Bit analysis of string

[Dave Biggs]

Sometimes I need to 'deconstruct' data that has been squeezed into a few bytes of serial data. (Typically position reports sent from a mobile unit).
I stuff all the 'bits' into an array and pick through that to get the individual values. Something like..

 sTest = "SomeBytes"                              ' Raw data 
 Redim BitArray(Len(sTest)*8)                     ' BitArray() As String ("1"s and "0"'s)
 bp = StrPtr(sTest)                               ' bp as Byte Ptr
 
  For b = 0 To Len(sTest)-1                       ' one byte at a time
    TempChar = Bin$(@bp[b],8)                     ' TempChar => 'Binary' string
    For c = 1 To 8                                ' one bit at a time
      BitArray(i) = Mid$(TempChar, c, 1)          ' build bit array
      Incr i
    Next
  Next
 
  For i = 3 To 6                                  ' e.g. group of bits 
    sGroup += BitArray(i)
  Next
  ExampleRes? = Val("&B0"+sGroup)

[John Gleason]

Here is a function to read various bits or groups of bits in the string.

Technical note: It does everything "big endian". That means the most significant bits start from the left. This holds true for the input string bytes as well. It can be converted quite easily to the reverse--"little endian"--if needed, where the most significant bits are on the right. It could even be split, for example, to a big endian output from a little endian input string.

#COMPILE EXE
#DIM ALL

FUNCTION showStringBits(stringIn AS STRING, BYVAL firstBit AS LONG, BYVAL lastBit AS LONG) AS STRING
   STATIC oneTime, ii, strLen AS LONG
   DIM bitTable(255) AS STATIC STRING * 8
   LOCAL bitConvertStr AS STRING
   
   IF oneTime = 0 THEN
      FOR ii = 0 TO 255                    'make an array one time only which shows binary version of all 256 bytes
         bitTable(ii) = BIN$(ii, 8)
      NEXT
   END IF
   
   strLen = LEN(stringIn)
   
   'do a little range validity checking:
   IF firstBit > lastBit THEN SWAP firstBit, lastBit 'if first bit asked for is larger than last bit, swap them
   IF firstBit < 1 THEN firstBit = 1
   IF lastBit  < 1 THEN lastBit  = 1
   IF lastBit  > 8 * strLen THEN lastBit = 8 * strLen
   
   bitConvertStr = SPACE$(8 * strLen)      'our initial string to hold 1's and 0's
   
   FOR ii = 0 TO strLen - 1
      MID$(bitConvertStr, 8 * ii + 1) = bitTable(ASC(stringIn, ii + 1)) 'convert to 1's and 0's
   NEXT
   
   FUNCTION = MID$(bitConvertStr, firstBit, lastBit - firstBit + 1)     'choose bits to display
      
END FUNCTION  
    
FUNCTION PBMAIN () AS LONG
   LOCAL strIn, bitStr AS STRING, firstBit AS LONG
   
   strIn = CHR$(51,52,53,54,55,56,57,58,59)
   
   bitStr = showStringBits(strIn, 1, 33)
   ? bitStr & $CRLF & "123456789012345678901234567890123"

   bitStr = showStringBits(strIn, 11, 11)
   ? bitStr
   bitStr = showStringBits(strIn, 12, 12)
   ? bitStr
   bitStr = showStringBits(strIn, 13, 13)
   ? bitStr
   bitStr = showStringBits(strIn, 14, 14)
   ? bitStr
   
   firstBit = 7
   bitStr = showStringBits(strIn, 68, firstBit)
   ? STRING$(firstBit - 1, "$") & bitStr & $CRLF & "12345678901234567890123456789012345678901234567890123456789012345678"

   firstBit = 17
   bitStr = showStringBits(strIn, 68, firstBit)
   ? STRING$(firstBit - 1, "$") & bitStr & $CRLF & "12345678901234567890123456789012345678901234567890123456789012345678"
    
   bitStr = showStringBits(strIn, 1, 1338)
   ? bitStr & $CRLF & "123456789012345678901234567890123456789012345678901234567890123456789012"

END FUNCTION

[Cliff Nichols]

Serial Port but at a "Bit" level???
Hmmmmmm, Interestinnnnnnng.......

[Paul Dixon]

John,
the following program shows one way to extract the bits for each character of a string.

Paul.

'PBCC5.01 program
FUNCTION PBMAIN()
StringData$="ABCdef"

FOR Char& = 1 TO LEN(StringData$)
    PRINT MID$(StringData$,Char&,1);
    Character? = CVBYT(StringData$,Char&)
    FOR WhichBit& = 7 TO 0 STEP -1
        PRINT BIT(Character?,WhichBit&);
    NEXT
    PRINT
NEXT

WAITKEY$
          
END FUNCTION

[John Thompson]

Check HELP for CVBYT, BIN$, BIT, etc.

[John Radford]

A newbie question; I recieve data from the serial port using Comm recv. I then need to work with the recieved data at a Bit level - ie. be able to read each individual bit value in the recieved string.

Regards
John