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: ıMultiple SCADA protocols, proprietary and standard. ıMultiple IED protocols for instrumentation, field controllers, meters, sensor networks, analyzers, etc. ıAdvanced 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 communications RTUs 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 LT Integral Modem GSM Spread Spectrum Radio PSTN Full Function RS232
Integrated Communications Comparison Rockwell MicroLogixSemaphore 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: ıWired Ethernet WAN ıUtility power or large solar panels ıSerial communications ıNo legacy protocols ıIntegral communications equipment not required Use an RTU in applications: Remote with: ıNo infrastructure ıMultiple IED interfaces ıLegacy protocol support ıData logging or reporting ıComplex communications ıMulti media communications ıAntenna height issues ıRBE or Push communications ıIntegral packaging
When RTUs Should Be Used In Place of PLCs Gray Matter Users Group Day Stephens CSE Semaphore Inc. August 19, 2010