c) Cards: Derivate elements and parallelism
As wires can be grouped, the relays are grouped in cards. Card is
a derivate element of relay which exploits parallelism processing
characteristics. The card has it physical counterpart similar as cards
in computers which are composed of different types of chips with the
same or different function and the external interface is the same for
every card. Figure 7 shows an example of a basic card where the
interface is common to every card (input, output) and the
functionality is added by choosing the relays included in the card.