Moving from Rockwell to Siemens

bornwild

Member
Join Date
May 2010
Location
Riyadh
Posts
429
Hello all,

All my years I have worked with AB PLCs and now I am starting to do Siemens S71500/1200. Firstly, I am trying to understand the Data Blocks. In Rockwell, when you want to make a change in the tag, you just change and download. What is different in siemens is that it has start values actual values and snapshots. Can someone explain why do you need them?

When do I need to create a snapshot?- Is it when only when I make a change in the DB or whenever the plc will go from stop to run.

What if all the tags in the DB are retentive? What about the tags which are setpoint. ?

Sorry, I read a few answers but I am not sure about them.
 
Start values are the initial values the programmer has chosen. In certain situations, the actual values can be reset to the start values for example when there is a memory reset.
When programming FBs, when you create a new instance of an FB, the new instance-DB is always set to the start values.

Actual values are the values that the variable has at any time. Confusing is that you can also have an offline actual value. Usually the start value and the offline actual values are the same, but in some cases you want to distinguish between when the program starts with reasonably good values (the offline actual values), or when it starts after a memory reset (the start values). If you want to force the operator to check all machine parameters after a memory reset, then set the start values to something that is not usable without the operator inputting a realistic value.

A variable can have different start value, offline actual value and online actual value.

A snapshot copies the online actual values, to a separate offline snapshot memory.
You can then offline copy the snapshot to the start values and to the offline actual values. You usually do this when you want to save the current state before loading a program update. This is important with S7-1200/1500 because TIA controls what to download. TIA loads everything that it considers necessary to keep the program consistent. The programmer cannot decide what to download.
 

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