The process image is the number of I/O that are automatically transferred from the physical I/O to and from the memory of the PLC.
Process image inputs are normally read from the physical inputs before the code scan, and process image outputs are normally written to the physical outputs after the code scan.
In an S7 the process image inputs are called Ix.x and the Qx.x
Typically the process image are the bytes from 0 to 255. On some CPUs the process image has a different size, and on some CPUs you can specify the number of bytes.
When the PLC code reads an input (i.e. A I3.0), then the status of the input adress used is not the currently real status, but the status when the process image was read before the scan.
When the PLC code writes an output (i.e. = Q2.0) then the actual update of the output happens after the code scan.
Peripherial I/O is the complete I/O table, including the I/O that is inside the process image as well as the I/O that is outside the process image.
To access the peripherial I/O, you add a "P" before the address. But you have to address at least a byte, i.e:
L PIB 24 // reads input byte 24 from the peripherial inputs.
...
T PQW 300 // writes output bytes 300 and 301 to the peripherial outputs.
When the PLC code reads a PI then the physical input address is immediately read.
When the PLC code writes a PQ then the physical output address is immediately written.
edit: added a little bit of info above.
