b) Control unit relay: Inheritance
The relay functionality is achieved by adding a control unit to
base relay. Most PLC programming software have not an inheritance
tool. So, to build a new relay that inherits the interface from the base
relay the process is as follows. Firs the base relay unit must be
copied and pasted. Secondly, the new object must be renamed.
Finally the code necessary to add a specific function is added. There
are two possibilities to add the specific functionality. On the one
hand by using a standard function Block defined by IEC 61131-3
such as timers (TON, TOF, TP), counters (CTU, CTD, CTUD), etc.
or other technological function available in the libraries offered by
the manufacturers. On the other hand creating its own function block
or adding the code to relay. Figure 6 shows an example of relay
which his specific function is the timer TON defined in the standard.