Question concerning pointers and BITADR()

2020-02-27T19:44:38.637000Z

Inside a word you can look for a bit if it is one by:
out:= WORD_TO_BOOL(inword and (SHL(one,bit)));
VAR
out : BOOL;
inword :WORD:=0; ( word to be tested)
bit:WORD:=0; (bit position in word)
one:WORD:=1; ( 1 to shift left)
END_VAR
this is nice 61131 and bitadr is not in the code
other ways are possible look at w www.oscat.de w for a nice library.

Question concerning pointers and BITADR()

2020-02-27T19:44:37.364000Z

Hi Vuli,

Thank you for your reply. It seems that BITADR is indeed very different from ADR.

Question concerning pointers and BITADR()

2020-02-27T19:44:33.702000Z

I guess I will answer my own question in case someone runs into this issue:

BITADR and ADR operators are very different

ADR returns pointer
BITADR returns bit offset

Example:

VAR
     var1 AT %IX2.3:BOOL;
     bitoffset: DWORD;
END_VAR
bitoffset:=BITADR(var1);

The value returned would be bitoffset = 19 (decimal)
If you consider that %IX2.3 just represents 2nd byte and 3rd bit
converting it to bits you get 2*8+3=19

Now...at the moment I am not aware of the way to get the actual bit state (true/false) of bit 19 directly.

I did figure out a decent workaround tho (needs cleaning up but it should work as is):

VAR
   byteOffset    : POINTER TO BYTE;
   address    : UDINT;
   dInputBase      AT %IB0 : BYTE;       (* FIRST INPUT BYTE *)
   dInputs     : ARRAY[0..100] OF BYTE;           (* ARRAY of HUNDRED BYTES to COVER IB0 to IB100 *)
   i      : INT;
   bitStatus   : BOOL;
   bitCount   : UDINT;
   byteCount    : UDINT;
END_VAR
   byteOffset := ADR(dInputBase);    (* GET A STARTING BYTE POINTER *)
   dInputs[0] := dInputBase;           (* ASSIGN A STARTING BYTE TO FIRST ARRAY ELEMENT *)
   
   (* LOOP AND INCREMENT THE POINTER, SKIP INDEX ZERO, ASSIGNED IN PREVIOUS LINE *)
   FOR i := 1 TO 100 DO
      address := byteOffset;
      address := address + SIZEOF(byteOffset^);  (* INCREMENT TO NEXT POINTER *)
      byteOffset := address;
      dInputs[i] := byteOffset^;                      (* ASSIGN INPUT BYTE TO ARRAY VIA POINTER TO THAT BYTE *)
   END_FOR
   
   (* GET BITADR FROM SOME ARBITRARY INPUT VARIABLE *)
   (* ON MY SYSTEM diAirPressure WAS MAPPED to IX76.4 BY TWINCAT *)
   (* BITADR RETURNED 612 WHICH IS 76*8+4 BASED ON TWINCAT MAPPING *)      
   (* DIVIDE 612 by 8 TO GET INTEGER BYTE COUNT *)
   (* DIVIDE 612 by 8 USING MOD to GET BIT COUNT *)
   byteCount := BITADR( diAirPressure ) / 8;
   bitCount := BITADR( diAirPressure ) MOD 8;
   bitStatus := GETBIT32( dInputs[ byteCount ], UDINT_TO_SINT( bitCount ) );

Question concerning pointers and BITADR()

2020-02-27T19:44:32.389000Z

Hey

Anybody figured out this one by any chance ?

Question concerning pointers and BITADR()

2020-02-27T19:44:31.083000Z

Hello, I am wondering about the following code:

Output : POINTER TO WORD;
Toggle : POINTER TO BOOL;

Output := ADR(%QW6);
Output^.1 := TRUE; - works

Toggle:= BITADR(%QX6.1);
Toggle^ := TRUE; - crash

If the ADR function is used on a pointer to a word, the value of the used address can be set using that pointer.
However, when a pointer to a bool is uses, the BITADR function should be used instead of the ADR() function. This results in an Illigal instruction.

Can anyone explain what the difference is between ADR() and BITADR()?
Is there another way to achieve the desired result? (The goal is to make an abstraction layer by refence to the physical IO of a plc, so that if different hardware is used, only the addresses need to be changed)

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