We are proposing a remote telemetry system with redundant Master terminal unit (MTU) polling more than 50 RTUs for data. The data will then be available to some HMI PCs by Ethernet LAN. I've seen some installations with this and it usually includes a hot backup PLC at the MTU acting as a data concentrator communicating over radio or telephone line to the RTUs.
Now we are thinking of using a pair of PC's running a Scada software like Intellution iFix for the MTU instead and poll using a terminal server with 50 ports. Does anyone have experience with this setup? what are the pros and cons of this?

 

Your configuration is fairly common and there are +/- for each configuration. Some of the decision making will revolve around what the RTU's are, the communications protocol and what is required out of them.

Using PLC's as data concentrators gives you the most control over the communications. Most HMI drivers/servers are "smart" and create their own "optimized" packets. These tend to work great in an inplant situation but they are difficult to control in a SCADA application. Using a PLC as a data concentrator forces you to create fixed communication block sizes and allows logic to control how often and when to communicate. This in turn makes it easy to calculate the throughput and performance. HMI driver/servers often create variable size packets and usually offer few controls and diagnostics.

The type of RTU and what your doing with it plays an important role in designing communications. If all your going to do is read registers out of the RTU and send a occassional command or setpoint then either technique will work. If the RTU's include report data, such as from a flow computer, then there are other considerations and communicaion requirements. There are few software companies that make very good "SCADA Servers" that handle many of the concerns in this type application.

One other consideration, there are some complications in using serial port terminal servers with redundant HMI's. It is not straight forward in a Windows environment to have both machines configured to use the same terminal server. Be sure to verify your configuration with your supplier before going to fair down the road with your design.

Good luck with you application.

Joe Malter
Pipeline Systems Inc.
www.pipesys.com