lompa said:
Can anyone explain why some plc's/outstations have isoloated I/O and some have non-isolated I/O's
What does the term isolated mean
and why are there two methods for PLC/RTU/Outstations?
I don't know anything about PLC's, but the terms are simple enough.
If devices are isolated, then there's no direct connection between them, the connection is made by some 'non-contact' method, such as optical isolation, or a transformer - or even a radio link!. Whichever method is used, the key point is that there's no direct connection.
To go back to a long recurring discussion, about earthing scopes!.
Assume you have an earthed scope, and you attempt to connect it to the chassis of a switch mode mains PSU - there will be a big BANG, because there's a direct connection between the scope ground (which is probably earthed - but not at my house!) and the live mains in the PSU.
To prevent this you can do one of three things:
1) Isolate the scope from ground by disconnecting the earth and using an isolation transformer (the scope probably already has a transformer anyway).
2) Isolate the PSU from the mains by adding an extra mains isolation transformer on it's input.
3) Isolate the connection between the scope and the PSU - this is the best in theory, but technically VERY difficult for a scope.
For your PLC's, option 3) will be the one the inputs use, the inputs are fairly basic (unlike a scope) and pretty simple to isolate, mostly using opto-couplers.