Tutorial: A Level Application

Refer to: SCAN 02-0100

I took the liberty, for my own edification to to identify/individualize the two (2) relay contacts identified as "1000"

Identified them as 1000(A) and 1000(B)

1000(A) latches the relay to complete the circuit when Lo-Level contactor (0000) becomes FALSE.

1000(B) latches the motor (0500) to complete a circuit.


Question: What took place between scan (2) and scan (100) to cause 1000(A) to become true.

The text states that "AFTER" 100 scans the oil rises above the low level sensor and becomes open.

It seems to me that when sensor "0000" is triggered by the oil level that "0000" and 1000(A) would change states simultaneously.

Thanks again for your patience
Bernard

Hi Bernard,

Check the animation link below for a scan-by-scan look.
animation

Review each scan instruction by instruction and you'll soon see that the contact A is evaluated before the coil 1000 and contact B is evaluated after the coil 1000. Therefore, B always follows the coil 1000 on the same scan. 'A' will be a scan 'behind'. Make sense?

Since it's an internal relay its status is updated immediately.

Enjoy,

Bernard, think of each rung as a boolean equation. The program becomes:

1000 = (0000 OR 1000) AND 0001
0500 = 1000

At scan 1, when both 0000 and 0001 are both true, but 1000 is still false, the equation for the first rung evaluates as:

1000 = (0000 OR 1000) AND 0001
1000 = (True OR False) AND True = True

The value of True for 1000 is used in the solution of the equation for rung 2:

0500 = 1000
0500 = True

Hope this helps.

Tutorial: A Level Applicaion

Thanks for the information
Bernard