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Writing a OPC Client

Permalink: 2014-03-07 00:00:00-06:00 by Keith Gray in work tags: python programming work

Introduction

OPC is something I see quite often in my job as a SCADA engineer. Up until recently, all I knew about OPC was that it relied on Microsoft Windows' Object Linking and Embedding (OLE). In fact OPC is short for OLE for Process Control. Even though OPC was always a problem on projects, I thought of it as a black box because of its relationship to Windows.

However, on a recent project, I had the need to automate some tests. I had two choices:

  1. Develop or purchase a DNP3 communication library to communicate with the device being tested.
  2. Use an OPC Server to speak DNP3 and then get the data out of the OPC server.

I had previously looked into what it would take to develop a DNP3 driver and it wasn't going to be cheap or easy. I needed this application and I needed it yesterday. Therefore, I decided to investigate what it would take to communicate with the OPC server.

Background

I have quite a bit of Python programming experience and I knew that the pywin32 library supported Windows COM. COM is what OPC uses for communication on a Windows system.

I spent one weekend poking around on Google, playing with pywin32, and using the demo mode of the Kepware OPC platform. I was able to connect to the kepware server and get data out of it.

OPC Client

The OPC client I am going to show here is by no means a complete implementation. However, this simple application met the needs of my project and is relatively simple to get set up.

Prerequisites

This application was written on a Windows 7 machine. I have started using the Anaconda python distribution on Windows because it is so easy to get the packages I need installed without having a full C/C++ development environment. The pywin32 library is required and can be installed via anaconda.

$ conda install pywin32

It is also required to have the OPC Server configured. In this case, I simply created a channel and device in the Kepware server. The channel used the Simulator driver and I named it 'Simulator'. I also named the device 'Simulator'.

The Code

The code below is really simple. It connects to the Kepware OPC client, creates a group, adds one binary tag to the group, retrieves the value, quality, and timestamp of the tag (which should be False), writes a value of True to the tag, then rereads the value, quality, and timestamp. Finally it disconnects from the OPC Server.

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from win32com import client
opc = client.Dispatch('OPC.Automation.1')
opc.Connect('KEPware.KEPServerEX.V5')
group = opc.OPCGroups.Add('Test Group')
group.IsActive = True
item = group.OPCItems.AddItem('Simulator.Simulator.K0000.00', 1)
value, quality, time = item.Read(client.constants.OPCDevice)
print("Value: {}; Quality: {}; Time: {}".format(value, quality, time))
item.Write(True)
value, quality, time = item.Read(client.constants.OPCDevice)
print("Value: {}; Quality: {}; Time: {}".format(value, quality, time))
opc.Disconnect()

What's Next

OPC has a lot more options than just reading or writing to a single point at a time. Your client can subscribe to events and get notified when a point value changes. I didn't make use of that functionality in my project but I probably will in the future.

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