Data transfer is the method used by a PLC to move data from one point to another. This can include transferring data from one memory location to another memory location or groups of memory locations, or it can also include the procedure required to move data either to or from an analog I/O module. Data from a single memory location, or register, may be transferred, or data from a group of locations, or table, may be transferred. Some PLC's refer to the transfer of more than one register as a block transfer.

Generally, moving data around within the same device is not too problematic, it’s really just a question of understanding good techniques and good practice. It’s moving data between devices that can be seriously problematic, because there are variables that you can’t control. PLC to PLC communications, comms between PLC and intelligent field devices such as drives, and perhaps most problematic of all, comms between PLCs and the wonderful wider world of IT is where the challenge of keeping things robust and reliable is much greater. Still a large part of it is keeping the exchange well controlled and logically organised when the link is present, but there is also the matter of how best to handle it when the link goes astray. Many a PLC programmer’s grey hairs may be directly attributed to data transfer!

Further more u can refer this link...

http://www.maplesystems.com/Allen-Bradley/Data_Transfer_-_PLC_Registers_to_HMI.htm

Actually, the PLC = Programmable Logic Controller, is a part of a distributed control equipment, and provides a local contro. Its job is to get information from the sensors, to do a local control of sub-system and to report and get dates to/from central dispatcher.

The sensors signals could be analogue, digital or digital communication. For digital communication between sensors (smart transmitters) and PLC there are specified many low rate industrial protocols: CANbus, HART, Foundation Fieldbus, Profibus DP, MODBUS, etc.

For communication with central dispatcher, there are used high speed protocols, many based on TCP/IP standard.

In order to understand data transfer between PLC and other entity you have to understand the PLC communication protocol.

Relay concept is the basic for all programmable logic controllers. The difference is relay uses the analog signals but the PLC uses the digital signals. These digital signals can be given from a computer through the simulation.

So, a program can be written to the controllers and depending upon the inputs it will give the output.

To Bharani Vasan

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The analog data available in the analog module is required to be transferred to CPU register so that the processing can be carried out fast.  Allocate specific registers to the analog inputs and then move the block of data.