This is Part 1 of our “25 Things to Consider when Choosing an OPC Tunnel” series. When choosing software applications to handle the tunneling of your data, there is more to making the right choice than simply a checkbox on a product datasheet that says “tunneling”. This series will introduce you to the considerations that will help you make an informed choice and choose the most effective tunneler for your specific application requirements. If you are unfamiliar with what an OPC tunnel is, please read our post “Tunneling OPC Data – What Is It?”
How are network drops and recovery handled by the tunneler?
If you are using DCOM, which is the Windows mechanism that transfers data between applications, and your network connection is lost, neither side of the connection will be notified of the problem right away. You may notice that your client application, such as an HMI, appears to freeze or ‘lockup’. This occurs because DCOM has a hard coded 6-minute timeout. In a production environment, you need to know right away if your connection is lost so that you can determine if your data is still valid. Experiencing an HMI locking up for no apparent reason because a remote data source or network connection is down could prevent critical interaction with your system.
An effective tunneler provides a means to detect a network drop, notify you of the drop, automatically recover in an organized way, and notify you of the recovery. Ideally, a tunneler will allow you to tune the settings for what connection loss length constitutes a network drop; this is known as a “time out.” The tunneler’s ability to allow fine tuning of its configuration settings will ultimately affect how quickly a drop is detected.
The effective tunneler will also have its own heartbeat signal that checks for the health of the connection without generating unnecessary data traffic. This is especially important if you are using cellular 3G/4G connections where you pay for all data transmitted. With a satisfactory tunneler, you will not have to set up your own heartbeat which, even with an integer value, can run up significant data charges.
Lastly, an effective tunneler will have a reconnection delay. This delay will allow you to specify a waiting period before retrying the lost connection. Using a retry delay, you can tune how quickly your tunnel will resume data transmission when the network connection has returned.
How Are OPC Values and Qualities handled under connection breaks?
When a network connection drops, do you want the OPC data Quality value in your OPC client application to be marked “bad” or remain “good”? The 16 standard OPC quality codes include: Good, Bad, Not Connected, Device Failure, Last Known Value Passed and Comm Failure. A connection break can happen between the OPC server and the device, or between two tunnelers. An effective tunneler will have the ability to provide a different OPC quality code based on which part of the connection is down. Providing this information will help the user understand that the data they are looking at may be stale, and help them find the root cause of the stale data.
Most users say they want the last known value to be displayed but the quality marked Bad so their client software application will detect a loss from the downstream device. Many HMI applications have the ability to advise the user that the value on the screen is stale or potentially inaccurate. The decisions that are made from that value can then take that added information into consideration. An effective tunneler supports this user interface mind-set.
However, some client applications require an OPC quality of “Good” before displaying a value at all. For these clients, the user often prefers to keep the OPC data quality Good so they can see the last known value. An effective tunneler will allow this option to be configurable so that you can make that decision for yourself based on your operational requirements.
How a tunneler handles network drops is just one of the many considerations when choosing an OPC tunnel. Choosing an effective tunneler that takes into account your application requirements will make a difference in your operational effectiveness, resiliency, and profitability. Learn about the other reasons in the free whitepaper “25 Considerations when choosing a tunneling solution”.
