PLC 

Introduction to PLC

PLC or Programmable Logic Controller was an intelligent system of modules which were introduced in the Control & instrumentation industry for replacing relay based logic. Exact history is not known to me but this is an accepted belief.
Over a period of time, better I/O handling capabilities and more programming elements have been added along with improvements in communication and thus this product (PLC) is capable of doing almost anything in I&C.

There are 5 or 6 languages used
Ladder Logic.
Statement List.
Sequential function charts (SFC).
Instruction list (IL)
Function Block Diagram (FBD)
3GL and 4GL languages are also supported by many.

Soft PLC is a product. Soft PLC if you mean Software PLC is a PLC implementation on a PC. The only difference here should be the use of commercial PC's instead of prop. hardware designs. The second design difference (on theory) should be that in a Software PLC the OS is commercial and there is a user program, while PLC has an embedded firmware. So any SOFTWARE PLC which embeds its software into the PLC and it becomes a firmware should then become a PLC.
PLC has proven itself in the market in very critical applications. Please note that here critical does not mean the same critical as in PC systems and servers. PC automation is still in its infancy stage and reliability is an issue. This has been overcome by many using Linux, raid disks etc. and we hope that it will pick up as the lead product very soon. PC automation is already established in several machine automation and in non critical data acquisition systems.

Backup Concepts of Processors.

The objective of a redundant system is to improve the amount of up-time of a machine or process by ensuring consistent availability of that machine, and by reducing costs associated with equipment failure. Bu using the backup system, you can guard your application against shutdowns caused by the programmable controller or processor.

Attention : Backup does not protect you from faults caused by programming errors or system timeouts because such an error or timeout will occur in the secondary processor.

The backup option is used where you must transfer the control of the process to a secondary system, without thereby interrupting the machine/process operation.

To guard against system shutdown, a backup system must provide: