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A PLC is a small industrial computer which originally replaced relay logic. It had inputs and outputs similar to those an RTU has. It contained a program which executed a loop , scanning the inputs and taking actions based on these inputs. Originally the PLC had no communication capability, but they began to be used in situations where communications was a desirable feature. So communications modules were developed for PLC;s supporting Ethernet (for use in distributed controlled systems) and the modbus communications protocols for use over dedicated (wire) link. As time goes on we see PLC’s support more sophisticated communications protocols.

RTU’s have always been used in situations where the communications are more difficult , and the RTU’s strength was its ability to handle difficult communications. RTU’s originally had poor programmability in comparison to PLC. As time has gone on , the programmability of the RTU has increased.

We are seeing the merging of rtu’s and PLC’s, but it will be a long time (if ever) before the distinction disappears.

What should I specify for my RTU’s

Temperature ratings for your application eg -10 to 65 deg cem.

Relative humidity 0 to 95% noncondensing.

Dust, vibration, rain, Salt and fog protection.

Electrical noise immunity.

Physical size – make sure it will fit in your site.

Power consumption.

An industry has grown up developing protocol converters and emulators. Recently some standards have begun to emerge for RTU’s Some Standards are IEC1131-3 For programming RTU’s.