Johnnie
These OB's are blank so that you may program them the way you want them. For instance, if OB80 is called by the PLC you might program a 'move' to 'pull' out the error code, then depending on what the error code is you could then either restart the PLC, put it into stop mode, bring up an alarm on a HMI etc.
These OB's are called by the PLC when it faults and if the OB is not there the PLC goes into stop mode. If you don't program these OB's then you will not know if the PLC is faulting, as it will call the OB, process it, even with nothing in it, and then carry on,
That's my understanding of these OB's, it might not be totally correct, perhaps someone could confirm this, maybe Jean Pierre could.
Paul