Can someone tell me what is the meaning of Redundancy in I/O Cards of PLC System.

PLC IO redundancy is a common property of safety PLCs in which each IO has a redondant signal processing.

For example, several SIL3 PLCs use a Triple Modular Redundant (TMR) architecture with a 2oo3 voting on each IO. You can also find Double archirtecture with 1oo2D voting or QMR architecture with 2oo4 voting.

You will find examples of such products if you look at manufacturers like Triconex, ICS triplex and HIMA.

You can get I/O redundancy in non-safety system PLCs also like those from Siemens or Allen Bradley. I/O Redundancy can be implemented in different ways by different vendors. For the user it means that even if any one channel or one entire I/O card fails, the loop will not fail because it will automatically get connected to the redundant I/O. In the absence of this feature, if an Input card fails, you may lose a measurement signal and thus go blind completely until you replace the card. It becomes worse if it is a closed loop since there could be a possibility of loss of control.
I could of course go on and on but I guess by now you get the general idea!

I have also been involved with PLCs designed with a second rack (with I/O cards that mimic the other rack (cards in the same slot position), then the individual loops are marshalled together to both I/O cards via a terminal strip. If you lose an I/O card or a point, the second (redundant) is still receiving the signal.

Dear Lionel, Mandar and Marvin

Thanks a lot for your inputs, I am planning to use Omron PLC CS1D, In this system, there is 16 DI & 16 DO, client has asked redundancy in CPU, I/Os....
For I/Os I am planning to use two nos of 16 DI/16DO Modules, Say, for a 1 DI from field it will be connected in two DI of two Modules. Is this correct.

Thanks a ton!

Hello Shiv,
The IO Redundancy is basically you get 2 (redundant) signals to of same signal type fed to the controllers. In case of a failure of a IO card in one chain , you still have the redundant one. But do keep remember, It all depends on the how you vote the signals, and how healthy are the signals that are fed to the controller.
I meant here is, Say suppose, there is a Logic One fed to the One chain due to IO card Failure and you really have a Logic Zero at the redundant chain, then you need to decide through your proper voting logic you implement and reading the healthiness of the cards.
In case of the TMR , the voting logic is still complex. You also have Hardware voting and Software voting to decide in selecting your Redundant IOs.
Hope i made few basics clear to you.
Thanks.!

Dear Sir, today i was talking with local scmercible eastern region engineer. he told me their safety plc have double channel I/O, which is a basic property of safety PLC did you worked on it.