Difference Between IED and PLC

Huh, never heard the term IED used in that context. Here in the US, and likely other English speaking countries with armed forces fighting terrorism in the Middle East, IED means Improvised Explosive Device, the things the terrorists use to kill people unawares by connecting cell phones to mortar shells for example. Not the kind of thing you want in your electrical equipment.

I, too, had never encountered the IED term, as defined by Wikipedia, before.

An IED is likely to include special dedicated functions for whatever application the IED is intended.

An instrumentation analogy might be a 'dumb' pressure transmitter versus a 'smart' pressure transmitter. The dumb unit is a fixed range, analog output, no digital comm for diagnostics or configuration. The smart unit can be ranged over a 50:1 span, identified with a tag, has diagnostics and can be configured via a digital comm protocol.

Problems with substituting a PLC for an IED

- are all the application specific features are even known to the PLC programmer? Can a sequence logic device perform the needed IED features? For example, attempting to execute a Fast Fourier Transform in a PLC is a serious Kludge, whereas a dedicated unit can be designed for a real-time FFT.

- I/O and sensors needed to supply the device data to the PLC - can the I/O be packaged into the device?

The PLC is an outside stranger, not part of the IED product itself and mimicing the IED's feature with I/O and a PLC is likely to be far more costly than an IED that is an integrated package.