I want to know what is the perception in the minds of PLC users regrading if there is any difference between Redundant PLC system and Hot-StandBy System.
These are terms normally used in the Safety PLC industry
Redundant PLC
Usually completely separate signal paths from input to output with multiple processors running the same application and voting the result of
any action (SIFT) software implemented fault tolerance. Sometimes there will also be hardware voting on the outputs such a 6 element voter (HIFT) hardware implemented fault tolerance. Normally there are 3 signal paths and all data is voted 2oo3. This gives fault tolerance and high
availability (one third of the system may be lost) without loss of control.
Hot standby
Input is split into two paths, one path is active the other path (may) monitor the actions of the first, in the event of a failure the standby
(loss of watchdog or by diagnostics) the second path take over. There are many configurations 1oo2, 1oo2D (the D stands for diagnostics)
I don't think there is an industry standard definition of these terms, but rather several vendor specific definitions.
Personally I would consider hot-standby a subset of redundancy, where you synchronise two (or more if your really brave) CPU's on a common I/O network to achieve a bumpless transfer on failure. In practice this can be difficult because CPUs have some finite limit on the number of registers that can be synchronised.
Basically, redundancy is a general term describing the use of multiple elements to eliminate or reduce process malfunction in the event of system failure.
A "warm-standby" system (also a style of redundancy) might have two CPUs but not synchronised, so the transfer isnt bumpless. I have seen this before where hot-standby exceeded the sync capacity of the CPUs.