RTU or PLC?

What is the difference between an RTU and a PLC?

We recently asked the question in our group among the Software and Systems business marketing and product leaders. Here is what they had to say on the differences between Remote Terminal Units (RTU) versus Programmable Logic Controllers (PLC) used in SCADA applications. Both are used in Infrastructure Monitoring applications.

Mike Foley, director of SCADA products for Invensys:

What makes an RTU an RTU? Let’s start with the “R,” which stands for remote. In what might be considered a simple “typical” SCADA environment, which is difficult to define since there are many diverse applications, RTU’s are responsible for gathering data in remote locations, e.g., outside of a plant, providing local control and exchanging data with an MTU (master terminal unit). The MTU collects information from many sources and integrates data from the RTU’s, and passes the data into the plant control system. Both a small PLC and an RTU can collect data from sensors, run logic, change outputs and send message. So let’s talk about the differences.

Power:

PLC’s are typically designed to work with industrial power sources (+24V dc and 85 – 264V ac)

RTU’s can be located in areas where the nearest conventional power source is hundreds of miles away. They are designed to cover these same voltages, but also input power from low-power sources, like solar cells and batteries, to high voltage DC sources as well, e.g., >100V dc in utility applications. This power may be simplex or redundant (supporting applications that require battery backup power) for operations that are critical even when system power is lost. If power isn’t an issue in your application, either path will get you there. Otherwise, an RTU might be what you need.

Communications:

PLC’s are designed to communicate using many embedded protocols. They can also use commercial or industrial converters to talk across many additional protocols. But with PLC’s you count on the communication channels to work. 

RTU’s are designed to compensate for the fact that communications are expected to fail. RTU’s tend to have massive on-board memory capabilities to store incoming, time-stamped data. Once communications resume, this data can be forwarded (store and forward) to the MTU, where the data history from all of the RTU’s can be reassembled as if communications were never lost. The time stamping and reassembling the sequence of events can not only be used to keep an accurate history, but also to diagnose problems, for example power grid failures, as the data is time stamped at the hardware level and accurate in the millisecond range. PLC’s typically are not designed for this type of operation.

Availability:

This doesn’t mean whether or not one is “available” for purchase, but by what percentage of time the controller is operating. Although PLC’s can be used redundantly, many RTU’s are designed for redundancy, so that if a point (or an entire I/O module) fails, the module can be hot swapped and operations can continue. This may also be true for the processor, so that when one processor fails, the other one takes over, allowing operations to continue until maintenance can occur. This may be “nice” if the RTU is around the corner, but it may be critical if it is on a pipeline that requires a helicopter to reach.

Applications:

Is the device running simple logic or are you performing application specific calculations for process control? Will the device be in a controlled environment or will it have to run reliably for a long time in temperature extremes? 

The bottom line is that if all you are looking for is some remote data, there are lots of solutions out there. But if you NEED an RTU, you probably need an RTU.

Steve Garbrecht, vice president, Software marketing, Invensys:

From a software point of view, our solutions interface with PLC’s and RTU’s so that’s not a problem. As far as one versus the other, it is hard to tell these days. It is a little bit of a religious subject with many users. Both RTU’s and PLC’s log data, have some control capability and can speak over networks. But RTU’s are usually a little more purpose-built for harsh environments and also for “polling” over geographic networks. They can also be used in low-power environments and might also have more specialized SCADA protocols like DNP3. All that said, many facilities management specialists I speak with swear by PLC’s, which are used extensively in water and wastewater SCADA. Additionally, many PLC’s are less expensive than RTU’s.