Capture and compare serial data

[Dennis]

@Charles and Dave

I definitely agree with the checksum idea , in fact, it is the checksum concept that has led to the confusion

As a reference let's use this thread (thanks Art for his input!)
http://www.picbasic.co.uk/forum/showthread.php?t=12492

SO from that thread POST #2 one possible method of checksumming is described, since it's fairly straight forward we could use that as a reference.

So the bytes I would like to transmit are declared as
WNUM,XNUM,YNUM and ZNUM respectively.

***NOTE that this fairly trivial when comparing each byte of a single transmission BUT I am transmitting the same thing 4 times and that's whats confusing me.
The reason I want to do it 4 (or more) times is because from what I have seen in my tests the first one and/or second received line is usually damaged/corrupted so I figured it is probably best if I compare the third and 4th transmitted lines. Does that make sense?

Here's an example of the transmit code:

WNUM as byte
XNUM as byte
YNUM as byte
ZNUM as byte
CHECKSUM as byte
CHECKSUM = $3F

'now do the XOR checksumming
Checksum = Checksum ^ WNUM
Checksum = Checksum ^ XNUM
Checksum = Checksum ^ YNUM
Checksum = Checksum ^ ZNUM

'and now TX the data 4 times 
main:
	low led 'set led low
        If GPIO.2 = 1 then main 'check button
        
send:             
          
          for counter = 0 to 3
     
   DEBUG train,train,train,train,train,synk,WNUM,XNUM,YNUM,ZNUM,CHECKSUM,$d,$a
     	
           next counter
              high led 'set led high
        
             pause 200
    goto main
end

On the receiver side the code (as per the thread referenced earlier) would look like this:

'Variables begin here 
   
    SYNK    VAR BYTE
    SYNK = $7E
    WNUM    VAR BYTE
    XNUM    VAR BYTE
    YNUM    VAR BYTE
    ZNUM    VAR BYTE
    CHECKSUM var byte 'this holds the received checksum
    CHECKSUM_CHECK var byte 'hold the XOR'd value
    Checksum_Check = $3F 'this is the checksum value to XOR with 
    
  'Variables end here 

DEBUGIN [WAIT(SYNK),WNUM,XNUM,YNUM,ZNUM,CHECKSUM]
           ' wait for SYNK, when received store next 4 bytes in variables WNUM,XNUM,YNUM,ZNUM
           'then send these to hyperterminal or serial program or wherever 
            
            'checksum the received bytes using XOR with 3F 
            Checksum_Check = Checksum_Check ^ WNUM
            debug "xNUM checksum_check is ",HEX checksum_check
            Checksum_Check = Checksum_Check ^ XNUM
            debug "wNUM checksum_check is ",HEX checksum_check
            Checksum_Check = Checksum_Check ^ YNUM
            debug "yNUM checksum_check is ",HEX checksum_check
            Checksum_Check = Checksum_Check ^ ZNUM
            debug "zNUM checksum_check is ",HEX checksum_check
            
IF Checksum_Check = Checksum THEN
DEBUG "WNUM is ",DEC WNUM,"XNUM is ",DEC XNUM,"YNUM is ",DEC YNUM,"ZNUM is ",DEC ZNUM,$d,$a
ENDIF

BUT as you can see the receive code only does its thing for each received line. I want to store all 4 lines of data then do a compare of the received lines 3 and 4.

See the problem is if only one transmission is made (one line) and there is a corruption of a byte (checksum mismatch anywhere in the line) then the TX fails. That is why I thought to send and receive 4 lines (or more) of the same thing and then do a compare.

I am totally confused (again)

PLEASE HELP..........!

Kind regards

Dennis