Input read and output write scan

During the input/output (I/O) scan, the processor updates its internal input and output buffers with data being read from or written to I/O devices. Local I/O devices are the input and output cardsresiding in the same physical chassis as the PLC processor. Remote I/Odevices reside external to this chassis and are communicated with the processor's peer communications interface port.

I/O data for input and output cards used in the application are maintained in input and output image tables. Typically the PLC will organize the I/O image tables. This means that the inputs which are present will read into an area in memory. The program will write into another area of memory which is used to represent the outputs. It can be said that the input image table is representative of 'how the inputs are perceived', and the output image table is 'the desired state' of the outputs. These tables are accessible to the Ladder Logic program as data files. During the I/O scan, data read from input cards are placed in appropriate locations in the input image table. At the same time, output data written to the output image table by the Ladder Logic are transferred to the appropriate output cards.