Do you have a link on the web to share?
The only EDDI I could find in 3 minutes of searching was for "Error Detection by Duplicated Instructions", which has duplicate programs running concurrently and comparing the results to the core program, to verify that no bits are accidently flipped during the program execution due to bad memory locations.
Not suitable for PLCs (although it makes you wonder just how immune to corruption the little beasties are. I take it as gospel that a -( )- instruction sets a memory location to the value I expect it to be when I do a -| |-. Or, for that matter, when the PLC is executiong its program, that it doesn't accidently read -|/|- for one scan when I wrote -| |-.
But you've got to believe in something.... I believe I'll have another drink... (-Groucho Marx)