# Homework Help: How does the positive transition sensing contact work

1. Sep 14, 2014

### ongxom

1. The problem statement, all variables and given/known data
Draw the time diagram for this PLC ladder

2. Relevant equations

3. The attempt at a solution
I am trying to understand how does the positive transition sensing contact work.

Definition : The positive transition sensing contact has the value 1 if it receives value change from off to on in one scan.

According to the time diagram I can see that :
Scan 2 : PB1 raises from 0-> 1 (off to on) so PB1 = 1 for this scan only
Scan 3 : PB1 remains 1, no changes so PB1 = 0 for this scan only
Scan 4 : PB1 drops 1 to 0 so PB1 = 0 for this scan only
Scan 7 : PB1 raises from 0-> 1 so PB1 = 1 for this scan only
Scan 8 : PB1 no change (still on), PB1 = 0 for this scan only
Scan 9 : PB1 (on-> off), PB1 = 0 for this scan only
Scan 12 : PB1 (off-> on) PB1 =1 for this scan only
Scan 13 : PB1 remains on so PB1 =0 for this scan only
Scan 14 : PB1 (on-> off) so PB1 =0 for this scan only

Am I understand it correct ? I know how to draw the diagram, just confused in determining the correct values PB1.