Hot Stnad by PLC

Dear Friends
What is the defenition of Hot Stand by PLC? Is it a hardware feature or Software feature? How we can test this feature if the manufaturer claims the system is hot stand by? Also what is a True hot stand by system?

Here's an article that might help: http://ecatalog.squared.com/pubs/Power%20Management/8000AC0503.pdf

The definition of "hot standby" is a bit like the definition of "real-time". It's up to the user to decide what is or isn't acceptable.

If you have a process where you can not afford to have downtime, then, yes, you should probably consider some form of standby system in case the main system fails. What is the maximum period of time you could survive without effective control? If you're doing high-speed positioning for example, even losing control for a few milliseconds may be enough to cause problems. On the other hand, level control in a reservoir could probably survive hours without major symptoms. Obviously it's physically impossible to repair a system within milliseconds, but it could equally be impossible to get an engineer to a remote unmanned pumping station within the required deadline, so both systems could justify having backups.

For the first example, "hot" standby, where you have two CPUs mirroring each others activity constantly and capable of switching control from the main to the standby possibly within the course of a scan, may be the only answer. That will probably have to be implemented using hardware links and high-speed memory transfer etc. For the latter, there may other less-expensive "warm" standby options involving a standby system which only wakes up when the master has already failed. This could involve a few seconds or more of downtime while process values are updated and synchronised. It may even result in a few bumps in the process while devices come under control of the new system. Can you accept this? It's up to the process.

Any manufacturer who claims to have a hot standby system should be capable of demonstrating what happens when any element of the main system fails. Note that in the examples above I have only considered backup CPUs - what about backup power supplies, what about backup or redundant I/O, what about backup networks, etc? How expensive is the damage when your system fails, and how much can you afford to spend to keep things running? What is the likelihood of a failure of any part of the system (MTBF etc) and what are the consequences if that failure occurs?

Sorry - no real answers, just more questions.

regards

Ken.

BIJU_K said:

Dear Friends
What is the defenition of Hot Stand by PLC? Is it a hardware feature or Software feature? How we can test this feature if the manufaturer claims the system is hot stand by? Also what is a True hot stand by system?

Click to expand...

The Hot Stand by PLC are hardware feature and Software feature.
See Modicon TSX Quantum PLC. The Primary PLC copy all important data in the Standby PLC on the each scan. The standby is ready to assume control within one scan if the primary fails.

Omron have a hot standby system with the CS1D series. The processors and power supplies are both redundant but the I/O is not. Both processors reead the same I/O. It is true hot standby as far as the processors and power supplies are concerned.

I am lead to believe that AB have a similar system.

Be carefull of Schneider - as far as I am aware they have warm standby systems. A friend of mine got caught with a cheap price and it turned out to be warm standby. It cost him a lot of money and nearly sent him broke. They may have a hot standby but be carefull.