Allen Bradley SCADA master

[JMiller]

The SCADA system consists of 9 remote sites that have AB MicroLogix 1400 PLCs. They use CalAmp Viper SC 400 licensed freq setup for ethernet. We plan to use a AB 1400 as the master PLC. The master will poll the remotes and store all the data. The HMI (Cimplicity, WW or RSview) not selected yet, will read the master PLC data. One problem with the 1400 is it has only one ethernet port and AB does not provide additional ethernet modules for the 1400. My concern is that using a router between the master PLC, radio and HMI will bottle neck the communications. I understand that delays can be implemented but as our current system is migrated to this new SCADA system, up to a total of 75 remotes our poll time might get too long. Control Logix and compact Logix PLCs and software are a lot more costly. Was hoping for a maximum poll time around 20 seconds.

Is my concern about a bottle neck valid?
Is there a reasonably cost effective Allen Bradley solution?

 

[diat150]

why dont you just poll all of the sites directly? using a plc as a data concentrator has always been pretty annoying in my experience.

 

[JMiller]

Thanks for the reply I guess I had tunnel vision this is something we will look into. 10 years ago when developing our existing SCADA it worked very good to have a master PLC and it covered up a lot of communication problems. Our existing system uses MDS Transnet 900 spread spectrum freq hopping 902 to 928 mhz using modbus serial communication. The radio signal strength at a lot of remote sites are marginal but have been very effective for what we need.

 

[JMiller]

Ken Roach have any input please?

 

[Ken Roach]

Don't you have to say my name three times before I appear ?

SCADA design varies widely, so I can't say there's any one right way to design yours. There are definitely Forum users with current knowledge of the "state of the art", especially with IP radios. A system with 75 remote stations is pretty big and deserves some serious consideration.

But I can mention things I know don't work well !

First, the MicroLogix 1100 and 1400 are great controllers for small remote stations, but I would not use one as a central data collector or SCADA master for a system with more than a handful of nodes.

The MicroLogix online editing feature has an important exception: you can't do an online edit of a rung with a MSG instruction in it. If you are adding nodes as the system grows, you're going to need to stop and download to the master controller every time you want to change the communications routine. This becomes tiresome fast.

You also mentioned that the system will grow, up to 75 nodes. If you are using TCP/IP then you're going to run into a TCP connection limit on the MicroLogix 1400; it can create just 16 outbound TCP connections.

Even the SLC-5/05 will run into this limit: the 1747-L553 has the highest limit, and it can create 60 outgoing TCP connections.

You would need to go to a ControlLogix with a 1756-EN2T to get enough TCP connectivity for your whole system (the 1756-EN2T can create 128 TCP connections).

If you use a SCADA computer to communicate with the remote controllers directly, PC operating systems typically don't have such low limits. Windows XP, for example, had a limit of 10 simultaneous incoming TCP connections, but not practical limit on outgoing connections. A proper server operating system like Windows Server 2008 can make many hundreds of outgoing connections.

I'm going to leave the SCADA supervisory software selection as a separate topic, but I'll warn you that most salesmen and even technical guys (including me) don't have deep experience with low-reliability networks. The drivers and configuration are meant for good-quality 100 MB Ethernet, not for a licensed radio network. The credo of anyone working with wireless communications should be "we can deal gracefully with packet loss", not "PING works, so there's nothing wrong with the radios".

In my experience, Kepware is very good at handling networks that have frequent packet loss, with features like "polling demotion". RSLinx Enterprise (the preferred data source for FactoryTalk View SE) is very good at integration with ControlLogix, but it assumes a fast, high-quality connection. Even though it means some extra labor creating tag databases, I would choose KepServer over RSLinx Enterprise for a SCADA system OPC server.

Many Forum users will emphasize the importance of separating the business or enterprise network from the automation network, and this is absolutely crucial for a wireless network. The SCADA systems I've worked on that have persistent, years-long battles with their telemetry have one thing in common: the telemetry network is connected carelessly to the enterprise network with unmanaged Ethernet switches.

That's principally an expression of their casual approach to the automation systems, but it's also a source of some of the problems. I've run Wireshark on these systems and found streaming audio protocols, printer discovery protocols, router discovery protocols... all kind of stuff that both the radio system and the remote PLCs had to deal with.

 

[diat150]

Thanks for the reply I guess I had tunnel vision this is something we will look into. 10 years ago when developing our existing SCADA it worked very good to have a master PLC and it covered up a lot of communication problems. Our existing system uses MDS Transnet 900 spread spectrum freq hopping 902 to 928 mhz using modbus serial communication. The radio signal strength at a lot of remote sites are marginal but have been very effective for what we need.

well you are definitely going the right route by ditching the serial radios for ethernet. It would be impossible to have the update times that you are looking for using serial radios.

as far as using the data concentrator, Ken has pretty much pointed out many of the shortcomings. Unless you are paying out of the wazoo for data, like say over a cellular connection or some remote satellite connection, there is no logical reason to use a data concentrator. it will cause you far more problems and headache. Thats just my opinion though.

 

[JMiller]

Thank you Ken, I was hoping you would have some input. By the way we are considering making AB our standard which would be a big change for me. Also thanks to diat150 for a good suggestion.

You may find this interesting how we have setup and are using our existing SCADA. We have 50 sewer lift stations with about 10 store and forward sites for a total of 60 sites. Using Automation Direct 06 for remotes and a 260 as a master. At the remotes I move about 16 words of data into a consecutive block which is read by the master then write 2 consecutive words from the master to the remote, this varies slightly depending on the site. The master polling is done by using the special bit (SP116) AD has that is on when the PLC is communicating with another device. Using a SP116 to toggle a counter then using the counter value sequence in each rung of the reads and writes and when it reaches the last rung the counter resets and starts over. The serial cable on the radio to PLC has the CTS and RTS jumpered out. I have a bit toggled on in every remote which when read by the master resets a timer in the master program stopping a comm fail output for that site. This has been working wonderful for 10 years even though sometimes the poll does not get the data it does the next time around but mostly this works good on every poll. The total poll time is 20 seconds for 60 sites. As we do very little remote control I could eliminate the write commands and probably increase the polling to just over 10 seconds. I don't really know but using 902 to 928 mhz is probably going to be faster than the uhf 450 freq.