only why you need a redundant processor?
make a redundant PLC cost is three times of one.
and yes i can program them.
"Redundant Processor" is one of those terms that gives people a warm, fuzzy feeling. It sounds nice and safe and reliable, right? Until they think the system through, that is.
In my experience the causes of failure in control systens, in order of frequency, are:
1) Programming error. Redundancy does nothing to improve this, and actually makes it worse by adding complexity.
2) Wiring issue. No help here from redundant processors.
3) Field deivice fault - transmitter or actuator fault, often as a result of installation error. No improvement in reliability here, either, from redundant processors.
4) I/O module fault. To help here you would need a whole redundant PLC, which can be done. But then you need to make sure the module knows its failed so you can switch over. And pay for it, of course!
5) CPU fault. Not common, and when it occurs is usually the result of power surges etc. So, how do you know that
both CPUs won't be fried?
When aerospace went to fly by wire I believe they used redundant controllers. I also think they went with three units, not two, so there could be a "voting" process to decide which one was brain dead. Lots of $$$$$$.