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

[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

[mackrackit]

OK, how about this.

Receive the first two and do nothing with that data because it is most likely bad.

Do the checksum on data #3.
if good then ... STOP??
OR go on and run the check on data #4

If #4 check then GO ON.

No compare any where...

MAYBE...

[Dennis]

@ Dave

OK cool

So I Will
1. Capture all 4 lines into and array each one is 4 bytes and a checksum byte so array needs to receive 20 bytes total.
2. Check 4 for checksum match
Continue on ...

It certainly seems doable and will give it a bash right away!

Still just wondering if there is a way to capture and compare all 4 lines

Kind regards
Dennis

[mackrackit]

If you really want to do each line then I guess the way would be ...

IF array1 bit1 = array2 bit1 then
IF array1 bit2 = array2 bit2 then
IF array1 bit3 = array2 bit3 then
......on and on and on....

But something like Charles said
IF check1 = check2 then
IF check2 = check3 then
IF check3 = check4 then
Hey. it is done...

[Charles Linquis]

Depending on what your data looks like, you can also do some neat things with:

ARRAYREAD (name_of_first_string),(nocompare_location),(numbe r_of_chars_to_compare),[WAITSTR name_of_second_string]

[Dennis]

AHA...

That's where I'm stuck ....at step 1 I mentioned earlier ...

1. Capture all 4 lines into and array each one is 4 bytes and a checksum byte so array needs to receive 20 bytes total.

I this is the line I am using to receive the data

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

What must be changed in order for me receive it 4 times and build the array?

I was playing around with this as a start..but it's here where I am stuck!

For counter=0 To 19 
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 
         counter=counter+1
        Next

As always any help would be appreciated

Kind regards

Dennis