FUNCTION_BLOCK PUBLIC fbSaveMemoryBlock
VAR_INPUT
   Write : BOOL;               //Edge triggered
   Read : BOOL;               //Edge triggered
   MemLocation : POINTER TO BYTE;   // e.g ADR(%MB0)
   ByteLength : DWORD;            // byte count to monitor and store
   MinTimeBetweenWrites : TIME;   // Used to limit writing
END_VAR
VAR_OUTPUT
   WriteCount : DINT;            //Count showing number of times write was triggered
END_VAR
VAR
   LastWrite : BOOL;
   LastRead : BOOL;
   
   FileHandle : RTS_IEC_HANDLE;
   OpenResult: RTS_IEC_RESULT;
   WriteResult,ReadResult: RTS_IEC_RESULT;
   
   pCompare : POINTER TO BYTE;
   LastByteLength : DWORD;
   DifferenceFound : BOOL;
   index : DWORD;
   pWorking, pWorking2 : POINTER TO BYTE;
   FirstPass : BOOL;
   TimeBetweenWrites : TON;
END_VAR

(*********************************************************************************)
(* This function block is to provide an alternative to persitent variable values *)
(* on systems that do not have a UPS or NOVRAM                                   *)
(* CAUTION must be used because excessive writing can damage flash memory        *)
(* MemLocation is specified like ADR(%MB0)  or ADR(V1)                                      *)
(* Variable values to be saved are mapped to the block starting as specified     *)
(* by MemLocation                                                                *)
(* A method that works, but not guaranteed is to create a GVL only for the      *)
(* variables to be persisted. Use ADR(firstvar) and calculate the byte length of *)
(* all the variables in the list to use in ByteLength                            *)
(* EDIT : The method above will not work as variables are added unless a Clean All*)
(* is done to reorganize the variable memory assignments                         *)
(* The memory block values are monitored for changes which will trigger the write*)
(* The writing can be limited by using the MinTimeBetweenWrites                  *)
(* Read input can be set to TRUE to load values on first scan                    *)
(*********************************************************************************)
(******************************************************************************)
(* Read the file and store in memory - rising edge triggered                  *)
(******************************************************************************)
IF Read AND Read<>LastRead THEN
   FileHandle:=SysFileOpen('PFile.dat',SysFile.AM_READ,ADR(OpenResult));
   
   IF OpenResult=0 THEN
      SysFileRead(FileHandle,(MemLocation),ByteLength,ADR(ReadResult));
   
      SysFileClose(FileHandle);
   END_IF
   LastRead:=Read;
END_IF
(******************************************************************************)
(* Create a memory area for comparing a change in values                      *)
(******************************************************************************)
IF LastByteLength<>ByteLength THEN
   IF LastByteLength>0 THEN
      __DELETE(pCompare);
   END_IF
   
   pCompare := __NEW(BYTE,ByteLength);
   LastByteLength:=ByteLength;
END_IF
(******************************************************************************)
(* Check for a change in values                                               *)
(******************************************************************************)
// Do not set PT here so that first check occurs without delsy
TimeBetweenWrites(IN:=TRUE);
index:=0;
IF TimeBetweenWrites.Q THEN
   WHILE (index<ByteLength) DO
      pWorking:=pCompare+index;
      pWorking2:=MemLocation+index;
   
      IF (pWorking^ <> pWorking2^) THEN
         DifferenceFound:=TRUE;
         // Copy all values to working memory area for checking value chsnge
         pWorking^:=pWorking2^;
      END_IF
      
      index:=index+1;
   END_WHILE
   IF DifferenceFound THEN
      TimeBetweenWrites(IN:=FALSE,PT:=MinTimeBetweenWrites);
   END_IF
END_IF
(******************************************************************************)
(* Write Values to the file                                                   *)
(******************************************************************************)
IF (Write AND Write<>LastWrite) OR (DifferenceFound AND FirstPass) THEN
   FileHandle:=SysFileOpen('PFile.dat',SysFile.AM_WRITE,ADR(OpenResult));
   
   IF OpenResult=0 THEN
      SysFileWrite(FileHandle,(MemLocation),ByteLength,ADR(WriteResult));
   
      SysFileClose(FileHandle);
   END_IF
   
   LastWrite:=Write;
   WriteCount:=WriteCount+1;
END_IF
DifferenceFound:=FALSE;
FirstPass:=TRUE;