Does Siemens IW = PIW?

ljustesen

Member
Join Date
Aug 2003
Posts
2
In the Siemen's Step 7 world what is the difference between an Input Word (IW) and Peripheral Input Word (PIW)?

For example, if I right-click on an analog input module and 'Edit Symbolic Names' the symbols that show up are IW's (e.g., IW512). Yet, I can reference this same address as PIW512 within logic. Is there any difference? In this example it would appear IW512 = PIW512, but is this always true?

Thanks.
 
They are not the same.

Using I, IB, IW, Q, QB, QW, etc, you are reading information from the processor's image table, which is built just befor each OB1 scan, (just after for outputs).

Using PIB, PIW, PQB, PQW, you are reading directly from the module itself.
 
Thanks for the reply.

So to be sure I understand ...

Using my example, IW512 and PIW512 both get their data from the same place (i.e. a given analog input module channel), however, WHEN they are updated is different. IW512 gets updated prior to the OB1 scan and is static during the scan, whereas PIW512 would go get the current value from the card when its referenced (kinda like an AB PLC immediate input command) and so could be different than IW512.

Correct?
 
It is possible update peripherial to process image anywhere in program. There are not process image above IW255 automatically in all cases. Please correct if I'm wrong.

L PIW512
T IW512
or with MOVE-command

now you can ask some bit as I 513.0 ...
 
You dont have to use IW or QW (for S7-400)

Hi,

if you are using S7-400 AND it is one of the newer CPU's, then you can set up each analog card so that it automatically loads its values to IW or QW adresses. In this way you dont have to transfer the values yourself, or have to use PIW or PQW (these will expand the program scan time in order to process the adressed values).

The setup is done in "hardware configuration" for the module at "adresses ... Proces image". If "OB1" is specified, then the values will be available in your program as IWs and QWs.
I cant remember if you have to do it yourself, or if it is allready set as default.
 
S7 CPU 317-2DP problem

I have PLC S7 CPU 317-2DP and use Profibus DP (DP port master) with two slave:
- WM14-96.AVx.3.x.DG power meter (from Carlo Gavazzi) with input adrress IB272 to IB287, 16bytes
- Siemens SENTRON WL ACB with input address IB2237 to IB2280 and output address QB272 to QB273

In OB1, I call functions to update data from two above slaves, but when WM14-96.AVx.3.x.DG slave failure then PLC call OB86 to interrupt update data function of WM14-96.AVx.3.x.DG slave, but CPU still change from RUN to STOP and SF LED light, DP LED flash.

I disable update data function of SENTRON WL ACB in OB1 and when WM14-96.AVx.3.x.DG slave failure then PLC is RUN and only DP LED flash

I change output address of SENTRON WL ACB to QB2721 to QB2722 then CPU can not RUN, SF LED light

Please advise me to do

Thanks
 
Thanks for the reply.

So to be sure I understand ...

Using my example, IW512 and PIW512 both get their data from the same place (i.e. a given analog input module channel), however, WHEN they are updated is different. IW512 gets updated prior to the OB1 scan and is static during the scan, whereas PIW512 would go get the current value from the card when its referenced (kinda like an AB PLC immediate input command) and so could be different than IW512.

Correct?

Can someone confirm this is correct in that PIW interrupts the scan to read the input direct from the card before carrying on?
 

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