Difference between FB and FC's

Hello Sadiq;

First, you cannot force a value in a DB with Step 7. See the attachment, taken from the Programming with Step 7 manual. You can modify it qwth a VAT table, for example.

https://support.industry.siemens.com/cs/fr/en/view/18652056

Next: I do not see how you want to control the state of your valve with an input.

You have a pneumatic valve, connected to an AS-i input signal. This input tell the AS-i master the status of the valve (open/closed). 

If you want to send a command to the valve, you should have an output to control it, and the adequate circuitry. 

Hope this helps,

Daniel Chartier

Hi

FB can have a memory (instance data block). FC can not remember what was in the past (previous scans) if you need to count variables in FC you need to use IN/OUT arguments which are linked to STAT/DB/Marker in calling block.

to make life easier (for me and service engineers too) I use following standard for naming FB/FC local variables:

I_XXX - FC/FB Inputs

Q_XXX - FC/FB Outputs

IQ_XXX - FC/FB In/Outs [they work like VAR variables in Pascal or By Reference in Basic]

Xxxx - Temp Variables (upper/lower case). see warning below

XXXX - STAT variables in FB [upper case]

TEMP: NEVER read a temp variable before write.

Example BTmp is a bool temp:

1.(CORRECT)

nw1 

                             BTmp

----||----|\|---.....----()

nw3

.....

BTmp

----||----...

2. (WRONG - NEVER DO IT)

NW1:

BTmp  <<<<<<<<<at this point value of BTmp is UNKNOWN (RANDOM)

----||-----------------...

Temp values are NOT STORED when program leaves the block.

on Temp you must not use (S), (R) or latch functions like this:

   ON               OFF               BTmp <<<<<<<<<<this code will NOT wirh wirh TEMP

----||--------------|\|---------------()<<<<<<<<<<<<<<<<<for this code you MUST use STAT

BTmp |

----||----|

Also in FCs outputs are treated in same way as Temps.

in FBs Inputs, Outputs, IQs and STATs are stored in Instance DB.