Another Siemens Indirect Addressing Question

[rikcando]

Yes, I did search for information first. Yes I looked through the limited help files that came with my Step 7. No I do not have any other books.

What I have done:
Machine does 3 basic sequences. I have 3 different FB's based on each sequence used to determine the next command number to send out (1 byte). I have (on paper) written a fourth FB to handle the actual setting of the command output byte (QB22), monitoring the acknowledge inputs (IB22), and setting Q21.5 when the acknowledge is completed correctly.

Item to consider:
Machine actually handle 2 sets of 3 basic sequences. Two simialr processes each with 3 FB's, each with a command byte and an acknowledge byte to be handled in the same manner. First set is QB/IB22, second set is QB/IB122.

What I would like to do:
In Ladder, not STL, write one FB to handle both command/ acknowledge sets. I would like to send the address of the command byte to the FB instead of hardcoding it and having two FB's.

Is it possible?

 

[rikcando]

I think I have been thinking too hard on this. Now that I have read this I see that I should probably just use the IN/OUT variables as my QB/IB and that would do it. Sorry for wasting your time.

Edit:
But is it possible?

 

[KalleOlsen]

Yes, it is possible. But could still be dependant of the calling structure of your FBs.
Would have been nice if you posted a sketch.

Kalle

 

[rikcando]

Sorry for delay but I took a nap. Thanks for the reply. Here is a small sketch I put together .

Edit: to update picture

 

[rikcando]

The process FB's will have an OUT parameter of CmdByte (byte).

The Main FB1 would call 2 instances of FB30, passing CmdByte and a pointer to the correct QB address.
Edit: Actually, I just wanted to send an INT and use it as the pointer address since I have configured the QB and IB to be the same for each device.

 

[rikcando]

I have been playng with the pointers trying to figure out how they work. I've not had very much success. I can load an output byte and transfer to a memory byte (L QB 3;T MB 4) but if I stick a pointer in there it give me an error.

I tried moving the pointer to a TEMP variable first (type INT) but still it does not work.

The error I get when I try to 'L QB [#tmpPoint];' is "Statement not permitted for DWORD/DINT indir. instruction address".
What does this mean? Can the pointer be used with an output byte (QB)?

 

[KalleOlsen]

This pointer must be a doubleword.
The format is byte,bit. The 3 lsb's forms the bit no, and must be 0 by byte/w/dw access. ( L byteno / SLD 3 would to the trick).

Have no time to check your sketch now, I'm off to a meeting.
Maybe LD pops in.

Kalle

 

[rikcando]

Alright, I know that I said that I wanted to do it in ladder, and that is mostly due to customer demand to keep the STL to a minimum. Below is basically what I want to do. I would prefer to have it in ladder. The issue is that I wanted to make an FB (or FC) to do this but replace the hardcoded address with a pointer to the input and output bytes.

      L     #CmdByte                    //Load command byte
      T     QB    22                    //tranfer to outptu byte
      L     IB    22                    //load ack inputs
      L     QB    22
      ==I                               //if equal
      =     #Ackd                       //      set OUT ack bit
      AN    #Ackd                       //if not ack'd
      =     #NotAckd                    //set  temp for tmr IN
      CALL  #Faulted                    //run SFB4
       IN:=#NotAckd
       PT:=T#2S                         // after 2 seconds
       Q :=#FaultBit                    //    turn on OUT fault bit
       ET:=
      AN    #temp                       //temp bit not on 
      SAVE                              // save - set br

 

[rikcando]

Thank you KalleOlsen. I think I understand now. I needed to reboot the Step 7 computer but I will be testing that right away.

 

[L D[AR2P#0.0]]

Hold on - why do you think you need to use a pointer and not pass the QB as an IN_OUT parameter to the FB?

 

[rikcando]

I have not given the whole story. I was trying to keep it to a simple question. I am not looking for other to do my work for me, and I was hoping to be able to figure this out myself. I guess i like the challenge.

I have 3 different bytes to send. I could send one byte and increment to the other bytes as needed. I wanted to make it a bit more efficient and readable by not having to send in 3 output bytes and 3 input bytes. If I could just send in one byte, or even DWORD with the address it would be much more presentable.

I can use the IN/OUT. I would prefer not to. I would like to learn how the pointer works.

I created a small sample program but it keeps crashing on me. It tells me it needs OB121. Loaded it in (empty) and I get the following error.

Alignment error on reading. (FB3 which has pointer logic)
Cause:
A bit address (bits 0 to 2) was specified for byte, word, or double word access.
To correct or avoid errors:
Make sure that the bit address contains access address 0.

 

[rikcando]

Current FB3

      L     #Pnt_QB3                    //Load command byte
      T     #DWPntr                     //tranfer to outptu byte
      SLD   3
      L     #CmdByte
      T     QB [#DWPntr]
      L     IB [#DWPntr]                //load ack inputs
      L     QB [#DWPntr]
      ==I                               //if equal
      =     #Ackd                       //      set OUT ack bit
      AN    #Ackd                       //if not ack'd
      =     #NotAckd                    //set  temp for tmr IN
      CALL  #Faulted                    //run SFB4
       IN:=#NotAckd
       PT:=T#2S                         // after 2 seconds
       Q :=#FaultBit                    //    turn on OUT fault bit
       ET:=
      AN    #temp                       //temp bit not on 
      SAVE                              // save - set br

 

[L D[AR2P#0.0]]

I can use the IN/OUT. I would prefer not to. I would like to learn how the pointer works.

OK, but I would strongly suggest staying away from indirect addressing for something as simple as this.

 

[L D[AR2P#0.0]]

You need to post all of the .AWL for your FB3 as I have no idea what the variable types are in your code (e.g. IN_OUT/STAT/TEMP...)

 

[rikcando]

I think it would be nice to know how to do this is necessary.

I will write an FC to return the ack bit and handle a single byte. I will call it 3 times, once for the command, once for DataX, and once for DataY. I can set up 3 different Ack bits and test those. I will do this just to make some progress today. If I write a small FC is it easier to pass off the use of STL with my client.

But I still would like to know how to do this, and is it possible to do this in LAD.