S7 prgram scan

Hi all,
I would appreciate if someone out there can clarify when Flags etc are update during scans.
I understand/believe that in S7, at the start of the scan of OB1, the status of the I/P's are mapped. As program is executed the status of O/P's are mapped but not updated until the end of OB1's scan.
Assuming this is correct, when are the Flags (eg M0.0) updated? Is it at time of execution or at end of OB1 scan?
Also in edge detection (one shot) when a Flag is on for one scan, is the Flag on during the scan of execution or is it the next scan.
Any further clarity in this area will be greatly appreciated.

Steveojavascript:smilie('')
wink

programe is

1 Input Scan
2 Logic Scan
3 Outputs
4 house keeping

in input scan all I0:0 anf flags get scaned
based on logic output gets drived


Thanks

when are the Flags (eg M0.0) updated? Is it at time of execution or at end of OB1 scan?

Click to expand...

> at time of execution

Also in edge detection (one shot) when a Flag is on for one scan, is the Flag on during the scan of execution or is it the next scan.

Click to expand...

>during the scan where the status differs from the previous scan cycle.

Polle

The state of I, Q and F (and DBX) can change many times during the program scan.
When you test one of these addresses, it is evaluated with the state it has at the particular time.
In this example F0.0 is set ON and then OFF:

  F0.0         F0.0

--|/|----------(S)



  F0.0         F0.0

--||-----------(R)

Conversely, you can assign the state if Q, F and even I addresses many times during the same program scan. Yes, you can assign the logical state to I addresses.

For example:

  F0.0  I0.0   I0.1

--||----|/|----()

The special thing about I addresses is that their state (in memory) is written by the CPU before the program scan, based on the states of the hardware inputs.

The special thing about Q addresses is that the CPU reads their logical states (in memory) to turn on or off the hardware outputs after the main program scan.

(Quote
Also in edge detection (one shot) when a Flag is on for one scan, is the Flag on during the scan of execution or is it the next scan.

>during the scan where the status differs from the previous scan cycle.

Click to expand...

The flag will be ON in the scan after the position of the oneshot instruction. And it will stay ON until it meets the same oneshot instruction in the NEXT scan.

If the oneshot instruction isnt executed in every scan, then it can happen that the output of the oneshot instruction stays ON for even more scans. It would be considered a programming error in most cases, but it CAN happen.

Hi
Thanks to all replies.
Jesper et al, in the LAD below, which is a single Input toggling on/off an Output.
At rung 1,when I0.0 is on, M5.0 detects the edge and turns on M0.0 for one scan.
At rung 2, M0.0 is now on, Q0.0 is off, therefore Q0.0 is now on.
In the parallel branch Q0.0 is now on but M0.0 which is ON requires M0.0 to be in an off state.So how does the parallel branch latch Q0.0 on when it requires M0.0 to be off?
It might seem straight forward to you but it aint to me.

I0.0 M5.0 M0.0
---| |-----|P|---------------( )

M0.0 Q0.0 Q0.0
---| |-----|/|---|------------( )
|
Q0.0 M0.0 |
---| |----|/|----|

Steveo

So how does the parallel branch latch Q0.0 on when it requires M0.0 to be off?

Click to expand...

At the beginning of the Next scan Q0.0 will be true and M0.0 will be false (since it was a oneshot), it will latch the output on this scan.

So in simple terms, Q0.0 goes true on scan 1 and latches on scan 2.

Its a flip-flop.

Jesper, Ken
Much clearer now.
On the first scan when M0.0 is on, Q0.0 is switched on in the software. At end of first scan the outputs are updated, ie the physical output Q0.0 is switched on. On the next scan Q0.0 is on, M0.0 is now off, that path latches Q0.0 on until the next time M0.0 goes on again.

Cheers, Steveo

The real truth is input image, output image & flags are updated when executed in siemens the i/o table is the same as the memory bit table then the peripheral tables (true input/output) are updated outside the OB1 scan unless you force it with L PIW 0 T IB0
etc.