First of all, what is SCADA and PLC? A SCADA (Supervisory Control and Data Acquisition) system is a type of distributed control system that consists of a central station and remote sensors. The central station relies on the sensors to gather and report telemetry data. These sensors can be Remote Terminal Units (RTUs) or Programmable Logic Controllers (PLCs). The RTU or PLC collects information locally and then passes it on to the central station in real time. Once the central station has the remote information it can send out notifications, generate alarms, or initiate automated responses.
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.
So how would SCADA interface with PLC or RTU? 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.
A PLC usually has relatively limited input/output capabilities. In most cases this is nothing more than 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
Interfacing PLC with SCADA brings its own set of challenges. It is not uncommon for a PLC based SCADA systems 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. If you are planning a PLC based SCADA system, you should expect to spend a considerable amount of time configuring your units.
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.
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.
Choosing a PLC or RTU
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. However you should be cautious, some PLC units from major manufacturers are marketed as RTUs largely because of their input/output capabilities. However, 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:
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.
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