Tuesday, October 27, 2015

NO and NC concepts

NO and NC concepts

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 maybe mixed.

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.