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I/O Mirroring vs RTU Polling in Wireless Automation Systems


The following techniques:

  • I/O Mirroring
  • RTU Polling

are used in wireless automation systems.

“I/O Mirroring” aka “I/O Telemetry” is the technology where Analog Signals (4-20ma) and Digital (Contacts) Process Signals are replicated from a transmitter to receiver end by wired or wireless means.  RTU Polling is technique where a master station polls remote stations via a protocol such as Modbus.

I/O Mirroring is best suited for applications when there is a small number I/O points to replicate.  I/O Mirroring is used to replace a broken or failing wire in existing or legacy systems; or as an alternatively to cable. I/O Mirroring is used when there is no facility in the PLC or RTU for polling. There may not even be a RTU or PLC in a hardwired control system.  In this case, I/O Mirror is the only option.

I/O Mirroring is best deployed when the operation or company has limited internal PLC programming support staff. For example in towns or villages with small water systems, it is often better to use I/O Mirroring techniques.

RTU Polling is used when the application or operation has many sites, as in large Oil and Gas SCADA systems, or large water systems and they have Automation Staff familiar with PLC / SCADA Programming.  RTU Polling allows the minimization of I/O hardware and wiring at the master site.

With both techniques, appropriate error handling is to be understood for communications failure and for system startup. With I/O Mirroring one must look at use of I/O Fail States, or the use of a Communications Fail Signal. Similarly in use of RTU Polling approach the PLC programming logic must appropriately handle communications failures,  retries, and setting default register values in event of communication failure.

SCADALink RIO900 supports both techniques.