Difference bw Redundancy & HotStandby


Thread Starter

Asif Khokher

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.
My understanding of the difference is this:

Redundant PLC - two complete PLC systems including cpu and I/O for each scanning the same devices independently

Hot-Standy - two cpus sharing the same distributed I/O and the cpu scan clocks are
synchronized with one designated primary and one secondary

Just my 2 cents. Not sure if this is industry standard viewpoint.

Dennis Wright

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)

Look at the ABB website where both technologies are described.

Hope this helps

Dennis Wright
Technical Product Management
ABB Industri (Crawley)

Daniel Grice

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.
I see redundant PLC's as running the same program with neither being primary or secondary and providing a "bumpless" changeover when one fails.

Hot-Standby is where the primary PLC sends data to the secondary PLC which will "take-over" if the primary fails.
I suggest the IEEE Std 100-1992 "The New IEEE Standard Dictionary of Electrical and Electronics Terms" for the definition of Redundancy and Standby

BTW, if you don't have access to the std, let me know, and I'll send you the definitions!

Phil Corso, PE
(Boca Raton, FL)