RTU and CPU Used in SCADA and Benefits Using SCADA System

Remote terminal units

An RTU (sometimes referred to as a remote telemetry unit) as the title implies, is a standalone data acquisition and control unit, generally microprocessor based, which monitors and controls equipment at some remote location from the central station. Its primary task is to control and acquire data from process equipment at the remote location and to transfer this data back to a central station. It generally also has the facility for having its configuration and control programs dynamically downloaded from some central station. There is also a facility to be configured locally by some RTU programming unit. Although traditionally the RTU communicates back to some central station, it is also possible to communicate on a peer-to-peer basis with other RTUs. The RTU can also act as a relay station (sometimes referred to as a store and forward station) to another RTU, which may not be accessible from the central station.



Small sized RTUs generally have less than 10 to 20 analog and digital signals, medium sized RTUs have 100 digital and 30 to 40 analog inputs. RTUs, having a capacity greater than this can be classified as large.

A typical RTU configuration is shown in Figure below:



A short discussion follows on the individual hardware components. Typical RTU hardware modules include:
• Control processor and associated memory
• Analog inputs
• Analog outputs
• Counter inputs
• Digital inputs
• Digital outputs
• Communication interface(s)
• Power supply
• RTU rack and enclosure

Control processor (or CPU)

This is generally microprocessor based (16 or 32 bit) e.g. 68302 or 80386. Total memory capacity of 256 kByte (expandable to 4 Mbytes) broken into three types:
1 EPROM (or battery backed RAM) = 256 kByte
2 RAM = 640 kByte
3 Electrically erasable memory (flash or EEPROM) = 128 kByte

A mathematical processor is a useful addition for any complex mathematical calculations. This is sometimes referred to as a coprocessor.

Communication ports – typically two or three ports either RS-232/RS-422/RS-485 for:
• Interface to diagnostics terminal
• Interface to operator station
• Communications link to central site (e.g. by modem)

Diagnostic LEDs provided on the control unit ease troubleshooting and diagnosis of problems (such as CPU failure/failure of I/O module etc).

Another component, which is provided with varying levels of accuracy, is a real-time clock with full calendar (including leap year support). The clock should be updated even during power off periods. The real-time clock is useful for accurate time stamping of events.

A watchdog timer is also required to provide a check that the RTU program is regularly executing. The RTU program regularly resets the watchdog time. If this is not done within a certain time-out period the watchdog timer flags an error condition (and can reset the CPU).

Considerations and benefits of SCADA system

Typical considerations when putting a SCADA system together are:
• Overall control requirements
• Sequence logic
• Analog loop control
• Ratio and number of analog to digital points
• Speed of control and data acquisition
• Master/operator control stations
• Type of displays required
• Historical archiving requirements
• System consideration
• Reliability/availability
• Speed of communications/update time/system scan rates
• System redundancy
• Expansion capability
• Application software and modeling

Obviously, a SCADA system’s initial cost has to be justified. A few typical reasons for implementing a SCADA system are:
• Improved operation of the plant or process resulting in savings due to optimization of the system
• Increased productivity of the personnel
• Improved safety of the system due to better information and improved control
• Protection of the plant equipment
• Safeguarding the environment from a failure of the system
• Improved energy savings due to optimization of the plant
• Improved and quicker receipt of data so that clients can be invoiced more quickly and accurately
• Government regulations for safety and metering of gas (for royalties & tax etc)