Different sensors, keys, switches and other elements that can change status of a joined bit at PLC input can be hooked up to the PLC controller inputs. In order to realize a change, we need a voltage source to incite an input. The simplest possible input would be a common key. As CPM1A PLC has a source of direct voltage of 24V, the same source can be used to incite input (problem with this source is its maximum current which it can give continually and which in our case amounts to 0.2A). Since inputs to a PLC are not big consumers (unlike some sensor where a stronger external supply must be used) it is possible to take advantage of the existing source of direct supply to incite all six keys.