Step7 - Is there an easy way to create large DBs?

[RMA]

So its an array then

Yup!

If its the same as S5, it would wait until the end of the current block.

This could still effect AR2 as the block could be a stat in another block.

The 64,000$ question - is it the same? Berger doesn't seem to be too clear on this, just saying that the "main program" gets interrupted, but whether this is only OB1, as the diagram in chapter 3.1 implies, and not FBs or FCs called from OB1, is not clear.

This project is pretty small, so I'm not using any multi-instance blocks this time, so at least that won't be a problem!

 

[SimonGoldsworthy]

Interrupts can happen after every statement (so thats any time any place any where, [that's Martini]). An interrupt must do a full CPU context save (or we may as well give up and go home).

 

[RMA]

An interrupt must do a full CPU context save (or we may as well give up and go home).

What all does that include - is saving AR1 and AR2 enough, or is there something else which I need to think about?

 

[SimonGoldsworthy]

You don't need to save anything yourself, the accumulators status word, address registers and DB registers are saved in the I stack for you and then restored at the end of the interrupt. Note also that at the start of an OB, the registers are set to zero and no DB's are open, so even if you opened say DB1 in OB1, it will not be open at the start of OB35.

 

[RMA]

Nice to know that some things are easier than one might expect. I'm pretty sure that I've seen discussions over the need to save at least the ARs under some circumstances - is this not true, or are there certain, special circumstances where it would be necessary?

 

[Ken M]

Sometimes, the interrupts can cause strange radiation patterns to be emitted from the CPU memory areas. These rays are particularly pronounced in the address registers, and for these special cases they are known as AR rays.

 

[RMA]

Ouch!

 

[SimonGoldsworthy]

AR2 is used as the base address in the instance DB for function blocks (function blocks that are multi-instance capable). If you change AR2, all references to STAT data in the function block will access the wrong area.

AR1 is used to access FC parameters that are complex, for example UDTs or structures. So, for example if you load AR1 with a value and then access a parameter of type UDT, AR1 will change.

If you want some examples then I'm sure I can find some later.

Here's one for AR1 http://www.plctalk.net/qanda/showpost.php?p=104628&postcount=8

 

[RMA]

OK, I think I've got that. In this case, I'm using neither multi-instances nor UDTs, so it looks like I can use S7Guys FC103 substitute with impunity.

That's nice, now I can start trying to fathom out how to speak to a Bürkert valve block over Profibus - something else I haven't had to do up till now. I expect I'll be back tomorrow with the first questions!

Cheers

Roy