Is it required to use an OPC Server to Communicate with Controllogix?

Question

I'm wondering if for simply communicating with a PLC, like reading and writing tags, do I need all of the other heavy lifting that comes with an OPC-UA server?

I've tried writing a simple server in Python that talks to the PLC, but I get denied when requesting information from the PLC.

The Controllogix PLC I'm attempting to communicate with uses Ethernet/IP to communicate, so why doesn't a simple server/client script work? What is required exactly to communicate with an Allen Bradley PLC or PLC's in general?

Answer 1

There is quite a bit required to communicate with a PLC.

Each vendor has a driver, there are firmware compatibility considerations. Different protocols to think about.

OPC-UA makes it a bit more generic, but OPC-UA still has a set of things to work around when setting up communications.

Most of the OPC products I've worked with, needs to have their security adjusted to allow anonymous communication. It's generally bad practice to do this. (A network intrusion would be able to read/write to your automation layer) There is certificate signing and some encryption business that needs to be turned off if you're looking for simple communication. (Again, not a good practice but ok for learning)

After all that you have to have a notion of how your PLC is set up on your OPC server, there are channels, devices, namespaces etc. You'll point the OPC client to some opc.tcp://:

If you got this far you're almost done, I'm assuming your OPC server is running and has tags configured at this point. You can use your OPC-UA API to do a read. It can return just the value, or you can get an object back with tag health, timestamp, and a bunch of other data. Depends on the implementation. After that you can do subscriptions, writes...whatever else you need.

TLDR: OPC server not required, but may be the easiest method. Turn off security. (But turn it back on before exposing your control layer to the net)

Answer 2

Depends on several factors, if you want something simple to program you can opt for Modbus/TCP I think some AB PLC supports it without extra hardware.

However if you want something with more security for example for industrial use then OPC UA would be better choice but the programming has a complexity far higher than Modbus, even using the libraries of OPC Foundation or others. There is the option of using a commercial or free (if any) OPC UA server to save work, then you will need to program the client side only.

With Ethernet/IP it should also be possible, but the problem is that there is no clear specification and even different AB models talk different Ethernet/IP dialect ! , it is also far more complex to program than Modbus.

Answer 3

I am a little late to this discussion, but there are a couple commercial tools that make this a bit easier. The one that comes to mind for me when you say you are using python is Cogent's data hub. It is certainly not the cheapest tool out there, but they have already done all of the heavy lifting for PLC/PC communications & security.

Answer 4

If trying to read CLX data using Python, there are several open source implementations that will save you a lot of work. Such as this:

https://github.com/dmroeder/pylogix

If you use .NET and Visual Studio, you can use AdvancedHMI

Answer 5

to be able to read and write OPC Tags to ControlLogix platform is done via its communication Driver RSLinx. RSLinx acts as an OPC Server, it will need to be configured to communicate with the PLC and run on a networked PC on the same LAN. Several flavours of RSLinx are available (for WAN/VLAN also) but essentially this is the communications driver you need to talk to AB PLC's

Answer 6

I am also a little late to this conversation. If you are interested in coding your own solutions and don't want to use any of the commercially available standards, AdvancedHMI is a "mostly" open source solution written in VB.NET which is 100% free and provides communications to many different PLCs including the ControlLogix platform. Since I see you are programming in Python you may also be interested to know that the project does work under Mono on the Linux OS. I have used it to write gateways between EthernetIP and ModbusTCP and to pull data serially from OEM devices and push this data to a CLX PLC.

The forum is full of many helpful hints and is very active and supported.

Just trying to give you another option. DDE, NetDDE, FastDDE, OPC, DCOM, Suitelink.... These are all good, but mostly a pay to play adventure. As a programmer, it seems ridiculous to have to pay such an excessive amount of money just to talk to my hardware, IMHO. Sorry for the rant. Have Fun!

Update - Just wanted to also suggest the following open source project written in python:

https://github.com/dmroeder/pylogix

I have used this to write small programs for communicating with CompactLogix and ControlLogix. (Even to/from a RaspberryPi!)