It is best to keep it seperated from the actual input real world
operation and use ONLY a POSITIVE or NEGITIVE logic concept for all.
Sink or source does not matter and in the case of POS / NEG DC inputs
Myself I use POSITIVE, if the discrete input is ASSERTED ( active, if
you like ) the open ladder contact is closed to give power flow left to
right through the contact.
In the case of say a standard N.C. Emergancy STOP button, the normally asserted input is thus called "NOT" STOP:
STOP or /STOP and the logic scheme is positive.
In most cases this would be used as a N.C. contact in the ladder and would open when the N.C. STOP button is pressed.
In the case of a standard N.O. momentary Start buton, the input is asserted when the button is pushed and thus named:
START and the logic scheme remains positive.
In most cases this would be used as a N.O. contact in the ladder and would close when the START button is pressed.