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Old April 4th, 2006, 02:37 AM   #1
srik
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Question PLC to PLC communication

I have to Install two PLCs.In which one PLC is main controller and one is auxilliary.I have to decide auxilliary plc first due to that auxilliary equipment vendor has been confirmed and they have asked for PLC specification.we given specification for the auxilliary PLC as per our requirement.we asked for Communication as RS 485 with modbus protocol.I am not familiar with communications. I have to decide main PLC communication .Is it required to give information regarding auxilliary PLC communication details to Main PLC vendors?If main PLC vendors followed another type of protocol,Is there any problem to establish communication between two PLCs?
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Old April 4th, 2006, 03:15 AM   #2
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You main PLC must also have modbus protocol communications on RS485.
RS 485 is a hardware protocol, and if the other end is not alos RS485, it will not work (OK, you can sometimes make RS422 work with 485, but I wouldn't generally try it).
I would also strongly suggest that you use the same brand, and preferably, the same model PLC for the main PLC. Usually, vendors make communications easy between their own PLCs.
Finally, I would suggest getting hold of the two PLCs you intend to use, and bench test your idea. If you can't do this, suppliers or integrators of PLCs can often advise you of the best way to go about this.

Otherwise, you can post the PLC make and model numbers here, along with the type of cards you intend to use. Someone would probably have tried this before and be able to advise you on what will or won't work.
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Old April 4th, 2006, 03:38 AM   #3
srik
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Thank you mr.Doug_Adam.
The auxilliary PLC will be supplied by auxilliry equipement manufacturer.So still not yet decided the PLC make and type.We thought that there may be problem with communication with main PLC.So we specifically mentioned the RS485 port and modbus protocol in the auxiliray PLC equipment specification.There is no chance to hold this PLC upto main PLC confirmation.The main PLC will be decided after complete I/O list preparation of remaining total plant.
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Old April 4th, 2006, 03:53 AM   #4
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Hello srik;

Basically, to establish communications between PLCs, you have to decide on protocols, topology and data exhange parameters. Modbus is a very well supported protocol in the PLC world, and is available on most (if not all) PLC platforms. It is also relatively easy to setup. So there is no absolute need for both PLCs to be of same make and model as long as both support the same protocol(although programming issues will be much simplified if they are). One of the stations must be setup as Modbus master, the other as Modbus slave; addressing and connectivity (RS-485, 2-wire, at 9600 Baud, Even parity... for example) must be consistent on all stations of the network.
Note that there are other industrial protocola on which you can do the same: Profibus, Ethernet (make sure it is the same version on both PLCs)....
Then comes the hard part: setting up the data to be exchanged. Amount and type of data must be decided on, and exchange tables must be structured (data areas reserved for communications). Say, if you have a Start bit that you want to send to the auxilliary PLC, it is better to set it in the Input Exchange table, use that Input bit to command the process you need to start (in the auxilliary PLC program), and send confirmation back to the master PLC through the Ouput exchange table, for example.
Come back to us if and when you need to clarify things.
Hope this helps,
Daniel Chartier
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Old April 4th, 2006, 04:26 AM   #5
srik
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thank you mr.dchartier,

Is there any chance to use two different protoclos with the same PLC? I mean if Mian PLC vendor wants to use profibus to communicate other peripherals,Is there any chance to use modbus to communicate the auxiliary PLC?
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Old April 4th, 2006, 05:06 AM   #6
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Hello srik;

Ever wonder why PLC communication processors cost so much?
Each one is a complete mini-computer, dedicated to its communication link. It holds micro-processors, memory buffers, modems and interfaces to exchange data over a bus; also, it has hardware and software to connect to the PLC backplane and exchange data with the CPU.
So yes, you can have as many comm processors in your configuration as the backplane can hold, and the CPU can handle. They can be all different protocols, that does not matter.
In the last project I am doing, on a ControlLogix platform, I have one ControlNet processor (redundant), 2 Ethernet-IP processors, 2 Modbus masters and 3 Profibus masters. No problem at all for my CPU. It never sees data from the fieldbuses; that is handles by the comm processors. The CPU only sees the data the comm processor passes to it over the backplane.

Hope this helps,
Daniel Chartier
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Old April 4th, 2006, 06:20 AM   #7
srik
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Mr.dchartier,
thank you for valid information.I have one more question .If existing PLC have one Ethernet switch comprising some gatways to communicate with other peripherals,Is there any possibility or any equipment required to communicate with RS485 modbus compatible peripheral?
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Old April 4th, 2006, 06:36 AM   #8
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Hello srik;

Do you mean thee is a switch integrated in the PLC hardware?

Or do you have a Ethernet communication processor installed in the PLC backplane, connected through a switch to a Ethernet network, and communicating with peripherals?
In this case, my last answer applies, you can have multiple communication processors on a PLC backplane; it depends on the platform you want to use.
Now we are entering the realm of "what if?". Now you need to be more specific: what brand, what model. The rest is speculation, at this point.

Hope this helps,
Daniel Chartier
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Old April 4th, 2006, 06:59 AM   #9
srik
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Mr.dchartier,
It is a qurey only.If there is an Ethernet communication processor installed in the PLC backplane and connected to a switch to communicate to peripherals,Is it possible to commnicate with modbus Rs485 device without keeping any another communication processor in PLC? I want to tell once more ,Iam not familier with communications.
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Old April 4th, 2006, 07:30 AM   #10
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Hello srik;

I fyou wish to go that route, you can always use a protocol converter. Look it up in Google, or try Prolinx Gateways, Sixnet IO, Woodhead, or other companies that make such devices.
But unless you understand both protocols you are trying to interface, you face a very steep learning curve. I do not recommend it.
Hope this helps,
Daniel Chartier
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Old April 4th, 2006, 07:38 AM   #11
srik
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I dont have any intention to use ethernet to modbus compatible device.Just I want to know the possibilities.Thankyou very much for the clarifications.Now I can decide my self some thing about communication.Once again thanks.
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