How Ladder is Scanned question

[BigDB]

Could somebody please shed some light one the example "How a Ladder is Scanned" I don't understand why after scan 1 the contact referenced to the tag 1000 is false while the one directly beneath it is true.

 

[Mickey]

What brand of PLC do you have? Some do "left to right top to bottom"
"some do top to bottom left to right"

 

[Steve Bailey]

In scan 1 when the PLC evaluated the first rung, the conditions were:
0000 = True
0001 = True
1000 = False

As a result of evaluating the first rung, 1000 becomes true,

so the conditions prior to evaluating the second rung are:
1000 = True

As a result of evaluationg the second rung, 0500 becomes True.

 

[curlyandshemp]

Could somebody please shed some light one the example "How a Ladder is Scanned" I don't understand why after scan 1 the contact referenced to the tag 1000 is false while the one directly beneath it is true.

In a classic PLC, the PLC's scan is predictable. Input/Output images updated, PLC code scanned, CPU overhead stuff, then the cycle repeats.

In the Contrologix world, I/O updates and code scan can occur at the same time. Clgx has special instructions to capture Inputs , to ensure that an input that is ON for one rung, will also be ON for the the rungs below.

In the Modicon world, ladder is scanned from left most column top down, ending up at bottom right.

Which type of PLC are you referring to?

Ian

 

[Steve Bailey]

I think BigDB is referring to the example in Phil's tutorial. At least that's what my answer refers to.

http://www.plcs.net/chapters/scan13.htm

 

[parky]

The simplest way (as most PLC's work) is at boot, all checks are made etc... then on start of scan I/0 is checked for validity.

Input images are refreshed, then the main ladder is scanned & processed & at the end of cycle the outputs are refreshed, it then repeats the cycle.

However there are some events like interrupts (i/o or timed) that will stop scanning the "ladder", process a subroutine then return to the "Ladder" to continue.
Also jumps to functions or subroutines will have an effect on the scan of ladder.

 

[BigDB]

Thanks for the help and feedback