Help using FB_FileOpen for Twincat (Beckhoff)

Hi,

I want to write a string into a file. This has to happen within a funcion (FUN, no FB) since I want to use it to log stuff to a file, triggered by the mein program.

My problem is:

If I step through my code, it works fine, but as soon as I run it normally, the FB_FileOpen keeps bBusy HI until it gets a timeout, regardless of timeout values or twincat pace (I'm using the simulation, not the real CX). Any idea what I'm doing wrong?

Here´s my code:

FUNCTION LogInfo : BOOL
VAR_INPUT
sID : STRING;
sInfo : STRING;
END_VAR
VAR
sMsgToLog : STRING; Step: SINT;
hFileHandle: UINT;
fbCreateLogFile : FB_OpenLogFile;
fbFileClose :FB_FileClose;
CreateOrAppend: DWORD;
fbFileOpen : FB_FileOpen;
fbFilePutString : FB_FilePuts;
x: INT;
END_VAR



fbFileOpen(bExecute := FALSE);

fbFileOpen( bExecute:=TRUE,
sNetId := '192.168.1.2.1.1',
sPathName:='C:\ADSLOG.TXT',
ePath := PATH_GENERIC,
tTimeout := T#2S,
nMode := CreateOrAppend);

WHILE (fbFileOpen.bBusy) DO
x := x + 1;
fbFileOpen(bExecute:=FALSE);
END_WHILE

hFileHandle:=fbFileOpen.hFile;

sMsgToLog := 'Whatever I need to write';

fbFilePutString(bExecute := FALSE);
fbFilePutString(bExecute := TRUE,
sNetId := '192.168.1.2.1.1',
hFile := hFileHandle,
sLine := sMsgToLog);

WHILE (fbFilePutString.bBusy) DO
fbFilePutString(bExecute:=FALSE);
END_WHILE

fbFileClose(bExecute := FALSE);
fbFileClose(bExecute := TRUE,
hFile := hFileHandle);

WHILE (fbFileClose.bBusy) DO
fbFileClose(bExecute:=FALSE);
END_WHILE

The way you have coded this means the plc program will not execute it's logic while it waits for the file open/write/close - is this your intention?

The example I've seen on the Beckhoff site uses a state machine to allow other processing to take place whilst the file access is being done. My guess is that the file processing is done outside of the plc scan - when you single step the file access is done between steps but when you run there is no time allocated because the plc cycle never completes.

Thanks, that might be the problem. That would mean that I cannot implement it the way I wanted (open / write / close to a file within a single "FUN").

You cannot call function blocks from within a function when the fb instances are declared as local variables of the function - local variables do not persist between calls of the function (created on the stack, so discarded when the function exits)

Functions are intended to be lightweight POUs with no memory - they always return the same output given the same input

Put it all in a function block, and it should work fine. Look at Beckhoff library blocks for some clues on structuring a fb - they use a state machine implemented by a CASE statement. Do not forget to call fb's that are edge triggered with the execute (or read or write or whatever) input off,once they have completed or they will never detect the rising edge next time you use them! Single stepping should make things clearer.

Regards

Roger