple speeds (depending on what the hardware is capable of). By far, the most common speed for Ethernet today is 100 Mbit/s. At this speed Ethernet can use CAT5 or CAT6 cable. Commercial Ethernet cable is unshielded, but some industrial cable suppliers make a shielded version for noisy environments.

Regarding your comparison of RS485 and Ethernet: RS485 uses 3 wires; one wire is Common (AKA Ground). The other two wires are a differential pair that is used to transmit AND receive data. Therefore, RS485 is half duplex. Ethernet at 100 Mbit/s uses 4 wires. These wires are 2 differential pairs. One pair is used for Transmit, and one pair is used for Receive. Therefore, Ethernet is Full Duplex. As Keith pointed out, Ethernet does not require an additional wire for Common (AKA Ground) because it is coupled through transformers and therefore doesn't care too much about common mode interference.

Per danw2's post, RS485 is a multi-drop bus so you can connect many slaves to the same copper wires (line topology). Ethernet is a point-to-point connection so you need an Ethernet Switch to connect devices to each other (star topology).

Both Ethernet and RS485 are considered serial busses. Ethernet and RS485 are two different standards that explain how data should physically travel from one device to another. Modbus is a standard that explains how to decode the data and what it means.

In general, I would say Ethernet is more reliable than RS485. The transformer coupling does a good job of destroying any common mode interference.

 

[jraef]

Ethernet is serious overkill for a simple device however and if you have 32 field devices to connect, that is an expensive switch (or switches) whereas RS485 can be daisy chained (multi-drop) from one comm device.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[NWBeaver]

Another thing to consider is the field devices.
How many of them actually have an Ethernet port?

 

[danw2]

As to why?

I have clients who use devices which talk Modbus over either ethernet or RS-485, but they use the RS-485 specifically to avoid network problems.

Their data is compliance data and the Modbus RTU network over RS-485 is a critical back-up to sometimes problematic ethernet LAN.

 

[bdn2004]

Now what I've been told by Allen-Bradley is their PLC requires Ethernet I/P, and the Ethernet the field devices provide is not Ethernet I/P but some other speak. That's what I'm saying all along...ETHERNET is ethernet is EtHeRnEt...all not the same thing, but called the same thing and that is confusing sorry.

There is some expensive module that I'll need on both ends to translate. And it cost as much as the CPU module itself in the PLC.

Is it the same with RS-485? That is are there different languages with that too?

 

[dpc]

Yes, RS-485 is just the physical layer. There are any number of protocols that can communicate over an RS-485 link. Could be ModBus, could be something else.



David Castor

http://www.cvoes.com

 

[catserveng]

Ethernet IP used in Allen Bradley processors is a propritary "language". Ethernet, as explained above, is the physical serial bus, and EthernetIP in one of many protocols, or "languages" it can carry, MODBUS TCPIP, Profinet and TELNET are some others used.

Try this link,

http://www.bb-elec.com/tech_articles/introduction_to_industrial_ethernet.asp

B&B Electronincs has several good tech articles on industrial ethernet, their website is

http://www.bb-elec.com

You may find their articles will help you better understand some of the basics you seem to be struggling with, hope it helps.

Mike L.

 

[robertjo24]

It seems that with all the recent advancements in the industrial realm in regards to Ethernet enabled devices, you would think that the equipment manufacturers would do a better job of implementing their protocol stacks and back end communications schemes. It is as though everyone always assumes a 100MB LAN connected device everywhere, that will never traverse anything but an industrial network, or talk to other manufacturer’s devices.
If you are considering implementing Ethernet into your controls systems, you really need to spend some time learning how the device manufacturers implement their own communications protocols and how they play with other devices. Also, it would help to have a fundamental understanding of how these protocols affect the network as a whole. As an example, many PLC manufacturers use multi-casting for all their PLC’s to talk to one another on a network. If you have just two PLC’s and one HMI, you now have three devices on the network all multi-casting to each other. They way they have implemented this comm scheme isn’t very flexible, and can wreak havoc with a network if not carefully controlled and isolated.

Both serial and Ethernet connectivity schemes have their pros and cons, and their own place. I think network connected industrial control systems will be the dominating force in years to come, but to be most successful, we need to do our homework to better understand how to properly implement these systems into our particular environments.

 

[amptramp]

I'm not sure I agree with the "proprietary" part of the comment "Ethernet IP used in Allen Bradley processors is a propritary "language"." At one time, this statement may have been true; however, today Ethernet/IP is an open standard (this does not mean the specification is free) managed by ODVA (Open DeviceNet Vendors Association).

ODVA is an international association comprised of members from the world's leading automation companies. Collectively, ODVA and its members support network technologies based on the Common Industrial Protocol (CIP™). These currently include the network adaptations of CIP—EtherNet/IP™, DeviceNet™ and CompoNet™—and major application extensions to CIP—CIP Safety™, CIP Motion™ and CIP Sync™. ODVA manages the development of these open technologies, and assists manufacturers and users of CIP Networks through tools, training and promotional activities.

Common Industrial Protocol (CIP) encompasses a suite of extensions stacked on a group of network adaptations. A layer model for CIP can be seen below:

 

[controlsdude]

Ethernet
Each device adds a connection to the PLC. AB PLC can only have up to 255 max, but recommended is 180-200. I would try and stay with the RS-485. If you have too many devices over ethernet you will be adding another plc due to connection limits. To me this is the downfall to ethernet on AB PLC, besides the switches you buy. Also, when you go up in connections with the devices, the ethernet card in the rack can only handle so many devices depending on speed and RPI, another bottleneck.

I would stick with RS485, there are plenty of RS485 scanners that you can find that fit into an AB rack or whatever plc you use.

 

[djs]

Generally speaking you are limited to about 32 devices on a RS-485 electrical segment. So to havemore devices on a network, you will have to use repeaters.

 

[delxzen]

We recently had some problems with a Profibus instrument bus which is physically an RS-485. An expert came to analyze the bus, and the problem was located. The service PC with the analyzer tool was connected via a piggy-back service connector at one end of the bus. Nothing had to be disconnected and process was running all the time. Since all nodes in the RS-485 are physically connected to each other, the whole bus can be analyzed from one single point (end node).

In an office environment there is usually very little electrical noise, you can disconnect equipment, replace cables and mess around without too many complaints. Maintaining Ethernet in a production environment is just not that simple.