Step 7 - few questions

Hi Guys,
there is a project in the near future which I'm likely to be involved with. Based on S7-300 family and 3rd party SCADA system.

At this stage I'm trying to learn Siemens (have worked with AB before).
I've been reading a thread on this forum called 'I am forced to really learn how to programme a S7' - very helpful and interesting Got Berger book as well, Siemens manuals, etc..
My questions would be:

1) S7Guy has shown us his way of organising data using FBs and UDTs and call them unconditionally setting AR2 and opening relevant DB - fair enough, got it!
During the first scan he finds out the length of UDT and start address of the DB, as below:

What does the statement L W [AR1,P#2.0] do in this occasion?
In AR1 we have got the ANY pointer pointing the first 'important' element of the DB, so AFAIK, because this element is UDT type, the ANY pointer holds info about the entire element - Type, quantity, DB number, Area pointer.
Does is sound correct?
Berger books says: (regarding register-indirect addressing)

L MW[AR1,P#0.0] load the memory word whose address is located in AR1

Click to expand...

I assume AR1 was loaded before like this LAR1 P#30.0.
So we physically store data from MW30 to ACC1 - am I right?

In S7Guy example, by executing L W [AR1,P#2.0], we're trying to find out the length of the UDT. Does this mean that we loading ACC1 with NOT the data pointed by ANY pointer, but with the contents of AR1 (Byte n+2 and n+3)?
I don't 'feel' it very well, am I missing something important?

2) When S7Guy is trying to point the data within DB, he opens a DB, sets AR2 to point the first element and then calculating the offset and providing this to the function setting AR2:

Let's know assume that we've got 100 of this motors, it's very difficult to remember what motor's got what number. What would be your neat and convenient way of modifying the code to be able to 'choose' a motor by its tag name, for example:


3) When we creating a UDT and DB we would like to have ALL the info about one device under one UDT, one DB element. Majority of this parameters would be calculated only within the code, but some of the signals come from 'external' world, like Inputs, Outputs, etc.
Again, what is your neat way of assigning 'external' values to the 'internal' DB values? I do realize that this would have to be done 'by hand', but it would be nice, to at least use symbolic addresses (tags) instead of direct array access like
"DB Motors".Motors.Motor[15] = I1.2

I would be extremely happy if we all guys could talk about different approaches. Thanks for your help
btw. Excuse my English, I'm still learning


luccass

Can you post a link to where you got this project ?

Thanks LD[AR2,P#0.0] for your interest.

Link to the S7Guy Post
http://www.plctalk.net/qanda/showpost.php?p=330426&postcount=106

Do you have access to Step 7 and a plc or plcsim ?

I have access to Step 7 now.
I will have access to PLC shortly.

luccass said:

Let's know assume that we've got 100 of this motors, it's very difficult to remember what motor's got what number.

Click to expand...

You need to define 100 INTs with motor names and the INTs are initalized to the motor number.
Even if you were programming in C you would need to use #define, const, or enum to define all the motor names.

Make the motor name part of the motor udt - you could use a string if it helps makes things clearer.

What does the statement L W [AR1,P#2.0] do in this occasion?

Click to expand...

Loads the length of the UDT (in bytes) into Accu1.

Peter Nachtwey said:

You need to define 100 INTs with motor names and the INTs are initalized to the motor number

Click to expand...

Even if you were programming in C you would need to use #define, const, or enum to define all the motor names.[/quote]

Thanks Peter,
that was my initial idea on how to approach that task.
Then I thought: 'What if I make mistake with assigning numbers to each motor?' What I'm after is that I want to make sure, that MotorTag numbered 1 is 'connected' with FB's element 1, 2<->2, 3<->3, etc..
That case only one place I can make mistake is when I'm assigning field addresses to the relevant MotorTags - but this should be easy to troubleshoot.

Is that clear?

L D[AR2 said:

Make the motor name part of the motor udt - you could use a string if it helps makes things clearer.

Click to expand...

But to be able to use it within the code I would have to create a FC 'Give me an array element from a DB which holds in its .MotorName field value 'MotorTag'?? Or I'm looking at this from the wrong angle?

L D[AR2 said:

Loads the length of the UDT (in bytes) into Accu1.

Click to expand...

Thank a lot. Looks like I will have to hook up the PLC and digest slowly all theory I read about.

My thoughts exactly.

Not a huge Siemens fan but if memory serves me right you could
easily define a block of n motor data types (INTs or UDT or whatever) and name each element. I do this by creating the block in source and then compiling it. You should be able to access the block (of single UDTs) via pointer just like you would with an array of UDTs.

In order to see what the source will look like build a block manually and then generate the source for it.

When you add motors you can just redefine the block and update it with the new motor names.

Oh yea, you'd access the motors via code as follows: DBx."Motor1".element.

Correct me if I'm wrong.

Good luck,

(8{)} .)
(Yosi)