Remote I/O racks vs straight to PLC? 2

[bdn2004]

I'm a novice at PLC hardware requirements and am looking over a proposed plan. We have a large plant, with a central control room. We want to bring in field data from various switches. The proposal includes 50 remote I/O racks and three PLCs.

I guess all the field devices wire to the remote I/O racks located strategically in the field? Exactly what is in a remote I/O rack?

And how do you get from the remote I/O (multiconductor cable? fiber?) to the the PLC, where I suppose the processor is located?

 

[Henneshoe]

A remote I/O station has multiple I/O points and connects to the PLC via a fieldbus. Depending on your location the fieldbus is generally DeviceNet, Profibus, or some other serial protocol. Ethernet is becoming an increasingly popular option for fieldbus because of its speed and ease of wiring.

 

[bdn2004]

In my application we are using a Modbus Master/Slave Communications Interface Module to interface with a field device that is storing instantaneous information. Would the module be located in the remote I/O rack or in the PLC cabinet? I guess the question is..."is there any computing going on in the Remote I/O rack?" Or is it just a place to wire to?

 

[LionelHutz]

A remote I/O rack is typically a dumb device that allows you to connect all your field I/O and then provides a single communications link back to the PLC.

But, you could also use a PLC for remote I/O and even put logic into that PLC as well.

The main PLC rack can only hold so much I/O and then it needs to be extended to another rack. Obviously, the main rack is limited by the rack size and the number of I/O cards you can fit. So, if you are going to install another rack, it makes sense to make the single cable between racks longer and the many I/O cables shorter by locating this extra rack closer to the I/O.

 

[bdn2004]

Lionel,

I'm still confused.

We have to have these various modules..the M/S module for the app I describe. We also have some analog data coming in from meters, they say to get a Communications module for that app, and we have this synch clock another module, and other various digital and analog signals - all wired to the remote I/O rack.

Can these modules be be located in the remote I/O rack as described, and connected to the main PLC? These modules don't sound "dumb" to me. Thanks for the help.

 

[Henneshoe]

Remote I/O modules generally have a fixed function and do not run any user code (code written by you). They do come with many different I/O types (digital / analog / communications).

I/O modules are available with many different I/O types. Some examples are digital (PNP or NPN), analog (voltage or current), or more exotic options (RFID, HART, Encoder Input).

In your application, it sounds like the remote I/O stations will communicate the process data to the PLC via the Modbus fieldbus.

 

[PLCMentor]

Looking at this thread, it still looks like more information might clear up some of the confusion. Why dont you tell us some more specifics. What brand/model PLC are you using? What communication modules and other modules are you saying you think you need?

Also, when you think of remote I/O, generally you can think of it as a remote extension of the I/O in the rack. There are some cards that have local processing built into them, but they don't work any differently in a remote rack than they would in the local rack. As Henneshoe has mention, they generally dont require any local code. They usually just send a value or values back to the processor.

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[bdn2004]

One of the problems I have is that I'm reading a proposal in paragraph form from an outfit on how to do this that is install a PLC based SCADA system in our plant. They purposefully, I'm sure left out many of the details. I could be asking them all the questions but I'd rather get it straight in my head without their involvement...

I just want to understand the basic architecture and its function and where we will have to run cables, install boxes etc.

Here is what I know. An annunciator with SOE capability communicates through an Allen Bradley Prosoft Module MVI56-MCM in the RIO rack and communicates to the PLC via RS485. Metering data will come from various digital meters connected through a Prosoft MVI56-MNETCR module in the RIO rack and communicate with the PLC via Ethernet. Other random digital, analog and RTD inputs will also wire in. The PLC is Allen Bradley ControlLogix.

There will be an engineering workstation for programming the system. There will also be a SCADA LAN with connections to the plant's LAN and the new system can be pulled up on any authorized person's PC.

That' about all I know.

 

[PLCMentor]

Ok, it's fine to do your research and come up to speed independently. Make sure you also talk with the integrator that put together the proposal. Don't assume anything - ask and have it confirmed. The prosoft cards are not Allen Bradley, they are third party cards. Not a problem, you just need to understand that. The Prosoft MVI56-MNETCR is a Modbus TCP/IP interface card. It seems that they will be adding instrumentation that will communicate over Modbus TCP/IP to this card. The card in turn will send this data to the processor (ControlLogix). I would prefer Ethernet I/P interfaces on my digital meters and have them come directly into the ControlLogix. Modbus TCP/IP may be all that is available for some of the meters or it may be cheaper. You need to do some general research on both networks so you know what they are and the differences. I put "Prosoft MVI56-MNETCR" into Google and went right to the device/user manuals. You need to do that to get some basic understanding of the card.

While on the prosoft website, I did a search for the MVI56-MCM card. That card is a Modbus interface card. Modbus is a fairly standard communication protocol that may be the only communication method available for the annunciator. Look up modbus while you are researching networks. You will find that the Modbus TCP/IP is basically modbus encapsulated in TCP/IP so that it can be used with standard ethernet networking equipment.

You mention that something communicates to the PLC via RS485. Surely the remote rack is not communicating back to a control logix via RS485. I dont like that. Once again I would prefer Ethernet I/P for my remote comm link.

One question you may want to ask is why modbus. Is that all that is available for communications? Is it cheaper? Or is the integrator more familiar with non AB equipment and not selecting the best communication interface (my opinion)? How are the other I/O points being brought in? Are they going to cards, flex I/O, point I/O, or something else? What kind of SCADA are they planning? Is it something that will be easy to find help with in the future or is it something proprietary or some soon to be extinct system. Why did they pick it over the others out there. You need to know the answers to these questions.

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[bdn2004]

Excellent points PLCMENTOR. I will talk to them, I just want to be able to ask semi-intelligent questions when I do.

The RS485 link is from the annunciator to the the Prosoft Module in the RIO cabinet. The annunciator is used to provide the SOE monitoring and timestamp. Apparently the PLC is too slow to provide this function.

The annunciator is available with an option stating "Ethernet Port, T1 Connection (Used with Modbus or DNP Communications options. Must select option MB or DNP. Replaces 232/485 Serial Port). - You are saying this is a better option?

You are telling me that AB makes the same cards as these Prosoft's as called for? That doesn't make much sense to me for all the reasons you outline. A good question to bring up.

I don't know why Modbus, why is that inferior? The other I/O points will come into the PLC via conventional DI, AI, RTD cards I am assuming...

 

[PLCMentor]

Ok, so the annunciator talks modbus. The physical portion of modbus is RS485 if I remember correctly (never a good assumption). Is the annunciator a new system or existing? My question would be, can it be bought with a Ethernet I/P option.

Standard ethernet is different than Ethernet I/P. I doubt, given the text you quoted, the annuciator could talk with the PLC via that ethernet port.

I think you misunderstood my comment. The modbus cards are third party. I dont know if AB makes a similar card. I doubt it. I was just pointing out that they were third party.

I think Modbus is inferior; however it is compatible with everything. I think my watch has modbus comm ports on it (just kidding - i dont have a watch). Ethernet I/P is very easy to deal with and works at ethernet type of speeds. Modbus is much slower. Modbus TCP is also ethernet, but without some of the advantages of Ethernet I/P - especially with the ControlLogix. Ethernet I/P devices are extremely easy to work with in the Logix.

As for your other I/O points, I would suggest dropping I/O in places where you can group it effectively. If you have a place where a junction box makes sense, then a remote I/O block makes even more sense. I really like the Flex blocks in cases such as this.



Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com