Hello all.

My colegue use this sequence for executing application (TIA portal s7-1200).

1. Precalculation block

2. Processing

3. Postcalculation

Postcalculation is reserved for reseting Commands datablock.

On HMI he use Set On function to set command (bit).

But I noticed that sometimes the command is not executed?

If the command arrived after Processing block, it will be reset in Postcalculation block.

It mean, that command don't be executed.

DB arrives from HMI in any time interval, not in start of the cycle? Right?

When I use Momentary bit function without Postcalculation block everything works good.

I think that with momentary button bit is set only for one PLC cycle? Right?

But now, I'm realy interested in this questions. 

Thanks in advance for help.

Hi!

I do not fully understand your problem, but maybe this FAQ might shed some light on it:

why-is-it-sporadically-not-possible-to-operate-the-cpu-via-the-panel-when-you-parameterize-the-hmi-tags-with-the-input-and-output-parameters

why-is-data-of-the-hmi-system-or-the-web-server-sometimes-overwritten-in-the-s7-1500

Milosh

But now, I'm realy interested in this questions. 

Thanks in advance for help.

Milosh,

Kahl has already posted links that will probably help you figure out what's going on. I would just add that things became tricky with optimized data access. You should be very careful about how you receive HMI commands (optimized/non-optimized approach with InOut Parameters, elementary or complex data types, ...), as mixing two paradigms (optimized/non-optimized as InOut) may lead to loss of data - it's all described in first Kahls link.

There is also a remedy proposal by Siemens in that link.

Hope this helps.

Regards.

Thank's a lot a both of you.

DB is absolute addresing.

I only worry about the interval when the commands received from HMI, SCADA or whatever?

PLC wait the start of cycle or it can be any moment during program execution (to receive commands)???

Milosh

....

PLC wait the start of cycle or it can be any moment during program execution (to receive commands)???

S7-1200 System manual

6.4 Understanding data consistency

....

Check out also the picture from the second link .