In the water and wastewater industry in the US, Modbus is the most common telemetry protocol. Its great benefit is its simplicity – and almost everyone in automation and process control is familiar with it.

 

For a few years now, we’ve been promoting DNP3 for telemetry instead of Modbus. Why? What’s the benefit? For people new to protocols and telemetry it’s hard to know where to start. 

Today, we’ll cover the first benefit:

 

Date/Time Stamping – or Less Guessing

DNP3 adds a date/time stamp to every event when it occurs. Each event is added to the “event buffer” in the RTU with its own date/time stamp. Then when the RTU communicates with the SCADA, or “master station”, the data can be stored in a Historian (database) or displayed on the HMI with the actual time the event took place – not the time that the master station received the data.

I mention that point because a lot of SCADA systems, most of them, were so orientated around process plants with hard-wired high speed data connections (like ethernet or high speed serial) that the normal mode of operation was to log the data with the date/time that it was received by the SCADA (and ignore any date/time stamp from the field if one existed). It doesn’t matter too much in a process plant – the comms is almost instantaneous – and if any comms outage occur they are quickly fixed.

DNP3 tells you when the event occurred – not when the SCADA system found out about it! 

In wastewater collection and water supply systems almost all of the assets are distributed in the field. And communications are often disrupted. Even when they are not disrupted, you don’t want every event to be communicated when it occurs.

Here’s how a typical Modbus network works – the master station (typically the master PLC) “polls” each site in turn to get all of the current data.

“Get the current data” means read a snapshot of all of the Modbus “registers” or data points. Most of them won’t have changed, so you are using precious bandwidth to capture lots of unchanged data. What about the data that has changed? Unfortunately, you don’t find out when it changed. You can just see that a different value is there now.

Modbus fills up the bandwidth with data that hasn’t changed. DNP3 only captures the data that has changed. So it can communicate a lot more useful data in the same bandwidth.

Let’s use the example of an analog process value. The value that the SCADA last captured from the field was 6.8, this time around – 10 minutes later – it is 6.9. So when the SCADA trending client displays this value – or you produce some kind of report, the process shows 6.8 moving to 6.9. Is that what happened? Or did it go to 7.1 and then back down? You’ll never know. You might know your own process really well, or you might be guessing.

You might know your own process really well, or you might be guessing. Using DNP3 avoids the kind of detective work that often goes on, where the operations and engineering staff try to fill in the gaps!

And then lastly, what if comms to that site – or a number of sites – is down for 24 hours? With DNP3, when comms is restored, all of the events during that period will be communicated to the SCADA (check out the technical note below).
 

Do I need this?

Whe you build a house, if you have the right foundations and structure you can add another floor. If not, you have to knock it down and start again.

To begin with, just getting reliable data out of a SCADA system is a great relief. But once it’s working and providing data you start to ask more questions – because you want to make the system work better.

Did both pumps run together in Pump Station 18?
What’s the hydraulic delay between Station 19 and 18?
I know the radio failed but do we know how high the level got at Station 12? And so on.  

 

A Quick Technical Note

Designing your comms system always takes some work and this isn’t aimed to be a technical workshop. But one important point with your field device is to ensure that the event buffer has been configured with enough memory to capture all the data that you might need in an outage. And the other important point is to ensure that the DNP3 configuration is for SOE (sequence of events) – as it is possible to configure last event only.

I mention these points because it is often only when you really want the data that you find out it wasn’t configured to capture that data that you wanted. Best to check when starting out.

Learn More

There are two whitepapers about DNP3  on the main site. One covers DNP3 Security – “Keeping SCADA systems open and secure from cyber-attack” and the other is more general – “Benefits of DNP3″.

To access the whitepapers you do need to complete a short registration form.

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