These pages will give you a basic understanding of the workings of a PLC and some of the numbering systems used.
A PLC operates by continually scanning the program and acting upon the instructions, one at a time, to switch on or off the various outputs. In order to do this the PLC first looks at all the inputs and remembers their states ( i.e.. 1 or 0 / on or off) this information is then stored in memory. The PLC then scans the program instructions and decides if an output should be on or off BUT, and this is important to remember, the physical output is NOT turned on immediately. When the PLC has finished scanning the programmed instructions it will, finally, turn on the required outputs. This is called updating the process output image.
At this point the PLC then checks its own operating system and if everything is ok it then goes and checks the states of the inputs and starts all over again.
This is program scanning and the delay is called the program scan time.
From the above statement it should be seen that there is a delay from when an input is turned on; the program scanned and then a physical output being turned on or off. As the delay is only mS it is not usually a problem unless you are using the PLC in a high speed process. However it should be remembered that in a PLC with many thousand I/O an input could turn on and then off before the PLC has finished scanning the programmed instructions.
This could result in an output coming on for one or two scans when in fact it should be off.