Supervisory Control And Data Acquisition Remote Telemetry Unit; Why You May Want More Than A Programmable Logic Controller.

A SCADA (Supervisory Control and Data Acquisition) system, at the machine level, consists of a central station for gathering data and managing the overall operation. It also has sensors (these could be RTUs, or PLCs) placed near to where the action is. The RTU or the PLC collects the information locally and then passes it on to the central station. This can be located several miles away.

RTUs and PLCs today are capable of controlling the actions within their range of vision through closed loop feedback systems. The central station oversees the overall performance of the one or more RTUs/ PLCs under its control. SCADA systems also allow staff or supervisors to change the settings as appropriate at the level of the RTU/PLC or the central station. Alarming conditions like high temperature can then be stored and displayed.

First, let's review Remote Telemetry Units (RTUs) and the sometimes less expensive alternative, Programmable Logic Controllers (PLCs). Both devices collect data and send it back to a central master. While their primary purpose might be the same, there are differences with respect to their design, installation, and operation.

Capacity
A PLC has relatively small input/output capabilities; a couple of digital/analog inputs, a similar number of open-collector/relay outputs and sometimes a single interface shared for configuration. A RTU has more capacity with 30+ inputs, several outputs and a lot of communication interfaces.

Ease of Use
It is not uncommon for a PLC to require external programming or script writing in order to perform its monitoring role. While this can often be done with an included GUI application, the point is it must still be done. More than likely by someone with a lot of programming knowledge. RTUs, on the other hand, come pre-set for deployment into a SCADA system. A RTU will often include an embedded configuration web application that requires nothing more than a standard browser connection for setup. A RTU can also be accessed for configuration even while it is actively monitoring.

Space Requirements
Just because an RTU has a lot of capability compared with a PLC, that doesn't mean you'll need acres of space to install one. In fact, many medium SCADA RTUs are only one rack unit (1.75" or 4.4cm) tall, and less than a full rack width across (commonly about 10" or 25cm). If you don't have rack space nearby, ask the manufacturer about wall-mounting ears that will allow you to mount the RTU on any flat surface or wall.

RTU in SCADA Application Drawing
RTUs are a great choice for monitoring in SCADA apps.

So, which one should you get for your SCADA needs?
When it comes to your SCADA system, you want something that is reliable and is going to have all the capabilities and functions you need. An RTU is the superior choice because they are more robust and better suited to the high demands of a SCADA system. But, be forewarned! To confuse system designers, some PLC units from major manufacturers are marketed as RTUs largely because of their input/output capabilities. But they have the same awkward external programming required in order to perform basic monitoring and control functions. Be sure you know what you are purchasing and that the system covers all of your needs. Here are a few things to look for in a good SCADA RTU:

  • Good RTUs for SCADA contain a lot of digital inputs, a lot of analog inputs, at least a few control relay outputs, and two data reporting channels (the second is used as a backup if the first fails)
  • For most SCADA uses, a RTU that's housed in a durable metal case is far better than a less durable device housed in a plastic case. Powder-coated for durability and electrical insulation is even better.
  • You should also choose an RTU that accepts the input power you have. Based on your industry and region, this could be 110vAC, 220vAC, -48vDC, +24vDC or some other voltage.

With its many inputs, an RTU in SCADA contexts will use its LAN or serial data reporting channel(s) to send alarms to your central console or HMI (Human-Machine Interface). This makes the single RTU's data part of your management system. If you or your HMI determine that a change must be made to keep your process running smoothly, you can send a command through your HMI and back to your RTU. The command will then be done by the control relay outputs. Just about any piece of gear, button, or switch can be manipulated by wiring it into a standard control relay output.

Don't compromise when it comes to your SCADA system. Get an RTU that is reliable and suited to your specific needs.

Related Topics:
PLC vs RTU: When does a PLC makes sense?
RTU vs PLC: Your Industry can be part of the Choice.

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