Analog channel addressing in STEP 7 TIA

Jasner

Now, I see that addressing on analog input modules in TIA is IW, but I wonder whether the same integer range is used like in STEP 7.

Actually, my question is how to process 4-20 mA input signal from the analog card in TIA? Let's say the input is IW272.

 The same range. Search here or in the TIA help for NORM_X/SCALE_X functions.

Hi Jasner,

In general, the scaling of analog values has not changed between classic STEP 7 and STEP 7 in TIA Portal. There are some analog modules that allow you to change the scaling in the hardware configuration, but the default setting is always "S7 style".

You can still use SCALE/UNSCALE instructions, but if you are using an S7-1200/1500, it is recommended to use NORM_X/SCALE_X (see FAQ). Or you can use a more flexible version available from the support site.

Kind regards
Stefan

Jasner

Thank you for the explanations.

Can you also explain, why STEP 7 on S7300/400 CPUs used PIW (Peripheral Input Word) and S7-1500 use just plain IW?

Where can I find more information? 

PIW means direct access to specific module internal memory. It also exists on S7-1500, but syntax is different: IW<byte address>:P.

IW means access to a Process Image. Here syntax is identical for S7-300/400 and S7-1500. But in case of S7-300/400 it must be assured that specific address is covered by a Process Image first.

Here's how I think of the differences...  However, my thinking is absolutely not based on facts, but just to ease my mind about the spelling differences.

S7-300/400 acquired the value from the input module someone on demand.

S7-1200/1500 acquire values from the input module before start of PLC logic solving (scanning), and place values into the Process Image (buffer).

If you want to access the valueright nowin logic, add the :P to the IW# address, like IW#:P, and the PLC willstop what it is doing and go ask the input module for the right then current value.

Using the process image approach puts Siemens more in line with the very first PLCs ever made and how they processed I/O.

There are exceptions available for special conditions and special I/O capable of updating very fast relative to normal PLC scan times.

Jasner

Thank you for the explanations.

Can you also explain, why STEP 7 on S7300/400 CPUs used PIW (Peripheral Input Word) and S7-1500 use just plain IW?

Where can I find more information?