SCADA RTU Basics

SCADA RTU

An SCADA RTU (Remote Terminal Unit) is A stand-alone data acquisition and control unit. A small computer that provides intelligence in the field, and allows the central SCADA master to communicate with the field instruments. An individual device, or a collection of devices, that are installed in a remote site or location which monitors, controls and provides feedback of field devices. The RTU exchanges data between itself and a master unit or database via a wired or wireless (radio, satellite, microwave) communication link. The data being monitored can be analogue and/or digital depending on the field device/s in operation. Some industries that utilize RTU’s include mining, gas & oil, water treatment, agriculture and environmental.

Main functions Of RTU:

  • Control process equipment at the remote site
  • Acquire data from the process
  • Transfer the data back to the central SCADA system

There are two basic types of RTU

Single board RTU: The “single board RTU” which is compact, and contains all I/O on a single board. A  typical single board RTU normally has fixed I/O

e. g. 16 digital inputs, 8 digital outputs, 8 analogue inputs, and say 4 analogue outputs.

  • It is normally not possible to expand its capability

Modular RTU : The “Modular RTU” has a separate CPU module, and can have other modules added, normally by plugging into a common “backplane” (a bit like a PC motherboard and plug in peripheral cards).

The modular RTU is designed to be expanded by adding additional modules. Typical modules may be a 8 analog in module, a 8 digital out module. Some specialized modules such as a GPS time stamp module may be available.

RTU  Hardware features:

  • CPU and volatile memory.
  • Non volatile memory for storing programs and data.
  • Communications capability either through serial port(s) or sometimes with an on board modem.
  • Secure Power supply (with battery backup).
  • Watchdog timer (to ensure the RTU restarts if something fails).
  • Electrical protection against “spikes”.
  • I/O interfaces to DI/DO/AI/AO’s.
  • Real time clock

RTU Software Features:

  • Real time operating system. This may be a specific RTOS, or it may be code that started out life as one big loop scanning the inputs, and monitoring the communications ports.
  • Driver for the communications system i.e. the link to the SCADA Master.
  • Device drivers for the I/O system i.e. to the field devices.
  • SCADA application e. g. scanning of inputs, processing and storing of data, responding to requests from the SCADA master over the communications network.
  • Some method to allow the user applications to be configured in the RTU. This may be simple parameter setting, enabling or disabling specific I/O’s or it may represent a complete user-programming environment.

Diagnostics.

  • Some RTU’s may have a file system with support for file downloads. This supports user programs, and configuration files.

Types of RTU’s are as follows:

Tiny stand-alone systems that run off batteries for an entire year or more. These systems log data into EPROM or FLASH ROM and download data when physically accessed by an operator. Often these systems use single chip processors with minimal memory and might not be able to handle a sophisticated communications protocol.

Small stand-alone systems that can power up periodically and apply power to sensors (or radios) to measure and/or report. Usually run off batteries that are maintained by solar energy. The batteries are large enough to maintain operation for at least 4 months during the darkness of the winter in the far northern hemisphere. These systems generally have enough capability for a much more complex communications scheme.

Medium systems. Dedicated single board industrial computers, including IBM-PC or compatible computers either in desk-top enclosures or industrial configurations such as VME, MultiBus, STD bus, PC104 etc. 

Large systems. Complete Plant control with all the bells and whistles. These are usually in Distributed Control systems in Plants, etc and often communicate over high speed LANS. Timing may be very critical.

An RTU is a microprocessor controlled device which, in order to function as desired, must be programmed with some sort of software code. The code is just a defined set of instructions that the RTU understands and executes as required. Any error or ‘bug’ in the code may cause the RTU, along with any device it communicates with, to malfunction. Prior to dispatch, RTUs are programmed using an external device such as a PC or laptop and generally require very little maintenance over their expected life.

In order for the RTU to receive and implement the correct set of instructions, the programmer must have knowledge of both the software requirements and hardware elements including the field and telemetry devices. RTUs can have their software parameters modified via the telemetry link; however this may affect some, or all, of the respective field devices, so it’s imperative that any upgrades are managed correctly.

Some common network connections, communication protocols and field devices used with RTUs include

  • SCADA
  • PLC
  • Valves
  • Pumps
  • Motors
  • HVAC
  • ModBus
  • CIP
  • RS232/422/485
  • HART
  • Ethernet/IP
  • 4-20mA

To ensure the best RTU performance is achieved, especially when considering the remote nature of the installations and the cost associated with potential malfunctions, it is important that RTU programming is done by trained, qualified and experienced RTU engineers.