The eWON industrial router is an IP-based connectivity device that enables the complete integration of a remote terminal unit (RTU) into a central supervision system. In a traditional implementation of a RTU, PLC and SCADA, eWON technology leverages easy-to-use Web technologies for decentralized monitoring of many different devices. Thanks to its FTP client/server functionality, eWON technology makes it possible push data acquired from the RTU to an eWON industrial router acting as a data concentrator on the local LAN. This feature enables data aggregation from multiple RTUs without the expense of polling remote units via landlines links from a master control station. 

Pull mode 

A typical SCADA system approach in the industry utilizes the pull model. In a SCADA system, every RTU is configured with a dial-up number and the SCADA regularly polls the RTUs to fetch the data in real time. Although this concept is workable, it has some drawbacks:

• Due to the nature of the polling process, the SCADA has to dial up every RTU periodically in order to retrieve the data, even if this data has not changed (which could be the case when monitoring small assets).
• What happens if a unit cannot be reached? The model should include a fallback mechanism. 
• Costs may be rapidly become exorbitant, especially when there are only small or infrequent variations in the field measurements or when there are many RTUs to be polled. 

Push mode

Another approach is the push mode. In this case, an eWON industrial router acting as an RTU pushes the data to the SCADA via another eWON industrial router acting as a data concentrator. The principle is twofold: 

• Periodic update: using an internal timer, the eWON industrial router collects data regularly – for example, every hour – from the field and sends it back to the data concentrator.
• Instant update: data is sent only when there is a change that needs to be forwarded to the SCADA controller. In this case, the eWON industrial router sends a notification in the form of a file with the new value directly to the data concentrator. This approach is suitable for situations where the number of variables is small and the dynamic behavior of these variables is slow, such as monitoring small remote assets (e.g. a water pumping station or a water tank station).

With both of these approaches, if a connection cannot be established to a device, the eWON unit can buffer the data and retry later when a line is available. 

Data concentrator

In this model, the eWON industrial router acts as a data collector for multiple RTUs. The principle is the following: the eWON industrial router (RTU) in the field establishes a dial-up connection to the eWON industrial router acting as a data concentrator. When the data is received, the eWON data concentrator unit displays it for the SCADA by means of tags using: 

• tags for RTU variables; 
• a time stamp tag; 
• an RTU name tag; 
• a data available flag tag. 

When new data arrives from an RTU through the data concentrator, the data is displayed and the data available flag tag is set to 1. When the SCADA sees that the data available flat is set, it reads all the indicated tags and identifies their origin from the RTU name tags. When all tags have been read, the SCADA resets the data available flag tag. From this point on, the eWON industrial router knows that additional data can be displayed from the same RTU or another RTU.   

The only requirement is that the SCADA is able to read tags from the eWON industrial router via Ethernet using the Modbus TCP protocol. Every data collector can act as a Modbus TCP server.  

The model also provides several data concentrators that can be called by the eWON industrial router. Depending on the number of variables collected, it is recommended to have each data collector retrieve data from 20 to 50 RTUs. The internal configuration of the eWON industrial router allows it to call several dial-up numbers, so even if one data collector is busy, the eWON RTU can automatically try another dial-up number in order to access another data concentrator. 

Put HTTP: a web server as data concentrator

An alternative solution is to replace the data concentrator by a web server that collects data originating from every eWON RTU in the field.  

To send the data, the eWON RTU performs an HTML transaction using the HTTP Put command. This means that the eWON industrial router has to connect to the Internet and the web server needs to be visible on the Web. Then the eWON industrial router tries to connect to the web server and send its data in an HTML data stream. The web server receives data streams from the individual eWON industrial routers and can extract the data sent by the remote units from these data streams. 

The HTTP Put command should be used in the context of a Basic script. This script can be triggered on demand or periodically, depending on the requirements of the application. The advantage of this alternative solution is that it is highly scalable, since a single web server can serve thousands of eWON RTU units sending their collected data at their own pace. With a large number of deployed units, this is the preferred way to implement a push model for data collection.

Advantages of this solution

 
A single eWON industrial router can support several serial and Ethernet protocols, so the user does not need additional driver protocols; they are all integrated in the eWON unit.

The SCADA and the data concentrator are all centrally located. On the RTU side, it is sufficient to use standard configuration parameters along with a small amount  of scripting if the HTTP Get method is used.  

In addition, the data concentrator concept eliminates the need for a modem server, since each RTU is responsible for sending its own data and can do so from any location and at any time, independently of the SCADA and the OPC server application. 

Featured products

We recommend the eWON 4101CD industrial router (with the historical data logging option for data aggregation) with following modem options: 

• GPRS quad band: eWON ref. EW532x8
• PSTN: eWON ref. EW532x4

Conclusion

This application model is ideal for remote monitoring of many small assets using a centrally located SCADA system. It is easy to implement deployment on the RTUs because all necessary features are available as configuration parameters.