Remote Automation and Monitoring: PLC or RTU

Key Points

PLCs and RTUs are similar devices with somewhat differing functionality.

Key Points

PLCs are best suited for wired applications and high speed networks while RTUs are

most effective in wireless applications and operate well on both high and low speed

networks.

Key Points

The major differences between PLCs and RTUs are in remote communications and

data handling capabilities.

Key Points

Most RTUs provide significant data logging capabilities so that data is retained during communications outages or for reporting

purposes.

Key Points

RTUs offer sophisticated RTU to SDADA/DCS, RTU to IED and RTU to RTU communications:

ı Multiple SCADA protocols, proprietary and standard.ı Multiple IED protocols for instrumentation, field

controllers, meters, sensor networks, analyzers, etc.

ı Advanced peer to peer communications with store and forward as well as dynamic message routing capability.

Communications examples

Standard polling The master station continuously requests the real-time data values.

Exception reporting (RBE) The RTU is polled but only reports values that have changed since

the prior poll

Push Communications The RTU initiates messages on and event or time basis.

Peer to peer communicationsRTUs can communicate with the master station and also with each other (peer to peer) if there is a communications path.·

Communications examples

Store and forward communications

Messages can be passed on by one RTU to another for further process or forward transmission. This type of communication can be used to extend the coverage of point to multipoint style networks and also allows linear communications paths such as along distribution or collection networks

Communications

Dynamic message routing

RTUs are able to ‘learn’ of changes to network routing and dynamically change message routes. For example if primary communications are radio via communications port 1 and messages have been failing, the master station may use an alternative communications path (via an intermediary RTU or different network connection). If a new message from the master station is received on backup PSTN communications port 2 the RTU will change its routing information to reflect that master station communications are now via port number two. Some RTUs can also re-direct messages via alternative message paths and ports based on message success/failure statistics or by RTU configuration.

Key Points

Most RTUs provide fully integrated communications hardware in the form of dedicated communications modules (with options for communications redundancy)

or integral modems.

Integrated Communications

Integrated communications examples: Kingfisher

MC30 comms module with option field swappable option

cards

Integrated Communications

Integrated communications examples: TBox MS

PSTN Modem

GSM Modem

GPS Receiver

Integrated Communications

Integrated communications examples: TBox LT

Integral Modem

GSM

Spread Spectrum RadioPSTN

Full Function RS232

Integrated Communications Comparison

Rockwell MicroLogix Semaphore TBox

OR

Key Points

Low Power Operation

RTUs are available in low and ultralow power configurations that operate on batteries or solar power and extend the range of remote control and monitoring to sites without any infrastructure. Generally, PLCs are not low power devices.

Summary

Use a PLC in applications: In plant Remote with:

ı Wired Ethernet WAN ı Utility power or large solar

panelsı Serial communicationsı No legacy protocolsı Integral communications

equipment not required

Use an RTU in applications: Remote with:

ı No infrastructureı Multiple IED interfacesı Legacy protocol supportı Data logging or reportingı Complex communicationsı Multi media communicationsı Antenna height issuesı RBE or Push

communicationsı Integral packaging

When RTUs Should Be Used In Place of PLCsGray Matter Users Group

Day StephensCSE Semaphore Inc.

August 19, 2010