Capture and compare serial data

[Dennis]

In my frustration @ myself and my rush to post I really messed up the english for this line

I am trying to do a kind of wireless capture and compare but I have a problem on the receiving end that I lookings for some suggestions on.

What it should read is :
I am trying to do a kind of wireless capture and compare but I have a problem on the receiving end that I am looking for some suggestions on.

Looking forward to your replies

Thanks so much

Dennis

[mackrackit]

Hey Dennis,

An array(s) would be the way to do it. Could compare separate bits or the whole array sorta like doing a checksum. Lots of possibilities.

This may give you some ideas on how to set it up
http://www.picbasic.co.uk/forum/showthread.php?t=544

[Dennis]

Dave :-)

Thanks a million for the tip ...very helpful indeed.
Still bashing around with the options.

Now this is what prompted me to post this thread, if I use an array , should I use a byte or a word array ?
Which would be the better one?
Why choose one over the other?

No matter what type of array I choose, I think the comparison of bytes would become rather cumbersome not so ?

For example if I choose a word array then the first line received would receive the first 4 bytes(0 to 4) WNUM,XNUM,YNUM,ZNUM.
The second line would be held in byte 5 to 7 not so ?

Then to compare each line is where things start to get cumbersome I think and by that I mean lots of IF,THEN statements.
So for example to compare lines 1 (WNUM,XNUM,YNUM and ZNUM and line 2 (WNUM,XNUM,YNUM and ZNUM I might have to do something like :
MyWordArray var WORD [7]
If (MyWordArray.0 and MyWordArray.1 and MyWordArray.2 and MyWordArray.3 ) = (MyWordArray.4 and MyWordArray.5 and MyWordArray.6 and MyWordArray.7) THEN DoSomething

This doesn't look quite right :-(

That's the problem I have ... I am really battling to somehow compare each line.
Is there a way of doing this by sending and receiving everything as a STRING and comparing the entire string ?

Any more suggestions would really be appreciated.

Kind regards

Dennis

[Charles Linquis]

Sometimes, when the strings you have to compare are very long, or when there are a lot of them, rather than compare them byte-for-byte, it is easier to use a good CRC algorithm on each one, then compare the CRCs.

[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

[mackrackit]

Not sure exactly what you are trying to do?

Send the same thing four times and if any of them fail the whole thing fails? Even if one is good?

Even doing a compare will not tell you which one is good. Only the checksum (CRC) thing will do that.

[Dennis]

Hi Dave

Send same thing 4 times
so
1
2
3
4
and compare instance 3 and 4 'cos usually correction occurs on 1 and 2
If 3 and 4 match then TX was good.

Hope that helps

Kind regards
Dennis